AM Transmitter, 813 Style, Part 1 (PA Deck)

(This is the first part of a three-part series. Parts 2 and 3 can be found here and here.)

[Note: I've changed the circuit slightly from my original publication in this post. Refer to the 19 August 2010 Addendum below.] 

Some time ago I toured the shack of a friend, W7MS (Mike), in Reno, Nevada. I was very impressed by his collection of boatanchor equipment, but I was especially wowed by his RCA BTA-250M Broadcast Transmitter that he'd converted to 75 meter operation. 

RCA's BTA-250M was designed to generate 250 watts carrier output using a pair of 813s modulated by another pair of 813s. Mike had done a great job of restoring his radio, and the four 813s, lit up side-by-side, were beautiful. 

After I left I began thinking...I had a box full of 813s up in the attic. I wonder if... 

Well, skipping ahead...about half a year later I finished constructing my 75 meter AM transmitter. Like the BTA-250M that inspired it, it too uses four 813 tubes: two in the PA and two in the modulator. It's designed to be driven by an external audio and RF source, and I use a Johnson Ranger to drive mine (identical to how John Staples, W6BM, drives his 813 transmitter, as described in Electric Radio, issue 15). 

 My transmitter generates output carrier RF power in the range of 200-350 watts. Here's the schematic of the PA Deck :

 

(Click on image to enlarge)

Notes on the schematic: 

1. A large part of the design is based upon the 80-meter 813 amplifier described in "One-band Kilowatt Amplifiers," which can be found in the 1961 - 1968 editions of the ARRL Handbook. I designed a different pi-network using the equation in the Wingfield equations (reference recent ARRL Handbooks). 

2. Per the original "One-band Kilowatt Amplifiers" article, the amplifier doesn't require any neutralization on 80 meters, so none was added. 

3. There's a two-pole, three-throw rotary switch that's used to select screen-grid current monitoring (either the left tube, the right tube, or both tubes together). Monitoring screen current independently allows (allegedly) for tube matching. 

4. Originally, I didn't have parasitic suppressors in the plate circuits of the 813s, but, when I first started testing the deck, I was seeing a lot of high-frequency stuff on the 'scope I'd connected to the "RF Sample" Output BNC, and I thought that this might be parasitics, so I added the two plate suppressors. They changed nothing, and I later discovered (using my spectrum analyzer) that the high frequency crud was all harmonically related to the fundamental -- that is, it's the natural byproduct of a Class-C amplifier, and that there were no parasitic oscillations. I decided to leave the plate suppressors in (out of laziness), rather than remove them, but, per the "One-band Kilowatt Amplifiers" article, they shouldn't be needed on 80 meters. 

5. Given the high-frequency harmonic components that I was seeing on my spectrum analyzer (up to and beyond 200 MHz), I built a metal cage around the entire PA deck to minimize unwanted EMI radiation. 

6. The input network is the same as the one described in "One-band Kilowatt Amplifiers." C38 was changed from 0.001uF to 0.01uf to give a bit stiffer connection of the input network to ground (because there's no neutralization required, this capacitor doesn't need to remain 0.001uF that was used in the original article). 

7. The pi-network's inductor is a three-inch long piece of air-inductor stock that I had in my junkbox (2.5" diameter, 6.7 tpi, 12 gauge wire). This length gives a max inductance of about 15 uH, but I tap it at around 10.8 uH. 

8. To design the Pi-Network I first calculated the load that I needed to present to the plate using the equations for Class C RF Power Amplifiers found in the RF Vacuum Tube Amplifiers section of older editions of the "Radio Handbook," published by Editors and Engineers. (For my calculation I used 350 watts out (carrier) at a B+ level of 1650 VDC. This gave me a Plate Load (R_L) of about 2600 ohms.) 

Then, given this load and the desired Q (Q should be in the range of 10 - 20; I chose 12), I used the Wingfield equations from the ARRL Handbook to calculate Pi-Network components. I put all of these equations into an Excel spreadsheet to allow easy manipulation and experimentation "on paper." 

(Note: equation nomenclature changed in later editions of the Editors and Engineers "Radio Handbook" from that used in earlier editions, and I believe an error crept into the text. The best way to determine if an edition is in error is to compare the variable being solved-for in the description of the Class-C calculation steps (particularly steps 6 and 7) against the variable being solved for in the same steps of the "Sample Calculation" that follows this description. For example, the 18th edition of the book, the terms ebmin and epmin are swapped between their use in the description of the equations and their use in the "Sample Calculation" which follows the description. If you're putting your equations into something like Excel, watch out, or you'll have a problem! I fixed this by assuming the terms ebmin and epmin were correctly used in the "Sample Calculation," and so I swapped them instead when they were first mentioned in the prior description of the calculations.) 

19 August 2010 ADDENDUM: 

I noticed that, while transmitting, the RF power output would slowly increase from 250 watts (my initial setting) to 300 watts over a period of about 3 minutes of continuous transmitting. And this effect would reoccur after I let the transmitter idle for awhile (i.e. cooling down) and then began transmitting again. 

In other words, it acted suspiciously as though the Pi-network's "loading" setting was changing with heat (its capacitance decreasing). So I pulled the RF Deck out of the rack for some bench testing. 

If heating were an issue, as a quick test I transmitted for a few minutes, then powered-down (letting the HV decay to 0 volts!) and felt various components in the RF deck. Most felt OK, temperature-wise, but one capacitor, a 500 pf, 20 KV cap that I had placed in parallel with the "LOADING" variable-capacitor, was suspiciously warm. Hmmm...could this be the culprit? 

I was using two sections (out of three) of the loading variable-cap (both sections connected in parallel). I wired in the third section of this cap (giving me 1800 pf max instead of 1200 pf) and removed the fixed 500 pf HV cap that was in parallel with the loading cap. 

Powered back up, tuned the transmitter for 250 watts carrier output power, and after three minutes...it was still 250 watts! Problem fixed! Apparently the cap was lossy and, with the tank-circuit currents, it was heating-up and changing its capacitance. 

I was a bit concerned that, with the three sections of the variable-cap wired in parallel, adjusting the LOADING control for a desired power might be a bit touchy because the capacitance might change too quickly as I turned the knob, but it's actually quite acceptable (admittedly, I have BIG KNOBS on my controls, which help when making fine adjustments). In this new circuit configuration, LOADING adjusts power from a min of about 180 watts to a max of about 370 watts RF output (carrier only, as measured using a Bird 50 ohm dummy load). I typically run the power at 300 watts carrier output. 

The PA Deck schematic page (above) is now labeled "Rev. 2", to differentiate it from the original Rev. 1. The changes incorporated into Rev. 2, are:

1. Delete C81 (500pf, 20KV fixed cap).
2. Change C30 from a 1200 pf max variable cap to an 1800 pf max variable cap.

(Note: Other schematic pages are still at Rev. 1).

   

The culprit!

Fixed cap removed and 3 sections of variable-cap wired in parallel.

And here's the finished transmitter, up and running!

Some additional photographs showing construction of the PA Deck...

   

Building an RF "cage" around the PA using scrap sheet metal I purchased and had cut-to-size at a local metals recycling place.

 

I grounded the metal base of each 813 in the PA section at two different spots for each tube using flexible "fingerstock." (The fingerstock flexes out of the way whenever a tube is inserted or removed). I don't know if this is necessary, but I recall reading about it somewhere (can't recall where, though, at the moment).

    

The angled piece of black material between the tubes and the front panel is actually a rectangle of PCB material that I painted black and stuck into the PA Deck to deflect the fan's air up and out through the screen material on top of the case. 

 The finished PA Deck:

 

Other Notes: 

1. The plate voltage when not transmitting is about 2300 volts DC, but it sags down to around 1800 at 250 watts out. This sag is probably due to the transformer itself coupled with the capacitor-input filter (rather than choke input filter) that I'd decided to go with (hey, I already had the caps in the junkbox). Modifying the supply to a choke-input filter may give me more output power, but honestly, it's more work than I think it's worth, so I'm leaving it as it is. (Note to self, though: next time, do a load test on the transformer and filter before installing everything!) 

2. Screen voltage is about 350 volts idle and drops to 300 volts when transmitting. 

3. At 290 watts out, HV reads about 1775 VDC, Plate current is 230 mA, Screen-grid current is 51 mA, and grid current about 20 mA. (Therefore efficiency is about 71 percent). 

4. When running at a Pout of about 290 watts, plate voltage of 1775 VDC, ebmin of about 300 volts (assumed), and efficiency of 70% (plus other assumptions per the "Radio Handbook" equations) these numbers work out to a plate load of about 4000 ohms. For an inductance of 10.8 uH in the pi-network, Q (given a 50 ohm load) calculates to be about 17, so we're in the ballpark of a Q between 10 and 20. 

5. Note the following pi-network Q relationships (using the Wingfield equations):

• As output power increases (by changing loading capacitance), for a given value of pi-network inductance, pi-network Q will decrease.
• As frequency decreases, for a given value of pi-network inductance, pi-network Q will increase.

How does it sound on the air? 

You can listen to a clip of the 813 Transmitter on W6THW's website here. It's the track labeled "K6JCA 813 RIG (AM)". (The rig was putting out about 300 watts, carrier power. Mic is a Heil PR-40 run through a Beringer 802 Mixer/EQ box, which feeds the Johnson Ranger's microphone input.)

References: 

 Articles:

• "A Modern One Kilowatt AM Transmitter," W6BM, Electric Radio, #15, July, 1990
• "813 Triodes as Modulators," W6BM, Electric Radio, #57, January, 1994
• "An AM Kilowatt Using 813s 1989 Style," WA4KCY. Electric Radio, #5, September, 1989
• "One-band Kilowatt Amplifiers," ARRL Handbook, 1961 - 1968 Editions, ARRL
• "Class-C Amplifier Calculations," Radio Handbook, Editors and Engineers, 18th Edition (1970) [See note in text above re: error in equations.] Or one could use an earlier edition of this book, such at the 15th edition (pages 153-156) which doesn't have this error.
• "Tank Output Circuits,"ARRL Handbook, 1997 edition, ARRL, pages 13.5 - 13.9 (Describes the Wingfield pi-network equations.)

Websites, Transmitters

• WB4GWA
• K7JEB

Websites, RCA BTA-250M Manual

• BTA-250M Manual

Websites, 813 Data

• Detailed RCA data here or here.
• 813 Data, AA8V

Sequencer Designs:

• W2DRZ Sequencer

Caveats! 

Standard warnings apply: First, I may have made mistakes when writing this post or in my design. I cannot guarantee everything is correct. Second (and most importantly), this design uses high voltages that can kill you. Be cautious and BEWARE!

Hooking Up a Western Electric 211 "Spacesaver" Telephone

I picked up this cute little telephone recently at the local De Anza swapmeet.


From web research, I discovered it's a Western Electric 211 Telephone (their "Spacesaver" model). Typically you'll see these with dials mounted on top of the metal box (they actually look pretty cool). This one is sans dial, making it a "manual" (rather than "dial") telephone.

This set consists of a G1 case and an F1 handset. When I opened it up I discovered that the case only contains a hookswitch and nothing else! The drawing below shows a "generic" 211's circuitry (the simpler wiring of my "no dialer" telephone is shown in yellow). Note that the circuitry at the left is contained in a separate "subset" unit (which I will discuss further, below).

(Click on image to enlarge)

As I mentioned above, these telephones are meant to be used with an external "Subset" (such as the Western Electric 634 or 684), which contains a network for the anti-sidetone circuit as well as the ringer bell.

To test the telephone without a subset, I simply connected one side of the phone line to the "GN" terminal, and the other side of the phone line to the "Y" terminal. This will place the hookswitch, the mic element, and the receiver element all in series and connected to the Telco line. This implementation is shown in the photo below.


But there are several drawbacks when wiring the set this way. First, there's no anti-sidetone circuit, so you'll sound fairly loud to yourself in the receiver when you're talking into the mic. Also, there's a DC voltage across the receiver element, and thus there's the possibility (allegedly) that, over time, this might demagnetize it. Finally, there's no suppression of "clicks" in the receiver when going On or Off hook -- if you're holding the handset to your ear while "flashing" the telephone hookswitch, you'll hear VERY loud clicks.

So it seemed that the thing to do would be to find an appropriate subset. I looked around for one, such as the 634A, 684, or the 685A, but all the ones I found were fairly expensive, and I really didn't want to spend a lot of money on one.

Hmmm...could I build my own?

Well, I didn't need a ringer because I have a phone with a ringer across the room on my desk. I really only needed a network. If I could find a network, I could build my own subset...

One network I found was a "101A" network that was used in telephones such as the Western Electric 302 . This is just a transformer, and it requires an additional 2 uF cap to work with the telephone. But the 101A is a bit primitive -- it doesn't include any components to limit clicks. I decided instead to use a more modern network such as the 4228 or the 425 (the 4228 is found in Princess phones (I believe), while the 425 (e.g. 425E) is found in the 685A subset and 500-series telephones) . In the end I purchased a 4228 network from someone I found via the "Telephone Collectors International" List Server (on Yahoo).

Here's how I connected my 211 to my external 4228 network:

(Click on image to enlarge)

I want to stress, these connections are only for my "no-dial" 211 set! Here's a list of instructions for making these connections:

Required:

1. 4228 (or 425) network
2. 5-wire cable
3. 211 (sans dial) Telephone

Instructions:

1. At the 211 Telephone, move the Handset's BLACK wire from the "BK" terminal to one of the two unused terminals marked "B".
2. Using the 5-wire cable, connect one end of one of its wires to the "L2" terminal on the 4228 network, and connect the other end of this same wire to the "Y" terminal in the 211.
3. Using the 5-wire cable, connect one end of one of its wires to the "GN" terminal on the 4228 network, and connect the other end of this same wire to the "GN" terminal in the 211.
4. Using the 5-wire cable, connect one end of one of its wires to the "B" terminal on the 4228 network, and connect the other end of this same wire to the "B" terminal in the 211.
5. Using the 5-wire cable, connect one end of one of its wires to the "C" terminal on the 4228 network, and connect the other end of this same wire to the "BK" terminal in the 211.
6. Using the 5-wire cable, connect one end of its last wire to the "R" terminal on the 4228 network, and connect the other end of this same wire to the "R" terminal in the 211.
7. Connect one of the two Telco wires to the "RR" terminal on the 4228 network.
8. Connect the other Telco wire to the "L2" terminal on the 4228 network.

Here's a picture of my homebrew "subset" with the 4228 stuck to the bottom of a project-box with hot-melt glue. Telco comes in via an RJ connector on the left side. The gray wire bundle (5 wires) goes to the 211 set...

(Click on image to enlarge)

(There's a drawing HERE to help you identify the terminals on a 4228 Network. Note that "L2" is a dummy terminal (that is, it isn't connected to anything), and you can use its two screws to connect one side of the telco tip/ring pair to the telephone, as I've done.)

Here's the phone, mounted for use by the workbench!



Addendum, 2 August 2014:

Someone reported that they'd had a problem when applying these mods to a 211 telephone with dial, and that their phone would not dial.

The mods above are only for a 211 without dial.  Do not apply them to a 211 with dial.

To adapt a 4228 network to a 211 with dial, I believe the required changes will result in wiring that looks something like this:

(Click on image to enlarge)

Please note the following, per the drawing above:

1.  Assuming that the 211 has a 5H dial, the wire between the R terminal of the dial and the R terminal on the Hook-switch assembly should be removed (I believe this is a Red wire).

2.  The wire from the BK screw terminal on the dial should no longer terminate at the B screw terminal on the Hook-switch assembly.  Instead, you will need to connect the dial's BK terminal to the 4228 network (connection is shown above to the C terminal of the 4228).

Note that if you have a G7 body, you should be able to use the BL screw terminal on the Hook-switch assembly for this connection.

3.  Therefore, you will need 5 wires to interconnect the 211 with your new subset (i.e. 4228 network plus Line-In signals), rather than the previous 4.

For reference, you can find a drawing of the original wiring of the 211, with dial, here (note that it doesn't include the 61M filter): http://www.classicrotaryphones.com/forum/index.php?topic=2111.0
(Also see Reference 2 in the Resource section, below).

Important note:  I do not have a 211 with dial, so I cannot test this circuit.  There might be a mistake.  If you do try it, please let me know your results.


Resources:

1. Information on the 211 (and other old Western Electric phones) HERE.

2. Wiring & other info for the 211 (towards the middle of the post) HERE.

3. A source for old phones and parts (and subsets!) HERE.

4. 4228 Network information HERE and HERE.

5. "Telephone Collectors International" webpage and list-server.

Standard Caveat:

And of course, there's the standard caveat! I may have made a mistake, so use at your own risk!

Improving the Selectivity of the R-105A Receiver

I discussed my experiences getting my ARR-15/R-105A receiver on the air in a previous blog posting (here). As I mentioned in that posting, the selectivity is very broad -- so broad that, when using the receiver on, say, an 80 meter AM net, it really suffers from adjacent SSB interference.

I would like to eventually pair this receiver with an ART-13 transmitter I'm working on, but first I needed to improve its selectivity.

How best to do this?

The R-105A has a variable IF -- that is, the IF frequency varies from 450 KHz to 550 KHz, with it being 500 KHz at the BFO "detent." Interestingly, when in MCW mode (i.e. AM), the BFO is OFF when the BFO dial is in its detent ('0') position, but it turns ON as soon as this dial is turned away from its detent.

So, clearly, for AM operation the BFO dial should be left in its detent position (otherwise the BFO is ON and you'll hear it hetrodyning with the carrier).

OK -- so the IF frequency is 500 KHz when copying AM. I wondered if I could cobble a 500 KHz AM mechanical filter into the radio. But first, I needed to find a filter...

I looked around the internet to see if I could find one for sale. Finally I found one on the Fair Radio site (Lima, Ohio). Actually, I found two:

The first filter was contained within the AF Audio Amplifier Module (Fair Radio p/n 546-6053) for the Collins 618T HF Transceiver, and it was a 6 KHz wide, 500 KHz mechanical filter (F500 Y60 (526-9378)). The price Fair Radio was asking for this module, including filter, was $52.00.

The second filter was contained within the 500 KHz IF Audio Amplifier module (Fair Radio p/n 105-AA) for the RT712 / ARC-105 HF Aircraft Transceiver. Fair Radio doesn't specify the filter model number, except to say that it's an AM filter, and that the IF frequency is 500 KHz. The price for this module was $44.00.

(Fair Radio provided the latter with a copy of the module's schematic. I don't know if they also provide this if you order the first module.)

Fair Radio also mentioned that the ARC-105 is a pressurized version of the 618T2 transceiver, so I thought, what the heck, it probably has the same filter as the 618T series transceivers, which is the F500 Y60. And best of all, it was $8 less than the other module.

So I ordered it. When it arrived, I noticed that all of the boards within the module were covered with a clear conformal coating. This wasn't a real big deal, but it's probably one reason why this module costs less than the 618T module. And it kept me from determining what the filter part number actually is, because the conformal coating effectively glued the shield bracket to the filter, and the filter's label is under it.

Here's the module I received from Fair Radio (with its cover removed). The filter is the long cylinder in the upper corner...


Because of the filter's insertion lose, I would need some sort of amplification to compensate. I first tried using the amplifier built onto the filter module's PCB. It actually worked fine, and I initially considered using this PCB with filter and amplifier already built and working for my project, but the board itself is a bit too large for mounting within the R-105A chassis, and so I decided to roll my own amplifier.

I designed and bread-boarded the filter and amplifier circuit and, after some changes, I mounted them on a small piece of copper-clad PCB material. Here's the finished circuit, installed within my R-105A:


This design uses an existing hole within the receiver for mounting, so a purist can, at a later date, easily remove my modification and return the receiver to its original condition. I connected this circuit into the receiver's existing circuitry by unsoldering a wire from one of the pins of the IF transformer Z-119 and soldering in new wires in its place (and connecting another wire to the receiver's AVC line). Again, the new wires can be easily removed and the original wire reinstalled to return the receiver back to its original condition.

Here's a comparison of the receiver's bandwidth with the filter in and out (I've raised the filter's trace by 10 dB so that you can more easily see the difference).


(The above measurement was made by driving the receiver's antenna input with a wideband noise source (General Radio 1383 Random Noise Generator), and then, using a FET probe attached to the grid of the first IF amplifier, capturing the frequency response on my HP 8568B spectrum analyzer).

Here's the schematic:

(Click on image to enlarge)

Notes:

1. I don't have a data sheet for the filter. I'm leaving its drive impedance high (it's the first mixer's load, which, when the filter is switched-in, is simply receiver's IF transformer, unloaded). And I've set the filter's load impedance to 2K ohms. Correct? I've no idea.

[Update, 10 Jan 2010: I just came across the following schematic in Service Bulletin 2 for the Collins 51S-1 receiver in which they install a F500 Y60 filter into the 51S-1. Note that the newly-installed AM filter (bottom half of page) uses a total of102 pF (51 + 51) at the input of the filter and 113 pF at the output (51 + 62). Source impedance is a 10 mH inductor (coupled to filter via 1000 pf) and load impedance is 220K ohms (coupled via 470 pF). I've experimented with using a 10 mH inductor that's switched-in (via the relay) to be the load, in lieu of using the existing transformer (as I'm doing now), and I've also experimented with different values of filter input and output capacitance. When I varied the capacitance I really couldn't see any significant change in filter shape/symmetry, and different 1sy Mixer loads produced different gains, but...nothing else of any real significance, so, at this time, I don't plan to change my design. But I'm including the Collins schematic below in case you'd like to experiment, using it as a basis for your design...]

(Click on image to enlarge.)

2. I installed variable caps (and some additional capacitance using silver-mica caps, similar to what is used at the filter input and output on the RT-712 module) at the filter input and output of my circuit, but during my testing I couldn't really find much difference in filter shape and symmetry with the caps installed or not installed. So I removed the additional silver-mica caps, but left the variable caps (which were peaked to give max response). These could probably be removed, too. The schematic reflects the current implementation that uses only the variable caps.

3. Because of the conformal coating on the PCB, you need to remove the filter and its shield bracket together. But this isn't too difficult. I simply clipped the bracket's four mounting tabs flush to the PCB with a pair of diagonal cutters. The stubs of the tabs that remained on the bracket were sufficiently long enough to allow me to easily solder them to my copper-clad board to hold the filter in place.

4. The relay came from the RT-712 module, too.

5. The filter is switched into the circuit when when the receiver's front panel Channel switch is switched to Channel 10, and only Channel 10. For all other channels the filter is out-of-circuit, and the receiver's selectivity is back to its original bandwidth. (Note: The filter could be switched-in for more channels -- simply connect the additional channels at the Channel switch using individual diodes in a wired-OR wiring.

6. The pot is set to give the same AVC voltage, for a given signal, when the filter is in-circuit, compared to when it is out-of-circuit.

7. [Note (7 January 10): I changed the design slightly (raising the source impedance for the source feeding the filter, lowering the gain of the first transistor stage) from when I first published this post, so the photo above of the implementation doesn't exactly match the schematic, and the schematic revision is Rev B, not Rev A. The following discussion pertains to this new revision]

I first designed the amplifier with only one transistor, but when I tested it I found that there wasn't quite enough gain, so I added the second transistor. I didn't rebalance the gains when I added the second transistor, so the first transistor supplies the majority of the gain (41 dB, assuming an Ic of 0.9 mA and a load of 3.9K || 100K || 33K (Av = gm*Rl)). The second transistor is variable gain, and in my case the pot is set to 3.8 Kohms, so the gain of this second stage is about 9 dB (Av = Rl / Rf). Therefore, the overall gain calculates to be about 50 dB.

50 dB is a lot of gain. I don't know why it needs so much. Hmmm...could I have made a math error in my gain calculation above?

Nope. I just simulated the amplifier using LTSpice IV (a great program, available here for free), and at 500 KHz the gain is 49.3 dB in the simulation.

Why so much loss? I don't know, but...I don't plan to investigate any further: the mod works to my satisfaction, and I've other projects to work on!

8. You can find schematics for the 618T series HF transceivers here.

9. And, as a reminder, my earlier post on the R-105A is here.


And of course, the following always applies...

Standard Caveat...

I hope you find this information useful, but please, use these modifications at your own risk -- although they worked for me, I cannot guarantee that they'll work for you. (After all, I could have made a mistake in transposing them from my lab notebook to this post.)

If you do find any errors, or if you have any questions, please let me know. Thanks!


Note:  November 2, 2013:

I've just received a very helpful note from Cliff, WB6BIH.  He says: 

There is no reason for needing extra gain to compensate for the mechanical filter.  I fear that you may have had a defective mechanical filter.  I bought a few of these several years ago, and found one that I had marked "bad" and it was the SSB filter. The wide AM filter I have works fine.

This is all you need; a .01 from the top of the mixer plate to the filter and I used a combination of disk capacitors of about 120 pf to resonate the filter input coil.  At the grid of the first IF I used a 100 pf mica (that probably would have worked fine for both).  There is more than enough overall gain after a second alignment using a Simpson 260 on the AVC voltage as an indicator. The shield cover goes on over this.  Also, larger capacitors on the last two audio stage cathodes will gain quite a lot more signal.  I will dig into that later, mostly to remove all the odd and useless controls and other connections to switches that only keep it from working.

I worked for the Navy before I retired and somewhere I have the Collins source control drawing on these filters.  I will send you a copy if I happen to come across it.  I spent a lot of time trying to get a smooth passband for these mechanical filters by varying the load and source impedances, but I don't think it matters (helps) much.  Just resonate them with about 110 pf in and out. 

This thing is the darndest technical oddity I have ever seen that you could call a "radio" and I appreciate your notes on getting started. 

Thanks very much, Cliff.  This is a simple mod, and I'll need to give it a try!

- Jeff, K6JCA

Revisiting the Heathkit Cheyenne Transmitter

[Note, 10 July 12: Another schematic error fixed:  The 1uF paralleled with the 25K pot should be between the cathode of the 6DE7 (pin 8) and ground, not the filament. That's now been fixed, and I've updated the schematic revision to Rev. 3.  - Jeff]

[Note, 24 Feb 10: I've just updated my schematic to fix an error. I'd forgotten to break the original connection from V6 pin 5 to ground. That's now been fixed, and I've updated the schematic revision to Rev. 2. I've also added Notes 5 & 6 at the end of this post. - Jeff]

(Cheyenne, now with knob and case. HP-20 power supply by the side)

In my earlier Cheyenne posting (click here) I described the modifications I'd made to the Cheyenne's audio stage to improve audio fidelity. This new post will describe some additional changes I've made since that original post, and I'll also update the MT-1 schematic.

I had been noticing that, during transmit, the 6CL6 tube (driven by the 6AU6 VFO) was getting very hot. I temporarily put a 0.5 ohm resistor in its plate circuit so that I could measure its plate current, and I discovered that it was drawing on the order of 64 mA, which, given its plate voltage of 322 volts, was 21 watts of plate dissipation.

The 6CL6 has a maximum plate dissipation of 7.5 watts, so this was far beyond where it should operate.

I took a look under the chassis and immediately noticed that the 6CL6's screen-grid resistor was 6.8K, rather than the 27K shown on my Heathkit schematic. But, when I referred to the assembly manual for verification, the instruction for the installation of this resistor describes a 6.8K resistor. In other words, the assembly manual's instructions match the wiring of my transmitter, whereas the schematic does not match the wiring.

I decided that the easiest way to lower the 6CL6's plate current would be to lower its screen-grid voltage. To do this, I added 47K ohms in series with the existing 6.8K ohm resistor. This drops the plate current to about 23 mA on 80 meters (7.4 watts plate dissipation) and 15 mA on 40 meters (4.9 watts).

Also, I found that I needed to change the value of the screen-grid resistor to the 6146 PA. I had initially changed this to 50K (please refer to my first post), but, after lowering the 6CL6 plate current (and replacing the 6146 tube) I found I needed to double this value to 100K ohms to give me my desired "sweet spot" of 10 watts out when the pot has been adjusted so that there's about 6 volts on the cathode of the first stage of the 6DE7 modulator (V6 pin 5).

While making these changes I also discovered other differences between the wiring of the Cheyenne's RF stages and the schematic. Again, the wiring within my Cheyenne corresponds to the instructions in my assembly manual, rather than the schematic

So I've modified the schematic to include my mods as well as the differences that I've found (to date) between it and the actual transmitter wiring (as described in my assembly manual). My mods are in red, and the wiring differences are in green:

(Click on Image to Enlarge)

These differences between the schematic and the manual are:

1. V2 screen-grid resistor is actually 6.8K, not 27K.
2. V3 screen-grid resistor is actually 4.7K, not 27K.
3. The tuning circuit for the PA grid is not a parallel L-C circuit, but instead it is a pi-network. 300 volts DC is fed to the plate of V3 (5763) via a 2.5 mH inductor. The signal from this plate is then coupled to the input of the pi-network via a 6.8 pF cap, and the output of the pi-network's inductor is directly connected to the PA grid (V4 pin 5), to which a grounded 47 pF cap is also attached.

There may be other differences, too, but I've not yet come across them. And unfortunately, I don't know which set of changes are the later (and thus, I assume, better) changes.

Additional Notes:

1. I used a Krylon spray-paint that I found at the local hardware store to repaint the Cheyenne's cabinet. It's Krylon Indoor/Outdoor Gloss Hunter Green. This isn't an exact match for the original Heathkit paint (which is slightly lighter and, in my opinion, contains a bit more blue), but I find it is close enough for my tastes.

2. With a new 6146W and the 6CL6 mods, for 10 watts out on 80 meters, the 6146 Screen Grid voltage is now about 94 volts (compared to the (roughly) 52 volts I describe in my original Cheyenne posting (for 9 watts out)).

3. There is a change to my tuning instructions in my original post: Now, when tuning, I peak the Drive control (grid) to give me maximum power out. Note that there may be two peaks (one lower than the other) as you rotate the Drive control. On 80 meters I get max power out with a grid current of about 2 mA, which is not the max grid current that I can get by rotating the Drive control.

4. The audio, in my opinion, sounds quite nice. I'm using a Heil PR-40 mic that is equalized using a Behringer 802 mixer/equalizer (the PR-40 the highs and mids need to be accentuated), and the Behringer level is set such that the Cheyenne's Audio pot is at about 2 to 2.5 on its scale. The Cheyenne's RF output is then fed into an Ameritron AL-811 linear.

5. If your power supply supplies a different high-voltage than my HP-20 (660 VDC), you may need to make some component value changes. For example, if you find that the final resistance of the 25K pot (from V6.5 to ground), after adjustment, is very small, you should increase the capacitance value of the 1uF cap that parallels it, otherwise you could lose low-frequency response in your audio. Or, if you want to keep this pot at roughly mid-level, you may want to try lowering the resistance of the screen-grid resistor to give you more output power.

My recommendation would be that you adjust the pot to give you (very roughly) 6 volts from V6.5 to ground, and then, if you don't have adequate power out, adjust the PA screen-grid resistor value. (If the voltage at V6.5 is too small, you run the risk of clipping the positive peaks of the audio at the grid of V6, which is why I recommend that the voltage at the cathode be about 6 volts).

6. By the way -- placing the 25K pot in the first section of V6, rather than the second section of V6 ( where others have placed it when modifying DX-60's), means you don't need to have a high-power pot (which you do if you place this pot in the cathode-circuit of the other half of V6). Instead, with it connected to V6.5, this pot adjusts the operating point of the first half of V6 (thus there's a low voltage across it, about 6 volts). Because the plate of this first half of V6 is dc-coupled to the grid of the second half of the tube, changing the cathode-voltage at V6.5 also changes the cathode voltage of the second section of V6, and thus changes the screen voltage to the PA.

(Raising the voltage at the V6.5 cathode drives this section towards cut-off (lowering plate current), which raises the voltage at the corresponding plate, V6.6, and thus the voltage to the grid of the second section. Raising this grid voltage means more current will flow through the second section of V6, which raises the voltage at the second cathode and thus raises the PA screen voltage.)

7. And as a reminder, my earlier posting on the Cheyenne is here.

8. An audio clip of my Cheyenne driving an Ameritron AL-811 amplifier can be found here.

Standard Caveat...

I hope you find this information useful, but please, use these modifications at your own risk -- although they worked for me, I cannot guarantee that they'll work for you. (After all, I could have made a mistake in transposing them from my lab notebook to this post, or there may be other problems in my rig that make its performance different from yours. So always verify for yourself that the changes have improved, rather than worsened, performance!)

If you do find any errors, or if you have any questions, please let me know. Thanks!

- Jeff, K6JCA

How to Make Your Equipment Look Like a Million Bucks!

There's a lot of surplus equipment out there that can be used as the basis for a homebrew project. Here's a way to make it look more professional.

For example, let's take this...


...and make it look like this:


It's actually very easy. Here are the steps I follow.

First, For the panel, I measure its dimensions and the locations of all holes I want to use. Then, using an electronic drawing program (I use Adobe Illustrator, but others I know use Autocad), I accurately place all hole locations as well as any labels I want to add on my panel drawing.


I then print out the panel drawing. If the panel is larger than, say, 8.5 x 11 inches, you'll need to create a "B" size sheet (11" x 17"). My printer does not print B-size sheets, so I instead print two "A" size sheets, trim their edges, align them (note the alignment marks in the overlap area), and then tape them together so that they form a larger sheet. If you don't have a light-table for accurately aligning them, a window works fine...


I then go to the local copier store (here it's the "Kinko's" chain) where I copy my "composite" drawing onto a B-size sheet (so that it's a single piece of paper, not two taped together).

(If you can squeeze in an extra panel, do it, just in case you "goof.")

I then laminate it (the local Kinko's has a laminator).


And I cut out the panel overlay (and any large holes) with an Xacto knife...


Finally, I glue my overlay to the original metal panel using "Adhesive 77" spray adhesive (manufactured by 3M) and cut out the remaining holes, using the holes in the original panel to guide my Xacto knife.

And voila!

[Many thanks to Dick, W1QG, who taught me this technique. His great looking panels on the equipment he built were an inspiration.]

Converting an HP Counter into a Nixie Tube Clock

Hmmm...I had an old HP 5233L nixie tube counter with 6 nixie tube digits gathering dust in the corner. What to do with it...?

Let's see, six digits...why not make it into a clock? After all, nixie tube clocks are pretty cool. And the counter came integrated with almost everything I needed: power supply and even a case with nicely milled holes. Mechanical work would be minimal, which defines my ideal project!

And as a nice bonus, there was also a back-panel BNC input for an external frequency reference, so I could use my GPS-locked frequency standard to keep the clock's time from "drifting" over time.

Conceptually, I figured it should look something like this (this includes a simple method for setting time):

Block Diagram

(Click on image to enlarge)

Pretty straight forward. But there was a small complication: I didn't have schematics for the 5233L.

This really didn't prove to be much of a problem. HP used the same nixie "single-digit" decimal counter plug-in module in a variety of its counter products. And I did have a manual for an HP 5232A. A quick glance revealed that it had the all-important schematic for the decimal counter module (HP part number 5212A-4A -- and although my counter used 5212L-4A modules, the 5212L-4A design seemed to be very close (if not identical) to that of the 5212A-4A)).

That manual, plus some scope probing of various signals within my 5233L counter, told me pretty much all I needed to know.

Some things I needed to do were:

1. Change two of the counter modules to count from 0 to 5 (instead of 0 to 9), so that minutes and seconds would each count from 0 to 59, instead of 0 to 99.
   
2. Add a reset circuit to the two "hours" digits such that, if the hour count increments to '13', it immediately resets the two hour digits.
3. Modify the Hour LS (Least Significant) digit to reset to '1' instead of '0' (so that hours count from '1' to '12'.
4. Disable the "Storage" feature of the Decimal Counter modules so that we can see the module counting.
   

So first, to count from 0 to 5, the 5212L-4A assembly(or 5212A-4a, or other variants) can be modified as follows:

1. Remove R45, R50, and C10
2. Change C11 from 200 to 470 pF
3. Move R59 from CR12 to CR11

If you're trying to understand how the decimal counter circuit works from its schematic, note that it counts per the following pattern:

D C B A
0 0 0 0 (0)
0 0 0 1 (1)
0 0 1 0 (2)
0 0 1 1 (3)
0 1 1 0 (4)
0 1 1 1 (5)
1 1 0 0 (6)
1 1 0 1 (7)
1 1 1 0 (8)
1 1 1 1 (9)

So, to count from 0 to 5 instead of from 0 to 9, we can use the transition of B from 1 to 0 to set C to 0. And D must be always kept at 0.

(Important schematic note: Q1/Q2 control bit A, Q3/Q4 control bit B, Q5/Q6 control bit D (not C!), and Q7/Q8 control bit C (not D!)).


For item 2, the Hours Reset circuit needs a few more parts. I mounted them under the counter chassis.

(Click on image to enlarge)

For item 3 -- to modify the HOURS LS digit decimal counter module (HP Assembly 5212(x)-4A) to reset to a count of '1' instead of '0', simply modify that module so that R23 connects to the base of Q2 instead of to the base of Q1.

And finally, for item 4:

The "Transfer Line" signal to the counter modules controls the modules "storage" function (necessary for a counter to maintain a stable display while the modules are counting). But there's no reason to use a storage function when operating as a clock -- we can simply view the count while it's incrementing, and it makes the modification simpler

To disable the "Transfer Line", I cut the wire from the driver that drove pin 5 of the Decimal Counter card-edge connectors. This line will then float at about-12V, and the storage feature of the Decimal Counter modules will be disabled).

So...that's essentially it!

[I did add various switches and buttons to give me some functions that I wanted (such as turning off the display, but continue running the clock, to save on "wear and tear" of the nixies). But the modifications listed above are the main mods to the basic counter function.]

Here's the top view of the modified counter...

...with the modified decimal-counter modules:

And a view under the chassis. The HOURS reset circuit is wired between two of the edge connectors towards the right.

The counter, before I made an overlay for the front panel...

...and after:

(Martini time!)

From left to right, front panel controls are:

1. Toggle Switch: Mode, RUN/SET-Time
   
2. Push Button: Increment clock when in SET-Time mode
   
3. Toggle Switch: FAST/SLOW increment speed when in SET-Time mode
   
4. Rotary Switch, 3 Position. Selects Display Format: H:M:S, H:M, Display OFF (but clock still counting)
5. Push Button, Reset SECONDS count
6. Pot with Power-On switch. Only the power-on switch is used.

Other Notes:

1. HP Manuals are great. I've always found their "Principles of Operation" section to be an excellent description of how their equipment operates -- very useful and well worth checking out.

2. To use a 5212(x)-4A module in slot A19 of the 5233L counter, clip wire going to pin 8 of that slot's card-edge connector. (I no longer recall which module was originally in this slot).

3. You can use the "decimal point" neon lamps in the modules as separators to differentiate between hours, minutes, and seconds digits (refer to photo of my counter).

[Note: the 5212L-4A modules in the 5233L counter do not have neon lamp decimal points within the modules themselves (unlike the 5212A-4A module). Instead, the neon lamps are mounted on a separate small PCB that runs underneath the modules, next to the front panel.]

4. You could probably make this a 24-hour clock as follows:

• Do not modify the Hours LS digit to reset to 1.
• Change the Hours Reset Circuit to use the following 2 bits instead of the 3 bits shown in my schematic above:
  

Hours LS Digit pin 13

Hours MS Digit pin 9

(I haven't tried this, so take this recommendation with a grain of salt.)

5. Pin 7 is the Clock input pin for a 5212(x)-4A module. It is to this pin of the first module (the least-significant digit of the "seconds" modules) that you connect the 1 Hz time reference (and the faster clocks, when in SET-Time mode).

6. In an HP counter you should find a number of identical divide-by-10 modules that are used to divide down the time-base reference frequency (HP p/n 5212A-65C). It is on the card-edge connectors of these modules that you can find the 1Hz, 100Hz, and 1KHz signals (as well as others, should you need them).

Note 1: These boards have pin 7 as their input and pin 5 as their divide-by-10 outputs.

Note 2: Although my block-diagram above has 1 KHz as the "fast" set-time frequency, I might have actually 10 KHz.

7. After I'd mucked around with making an overlay and installing it on the front panel, the counter started "double-counting" (it would increment twice in one second -- on the rising and the falling edges of the 1 Hz clock). It turns out that CR9 on the first nixie tube module had opened up. HP's p/n for this part is 1910-0015, but unfortunately it didn't list the manufacturer's part number. I replaced it with a 1N4148 -- and although this new part is silicon, rather than germanium of the original diode, it fixed the problem.

8. A note regarding my "Reset Seconds" pushbutton. This button simply zeroes out the MS and LS seconds digit (and it also keeps the other digits from counting). But if one does this when the seconds count is greater than '19', the minutes count will increment by one. This isn't a big deal for me, and actually, it's kind of useful when setting time. Here's what I do:

1. I'll run up the time to the current time (per WWV), making sure that there are at least 19 seconds on the clock.
2. Then I'll flip the RUN switch to RUN and depress and hold the Reset Seconds button. This will set the clock to the next minute and zero the seconds.
   
3. I then wait, with button depressed, until WWV hits the minute mark before releasing the button.
4. The clock is now within a fraction of a second of WWV!
   

9. The +20VDC rail within my counter was way off (it read +31 VDC). Some probing revealed that a couple of transistors, as well as a zener diode, in the voltage regulator circuit had blown. The two transistors were both germanium PNP parts (one was an HP 1850-0062, which crosses to a 2N404A, and the other was an 1850-0105, for which I cannot find a cross reference). I replaced these both with 2N3905 transistors (PNP silicon). I replaced the blown zener with a 6.8 volt one, and, when finished, the voltage read +19.98 VDC. The voltages across the 2N3905 transistors are well within their range, and they aren't getting warm, so I believe everything should be copacetic.

Final note...this posting was written not to give detailed instructions for modifying an HP counter to be a clock, but rather to generate ideas and inspiration. If you have an old HP counter kicking around somewhere, consider giving it a try!

- Jeff, K6JCA

Improving AM Performance of the Heathkit MT-1 Cheyenne Transmitter

[Note (10 July 12):  The schematic shown below has an error in it.  The 1uF cap paralleled with the 25K pot added to the circuit should connect between pin 8 (cathode) of the 6DE7 and ground, not between pin 5 and ground.  (I would fix this drawing, but I no longer have the original).  - Jeff]

[Update (2 January 2010): New information on my Cheyenne can be found here]

I picked up this transmitter at a swapmeet earlier this year. Although the front panel was in nice condition, the topside of the chassis itself had oxidized quite a bit (as had the cabinet, which was in dire need of rust removal and repainting), and it was not very appealing. Never the less, the price was right, and I thought it might be a fun project to get on the air during the cold winter months!

The Cheyenne -- case removed for repainting and
needing an original knob (hint hint) for the Drive control.
(Click on image to enlarge.)

Hmmm...under the hood, not so pretty

The Heathkit Cheyenne transmitter was a mobile AM and CW transmitter that Heathkit marketed in the late 50's and, I believe, early 60's (its matching receiver was the Heathkit Commanche (MR-1)). With a design including a single 6146 PA, 12AX7 Mic amp, and 6DE7 as a "controlled carrier" modulator, it is similar (although not identical) to the later DX-60 series transmitters.

I powered up the transmitter with an HP-20 power supply and quickly discovered that its audio in AM mode left quite a bit to be desired -- noticeable distortion and a restricted audio passband. So I wondered...what could I do to improve its performance?

Well, the first thing to do: check to see if someone else has already been down this path. Unfortunately, a google search revealed no internet articles for improving the Cheyenne. But, because of the similarities between the Cheyenne and the DX-60, I wondered if I could apply any of the DX-60 modification articles to the Cheyenne...

First Steps...Make the Cheyenne more like a DX-60...

Electric Radio magazine has several very interesting articles by Bill Breshears, WC3K, on improving the DX-60 audio. I thought they might be a good starting point for modifying my Cheyenne, but to do so, I'd first need to correct those few differences between the Cheyenne's audio/modulator stages and the DX-60's stages.

Upon comparing the Cheyenne schematics with those for the DX-60B, the significant differences in the modulator section seemed to be:

1. The DX-60B's 6DE7 Cathode Follower has a 33K ohm resistor from its cathode (pin 9) to ground. This resistor was lacking in the Cheyenne (and indeed, its lack prevents the AC signal on the Cheyenne's cathode-follower cathode from going below about 50 VDC).
2. The grid of the first stage of the DX-60B's 6DE7 modulator (pin 7) has a 22 Meg ohm resistor to ground, compared to the Cheyenne's 10 Meg, and this grid is driven by the previous 12AX7 stage via a 5 nF cap, instead of a 510 pF cap in the Cheyenne.
3. The DX-60B's PA screen voltage is driven by the 6DE7 Cathode Follower through a 47K ohm resistor paralled with a 0.1 uF cap. The Cheyenne uses a 10K ohm resistor and a 0.25 uF cap.

I incorporated the changes in first two items above (although I used a 6.8 nF cap in lieu of 5 nF in step two, because that's what I had in the junk box). I left the third item for later, until I could incorporate the Carrier-Level adjustment pot that WC3K described in his articles.

One of my goals was to drive my AL-811 linear amplifier with the Cheyenne. I don't feel comfortable running this amplifier in AM mode at more than about 100 to 120 watts carrier output power. For this level of power output from the linear, I needed the Cheyenne's idle-carrier power output to be to be in the range of about 9 watts or so.

I initially incorporated the 47K ohm resistor in step 3 above (keeping the cap at 0.25 uF) and added a 25K ohm pot in series with it to the PA Screen Grid (the 0.25uF cap paralleling both) -- similar to the carrier control pot described by by WC3K in his DX-60 mods. Unfortunately, I felt I had a bit too much drop in power, and I instead replaced the 47K with the original 10K. (This would later change again. See below...)

The new pot (mounted conveniently in the rear-panel's Key Jack hole (after all, who needs a key for AM operation?) allowed me to easily adjust carrier level. But during testing I wasn't satisfied with audio performance -- there was a still a bit of "fuzziness" on the audio (pointing to distortion) that bugged me.

Examining the audio chain, it became quickly apparent that part of the problem was with the 6DE7 "controlled-carrier" modulator itself. This modulator adjusts the carrier level such that the carrier level is low for low-level signals and higher for high-level signals. To accomplish this "dynamic" carrier-level adjustment, the first stage of the 6DE7 modulator, in addition to being an AC amplifier, "clamps" the input AC signal on its positive peaks, thus causing an additional DC voltage to be impressed across the coupling cap (that couples the signal from the second 12AX7 stage to the input grid of the modulator). This DC voltage is proportional to signal level and thus drives the modulator grid (6DE7 pin 7) more negative with higher audio levels.

As this grid is biased more negative (relative to the grounded cathode), the tube conducts less, reducing the DC plate current. The plate voltage goes up, thus raising the grid voltage on the next stage (cathode-follower) and consequently, of course, its cathode voltage.

As this cathode goes up, the PA Screen Grid voltage goes up, and more carrier appears at the output.

BUT -- the key here, and the source of the distortion, is the clamping action at the grid of the first 6DE7 section. This clamping action essentially flattens the positive peaks of the audio signal at this grid, which in turn results in flattening of the "troughs" of the modulation on the output RF signal.

This flattening of the audio signal is easily observable at the 6DE7 with a scope (monitor the plate of the first stage, for example), and is quite obvious on a 1 KHz test signal. Not good. My rule of thumb is...if you can see the distortion, you can hear it.

I also had a problem in which, as I tried to adjust the Cheyenne's audio level towards 100 percent modulation, I'd get compression (i.e. distortion) on modulated RF envelope "peaks". Again, this was readily apparent by comparing the modulation on the RF signal with the audio signal driving the modulator.

Here's photo showing both of these two distortion mechanisms (exaggerated to make it clearer) . The top trace is the modulator output. You can see the clipping on the largest negative peak due to clamping by the modulator input grid. The bottom trace shows peak compression on the output RF envelope, which you can see by comparing the different levels of the positive peaks of the modulator output with the peaks of the RF envelope -- they're all the same level!



The Next Step...Improving the Cheyenne's Audio...

The fuzziness on the audio was just enough to make me want to keep working on the transmitter. One source of this distortion, as discussed above, was from the clamping action of the "controlled carrier" modulator in order to dynamically adjust carrier level.

Hmmm...suppose I eliminated this clamping action and thus the distortion that it created? Would audio be improved?

Why not? I only needed a carrier to be somewhere in the range of 8 to 12 watts to drive my linear. There's no reason why the 6146 PA in the Cheyenne cannot handle this. In other words, why not remove the "controlled-carrier" feature of the modulator (and thus the distortion that it introduces) and keep the carrier at a fixed level?

And I wanted the carrier level to be adjustable so that I could adjust the level to give me my 100 watt "sweet-spot" output from my linear.

One way to get around the "control carrier" feature is to bias the first stage of the 6DE7 modulator so that there is a fixed negative grid-to-cathode voltage that is large enough to prevent clipping on the positive peaks of the incoming audio, yet provide sufficient carrier to drive the linear. I already had a 25K pot mounted on the back panel of the chassis that I had intended to use to adjust carrier level (and had been wired in series with the original 10K power resistor to the PA screen grid -- see discussion above). I wired it instead into the cathode of the first stage of the 6DE7 so that I could adjust its operating point.

Through a process of iteration, I adjusted the PA screen grid resistor value and the position of the 25K pot so that, for the carrier output power that I wanted (8-10 watts), the cathode of the first 6DE7 was at a high enough voltage that full-modulation audio wouldn't be clamped by the grid. Thus, the PA screen grid resistor (from V6 pin 9) was changed from the original 10K ohms to 50K ohms. Although I used a robust power resistor (it was in the junk box), there's no reason why, say, a 2-watt resistor couldn't be used. And you can play around with the value of this resistor -- lower values of resistance will increase the maximum carrier power, while higher values will lower the maximum carrier power (maximum carrier power occurs when the 25K pot is set to its maximum resistance).

[Important Note: there's a trade-off when selecting the value of the PA screen resistor: for a given carrier output power, lowering the PA screen grid resistor value means that the resistance of the 25K pot in the cathode of the first 6DE7 section must also be lowered to maintain the same carrier output level. This in turn will bring the cathode voltage closer to the grid voltage (which is essentially at 0 volts), which means that it's more likely there will be audio distortion introduced at this 6DE7 grid due to grid "clamping" the positive peaks of the audio signal. I found that a 50K ohm PA screen grid resistor worked well for my application.]

I found that the value of the pot is about 8K-9K ohms for about 9 watts carrier (no modulation) RF output from the Cheyenne. Given this value of resistance, I added a 1 uF cap in parallel across the pot to bypass it for audio frequencies. (Note: This cap can be made larger, if it's desirable to run the Cheyenne at lower power (and thus a lower potentiometer resistance, which means you need a larger cap to maintain the low-frequency cutoff), but increasing its value will also increase the amount of time that it takes for this stage to reach its bias point each time PTT is pressed.)

By the way, with these mods made and the Cheyenne set for about 9 watts carrier output (no modulation), I measure the following DC voltages during Transmit:

• V6.5: 6 volts (6DE7, first cathode)
• V6.2: 110 volts (6DE7, second grid)
• V6.9: 200 volts (6DE7, second cathode)
• V4.3: 52 volts (6146, screen grid)

These changes gave me the ability to control the output carrier power from about 4 watts to about 12 watts. Note -- at low powers there may still be some peak flattening at the grid to the first 6DE7 stage (this occurs when the cathode-grid bias voltage is less than the audio peak voltage at the grid), but I've found that there's no limiting when the 25K pot is set to give me 8-12 watt carrier power output.

OK! Now that I had the distortion reduced, I next tackled the frequency response, which was a bit too restricted in the stock Cheyenne.

This was accomplished (in addition to the changes above) simply by :

1. Changing the 0.001 uF cap feeding the grid of the first 12AX7 stage to 0.01 uF.
2. Changing the 510 pF cap feeding the Audio Level pot (from the plate of the first 12AX7 stage) to 0.01 uF.

This gave me an audio passband with -3dB break-points at 100 Hz and 5 KHz.


The new schematic:

(Modulator Modifications -- Click on schematic to enlarge...)

[10 July 12 -- Please note that there is an error in this schematic!  The 1uF cap paralleled with the 25K pot should connect between pin 8 of the 6DE7 and ground, not pin 5.]

Other problems:

1. 6.3 VAC reading low on DVM at the terminal strip: only about 5.5 VAC (causing the relay to chatter):

• Bypassed the fuse in the filament line with a wire soldered to the fuse-holder's terminals (this fuse is not needed, after all, the power supply is fused, and this fuse added a few additional tenths of a volt of voltage drop).
• Cleaned the Function switch contacts.

2. The 0.02 uF cap attached to V6 pin 1 (600v bypass) "popped." Replaced.

3. AC Hum on AM signal which gets louder as mic gain is increased. The PTT signal of the Cheyenne's mic jack directly keys the Cheyenne's relay, which is powered by 6.3V AC. A mic with wired to a 4-pin plug to mate with the Heathkit mic jack shouldn't have an issue with this, because, assuming the mic and its cable have separate grounds for the PTT return and the audio return.

Unfortunately, a number of my mics have common PTT and audio grounds (and are terminated with PJ-068 plugs), which means that the 6.3VAC on the PTT line runs on the same ground line as the audio return and thus contaminates the audio signal with AC hum.

I really didn't want to rewire a mic with a Heathkit-compatible plug -- I preferred to keep them terminated with PJ-068 plugs so that they're interchangeable among a number of my transmitters. Instead, I made an adapter with a PJ-068 compatible jack and a 4-pin Cheyenne compatible plug. Because the common ground would create a hum problem, I decided to have the PTT switch control a DC, not AC, signal, thus removing AC crosstalk from the common return line.

To do this I added a second, 5VDC relay, and rectified the 6.3VAC filament voltage to provide the voltage to drive this relay (see the schematic above). The mic's PTT button now switches this DC voltage. The new relay then switches the 6.3VAC signal to the original Cheyenne relay.

The new relay also has an additional benefit -- I wanted some way to mute an external receiver during transmit as well as key an external amplifier, and the extra contacts on the relay now provides these functions. (A previous owner of the Cheyenne had rewired the 6-pin connector that connects to a receiver (such as the Commanche) and several of these pins were left unused, so I brought these two new signals (Receiver Mute and Amplifier Key) to these unused pins on this connector).

(Additional note: the 6.3 VAC relay has a coil resistance of only about 8 ohms. This means that it draws about 0.8 A when ON. The only 5VDC relay I could find in my junk box has contact ratings of 1A at 30 VDC. OK, 1A is greater than 0.8A, but personally, I'd prefer a bit more margin. It seems to be working well so far, though.)

(Mounting of the 5V relay)

4. The SPOT switch did not work in STBY mode. Incorrectly rewired by someone in the past, I connected it to pin 5 of the relay (300V when not transmitting, although it really should go to pin 4 of the relay (per the schematic), but the remaining wire wasn't long enough, and pin 5 is a good compromise).

5. VFO tracking way off. Re-adjusted, but during this readjustment I discovered that the bottom-end of 80 meters would quickly diverge despite the rest of the band tracking well. Because I intend to use the transmitter for AM only, I decided to leave well-enough alone and I adjusted the bandspread to track the VFO dial over the range of 3.7 - 4 MHz.


Still to be Resolved:

1. Oscillator: the 1.8 MHz fundamental at the plate of the oscillator 6AU6 (that's later doubled to provide the 80 meter signal) looks terrible, as can be seen in the top trace below (the bottom trace is the RF output (sampled via an attenuator)):

I'm surmising that the signal on the oscillator plate looks this way because the 8.5 uH inductor in the 6AU6 plate circuit is differentiating a 1.8 MHz plate-current pulse-train from the 6AU6 (after all, v = Ldi/dt). But is this the way it really ought to look? I've no idea.

Never the less, the transmitter seems to perform OK and I cannot find anything obviously wrong in the oscillator circuit, so I'm going to reserve judgment...

(Additional note: The waveform at the grid of the oscillator looks great -- a nice sine wave. Just the plate signal looks weird. Ought to have a resonant circuit to make it look nice, I think.)

2. There is a bit of audio roll-off from about 1.5 KHz (down 1 dB from 1 KHz) to 5 KHz (down 3 dB from 1 KHz). The first place this roll-off appears is at the plate of the second 12AX7 stage (yet it looks fine at this stage's input grid). I've yet to identify the cause -- I suspect it might be roll-off caused by the RC network formed by the AUDIO pot and the 12AX7 grid capacitance at pin 2, but...

3. I'm not sure if this is a problem or not, but carrier power does increase a bit as I approach full modulation, even though I've disabled the "control-carrier" feature of the modulator. I'm not sure what the cause is. Perhaps a non-linear PA Screen Grid transfer function? Or...?

4. VFO drifts.


Tuning up the Transmitter:

It's important that the transmitter's loading be properly adjusted. If the loading is too light you'll get peak compression on the RF envelope. I find that adjusting loading for peak power in CW mode actually puts it about where it needs to be for AM.

Here's the procedure I use. It seems to work.

1. Rotate LOAD and AUDIO controls fully counter-clockwise.
2. Place function switch in GRID position. Press PTT and adjust DRIVE for 3 mA (or for peak reading if 3 mA cannot be reached).
3. Place the function switch in PHONE and the meter switch in the PLATE position. Press PTT and dip the plate using the FINAL control.
4. Switch the meter switch back to GRID. Press PTT and ensure grid drive isn't exceeding 3 mA. (Important note: I've found that as I rotate the DRIVE control through 360 degrees, I hit the 3 mA level at four positions. And for two of these four locations, the output power is greater than for the other two locations, despite the equivalent 3 mA drive level. When performing the final DRIVE adjustment, be sure to select one of the two DRIVE positions that results in greatest output power (at 3 mA drive)).
5. Switch the function switch to CW. Press PTT and advance LOAD to peak the power output. Dip plate current again, just to be sure.
6. Switch the function switch back to PHONE. Press PTT and adjust the new CARRIER LEVEL pot (on the back panel of my Cheyenne) to the desired carrier power out (no modulation). Then, while talking into the microphone and monitoring the RF envelope on a scope, advance the AUDIO control until the "troughs" of the modulation envelope are just on the cusp of flat-lining. If you find that the peaks of the envelope reach their max level before the troughs reach their min level, you have too much carrier and you should back down the CARRIER LEVEL pot -- you're just wasting power.

You're done!


Other Notes:

1. One goal was to make my modifications without drilling new holes in the transmitter, so that if someone, at a later date, wished to return the transmitter to its original condition, they could. Fortunately, it was fairly easy to add additional terminal strips using existing screws, and the key-jack hole on the back of the chassis was an ideal place to mount the Carrier-Level pot.

Additional terminal strip for new components. No holes drilled!

2. For proper AM operation, the Loading control must be adjusted for peak power out (and perhaps even a bit beyond, to be on the safe side), otherwise the positive modulation peaks will be greatly compressed and your audio will sound lousy.

3. The 1 uF cap across the new carrier-level pot can be made larger if it's desirable to run the Cheyenne at lower power (and thus a lower potentiometer resistance) and to keep the low-frequency cut-off. But increasing its value will also increase the amount of time that it takes for this stage to reach its bias point when PTT is pressed.

4. At higher carrier levels the Cheyenne has somewhat less measurable distortion at close-to 100% modulation than it does at lower carrier levels (as long as you aren't exceeding the capabilities of the PA on voice peaks, of course). I attribute this to non-linearities in the PA's screen grid transfer function (hypothesized, but not proven).

5. Here's a transfer curve that I've made, using my Cheyenne transmitter, showing RF output voltage (attenuated by my RF "sampler") versus PA Screen Voltage (note: I'm using a 6293 tube in lieu of a 6146). Test conditions: PHONE mode, 80 meters, no modulation.

You can see the curve bending at the high voltages (this results in compression of RF envelope peaks). It looks pretty linear at lower voltages (the slight burbles are most likely due to measurement error (of either screen voltage or RF amplitude) on my part).

(Click on image to enlarge)

6. WC3K's articles in Electric Radio (regarding the DX-60, see below) describe a neat modulation monitor using an LED. There's no reason why this can't also be used with the Cheyenne. I didn't install it, because I didn't want to drill a hole in the front panel and, besides, I use a scope to monitor my modulation. But I recommend taking a look at it. (You can find similar circuits in some of the web sites discussing DX-60 mods, too.)
7. A reminder: Update (2 January 2010): New information on my Cheyenne can be found here.

Resources:

Heathkit MT-1 "Cheyenne"Information HERE

Heathkit Schematics HERE

Unfortunately, I couldn't find any information regarding modifying the Cheyenne on the web. However, its design is similar to the DX-60, and there are articles that discuss improving AM performance of the DX-60...

Electric Radio articles on DX-60 improvements:

1. "Fun with a DX-60," Bill Breshears, WC3K, Electric Radio, Issue 133, May, 2000
2. "More Fun with a DX-60," Bill Breshears, WC3K, Electric Radio, Issue 138, November, 2000

Websites with DX-60 improvements or discussions:

• WC3K Improvements (note: the 50 ohm pot should be 50K ohms)
• K4TAX Improvements
• KS3K Improvements
• N4JK Modifications
• K4TLJ Modifications
• LA8AK

Let's see...where did I put that screwdriver?

Standard Caveat...

I hope you find this information useful, but please, use these modifications at your own risk -- although they worked for me, I cannot guarantee that they'll work for you. (After all, I could have made a mistake in transposing them from my lab notebook to this post.)

If you do find any errors, or if you have any questions, please let me know. Thanks!

- Jeff, K6JCA