Monday, January 4, 2010

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)


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

Saturday, January 2, 2010

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