This post describes an AC power supply I designed and built for powering old military ARC-5 receivers that are either in unmodified condition (i.e. designed for 28VDC operation) or modified for either a 12.6 VAC or 6.3VAC filament voltage)..
Power Supply Requirements:
+HV (High Voltage): Must be between 200 and 250 VDC
LV (Low Voltage): This is a receiver's filament voltage. Three different filament voltages (25.2 VAC, 12.6 VAC, and 6.3 VAC) should be available for powering ARC-5 receivers.
Audio: RCA jack for attaching an 8-ohm speaker, and an internal Impedance Matching transformer to transform the speaker's 8-ohm impedance to 600 ohms as load for the receiver's audio output signal.
BFO ON/OFF switch (front-panel mount).
GAIN Control (front-panel mount).
All output voltages and control signals available both on a female Octal connector and on an eight-terminal terminal strip.
Power ON/OFF switch (controls both +HV and LV) and Indicator Lamp (ON when power is ON) both mounted on the front panel.
Schematic:
The schematic is shown below:
Schematic Notes:
The Transformer's secondary specification is 300 Vrms, center-tapped (Vprimary = 115 VAC, Secondary Load = 86 mA). Assuming full-wave rectification (i.e. center-tap tied to ground) and a capacitor-only filter (which acts as a peak-detector) the peak DC voltage (and ideally the output voltage, if the capacitor is large enough) should be the peak of one-half of the secondary's Vrms spec, which calculates to be 212 VDC.
If the secondary's peak voltage is 212 VDC, the diode Peak Reverse Voltage (PRV, or PIV) should be at least 2x 212 VDC, or 424 VDC, plus a good margin to account for AC Line voltage variation, etc. 1N4005 diodes (PRV = 600V) should suffice, but I went with higher-rated diodes (1N4006, PRV = 800V) from my junk box.
The higher the capacitance attached to the full-wave rectifier, the lower will be the AC ripple on the DC line. My BC-454-B receiver's frequency varies if +HV changes (the frequency change is *very* roughly 60 Hz per 10V change in +HV), so if +HV had significant ripple, there could be FM'ing of the audio signal.
The 68K bleeder resistor has a time-constant of 32 seconds in conjunction of 470 uF (and especially important, it was in my junkbox). And its power dissipation across 212 VDC is about 0.7 watts, so the resistor's 2 watt rating provides a nice margin.
The Audio transformer's primary will have an impedance of 1.2K ohms if terminated with an 8-ohm load. The receiver requires a load of 600 ohms, which is available at the primary's center-tap (i.e. its impedance will be one-half of the total primary impedance of 1200 ohms). Note that the selection of this transformer (a Xicon 42TM003-RC transformer) is discussed further in my BC-454-B post.
I chose a lamp power-on indicator (with a #44 bulb), instead of an LED to match the era of the ARC-5 receivers.
Front View:
On the left is the POWER ON/OFF switch and above it a panel-lamp (with a #44 bulb) to indicate when power is ON.
The middle switch is BFO ON/OFF.
And the right control is the GAIN potentiometer.
Back View:
Across the top of the back panel is a Terminal Strip consisting of 8 screw terminals.
Below it, from left to right, are:
Octal socket (with same signals assigned to the same pin numbers as those on the terminal strip)
Fuse Holder
RCA jack (connect to an 8-ohm speaker)
AC Power Connector.
Bottom View, Looking Towards Rear Panel.
(For reference)
At the upper right is the audio impedance-matching transformer (mounted on a piece of perf-board).
Just below it is the 470 uF capacitor, the 68K bleeder resistor, and the two diodes.
Bottom View, Looking Towards Front Panel:
(For reference)
Terminal Strip, Identification of Terminals:
Note the color coding under each screw-terminal's name -- I used the color-code to identify terminals for cabling purposes -- the associated spade terminals (that connect at the power supply side of the cable) are color coded to match their screw terminals.
For example, the first terminal on the left (Terminal One) is the +HV terminal, and its associated color is BROWN (this might not be obvious from the photograph, but there is a brown line drawn under the "+HV" text. The associated +HV wire (in the cable bundle) from the receiver that attaches to this screw terminal has its spade-terminal color coded with a matching BROWN.
The next terminal to the right is Terminal Two, and its color is RED. It is the GND terminal.
The Terminal Numbers increase as we move to the right and the color code increases to match the numbers. At the far right is the eighth terminal (BFO On/Off). Per the color code, this should be grey, but I did not have a grey Sharpie, which would have been the appropriate color per the color-code, so I instead colored it with alternating black and white lines.
Note that the numbers assigned to the terminal strip match the pin number of the octal socket. For example, Terminal One (+HV) of the Terminal Strip connects to the octal socket's pin 1, Terminal Two (GND) to the octal socket's pin 2, etc.
Interconnect Cable (Power Supply Terminal Strip to Receiver J3):
Both ends of the cables that interconnect the Power Supply's Terminal Strip to the Receiver's J3 connector (on the back panel) are color coded. For example, in the image below, the spade terminal that attaches to the Terminal Strip has a red band, identifying that it should attach to the second terminal (from the left) of the Power Supply's terminal strip (i.e. the GND terminal).
On the other end of the wire is a mini Banana Plug (2.5 mm diameter), which will insert into Pin 1 of the Receiver's rear panel connector, J3. Note that it has a brown band, identifying that it should be inserted into pin 1 of J3 (i.e. the receiver's GND pin).
I sourced these miniature Banana Plugs, from Amazon:
Note: not all 2.5mm Banana Plugs are compatible with the receiver's jack. I purchased a different set (that were only metal -- there was no plastic covers) and they did not fit. So you might need to do some experimentation.
Interconnect Cable (Power Supply Octal Socket to Receiver J3):
Below is an interconnect cable that uses the power supply's Octal socket in lieu of its terminal strip. This specific cable is for radios that require a filament voltage of 12.6 V.
Banana Plugs Inserted into a Receiver's J3 connector:
The image below shows six miniature Banana Plugs inserted into a receiver's J3, which is a seven-pin jack. Note that only J3 pin 5 (+screen grid voltage) is not used, and therefore it does not have a Banana Plug inserted into it.
Receiver J3 Pinout:
The partial schematic, below, identifies the signals at each pin of an ARC-5 receiver's J3 rear panel connector:
Note that pin 5 is not used -- therefore only six miniature Banana Plugs are required.
Measurements:
Given a measured AC line voltage of 118 VAC, I measured the following power supply output voltages:
Connected to a BC-454-B, powered ON, with filaments wired for 28V:
+HV: measured (loaded) 212 VDC with 0.15 VAC ripple.
25.2 VAC: measured (loaded) 28.2 VAC.
12.6 VAC: measured (unloaded) 14.1 VAC (13.6 VAC when loaded with an R-27 Receiver).
6.3 VAC: measured (loaded with panel lamp, but no receiver): 7.2 VAC.
Connected to an R-27 receiver, powered ON, whose filaments have been rewired for 12V:
+HV: measured (loaded) 212 VDC with 0.15 VAC ripple.
12.6 VAC: measured (loaded) 13.6 VAC.
Notes on the measurements:
The loaded 25.2 VAC filament voltage (when attached to the BC-454-B) measures 28.2 VAC -- almost 10 percent higher than the recommended filament voltage of 25.2 VAC (for two equal filaments, wired in series). However, this RMS voltage is essentially the same as the DC voltage used to power the receiver when a Dynamotor is used (28 VDC). Note, too, that it is close to RCA's recommended voltage tolerance of +/- 10%. If the AC line voltage were to increase above 118 volts, this 10% specification would be exceeded.
The loaded 12.6 VAC filament voltage (when attached to the R-27 receiver) measures 13.6 VAC, or 8% above the recommended filament voltage. If line voltage were to increase, then RCA's 10% maximum variation spec could be exceeded.
If it is important to a user that the filament voltages, when loaded, measure to be 25.2, 12.6, and 6.3 VAC when the AC input line voltage is at its nominal value (typically 120 VAC), resistors could be added, within the power-supply, in series with each filament line to provide the necessary voltage drop. The resistors would probably only be a few ohms each, and probably spec'd at a watt, or less.
Other Notes:
L14 is in series with the LV (filament) voltage -- does it have an effect on filament voltage? This part is an RF choke whose inductance is 112 uH. At 120 Hz its reactance is only 0.084 ohms -- in other words, negligible.
I strongly recommend covering pin 2 (LV) and pin 3 (+HV) on the Receiver's J2 connector (the Dynamotor connector) to prevent accidentally shorting these pins to ground (or shocking oneself). Perhaps use heat-shrink tubing?
Standard Caveat:
As always, I might have made a mistake in my equations, assumptions, drawings, or interpretations. If you see anything you believe to be in error or if anything is confusing, please feel free to contact me or comment below.
And so I should add -- this information is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Back when I was a young ham, I was given an ARC-5 receiver, with power supply, that covered the 80-meter ham band.
That receiver provided hours of listening enjoyment, and it, along with its power supply, were small enough that I could take them on family vacations for listening during the evenings.
Somewhere along the way the receiver went one way and I another. But I've always had fond memories of it.
In more recent years I've picked up a few ARC-5 receivers at various swap-meets, and I've finally decided to see if I could get any of them back on the air (some are more heavily modified by previous owners than others, and those might not be worth the trouble).
My ARC-5 receiver stable currently consists of the following six radios:
CCT-46129 (0.19 - 0.55 MHz). One of the ARA series of receivers.
CCT-46104 (1.5 - 3 MHz). One of the ARA series of receivers.
BC-454-B (3 - 6 MHz). One of the SCR-274-N series of receivers.
R-26 (3-6 MHz). One of the AN/ARC-5 series of receivers.
CBY-46106 (6 - 9.1 MHz). One of the ARA series of receivers.
R-27 (6-9.1 MHz). One of the AN/ARC-5 series of receivers. (Somehow this one wound up with a cover from an R-23 receiver).
In this blog post I will:
Introduce the three receiver series that comprise the ARC-5 receiver family.
Discuss the differences in design between these three series.
Describe how to recognize which series a receiver belongs to.
Describe how to power receiver.
Describe the receiver's connectors and their signals.
Describe controlling the receiver.
Describe how to interface with an 8-ohm speaker.
List the steps to take when first checking out a receiver.
And finally, describe of my efforts in bringing my BC-454-B back to life.
Introduction, the ARC-5 Receiver Series:
Per Wikipedia, the term ARC-5 is used to describe three similar (yet different) series of receivers deployed during World War Two. Of these three series, only one series has the "official" ARC-5 nomenclature (the AN/ARC-5series), which was actually the last of the three series to be introduced (in 1943).
The other two series, unofficially, are also referred to as "ARC-5" receivers. The first series, developed for the Navy prior to World War Two, was the "ARA" series. This series was followed in 1941 by the "SCR-274-N" series, which was initially adopted for the US Army Air Corps.
The table below (from the AN/ARC-5 Wikipedia entry) lists the receivers of each of the three series that cover the medium through high-frequency range of 0.19 to 9.1 MHz. This frequency span is typically divided between five different receivers (note that the SCR-274-N series does not have a receiver covering 1.5-3.0 MHz).
MHz
ARA-ATA
SCR-274-N
AN/ARC-5
Navigation Receiver (Beacon Band)
0.19 - 0.55
CBY-46129
BC-453-(*)
R-23(#)/ARC-5
Navigation Receiver (Broadcast Band)
0.52 - 1.5
CBY-46145
BC-946-(*)
R-24/ARC-5
Communication Receiver
1.5 - 3.0
CBY-46104
R-25/ARC-5
Communication Receiver
3.0 - 6.0
CBY-46105
BC-454-(*)
R-26/ARC-5
Communication Receiver
6.0 - 9.1
CBY-46106
BC-455-(*)
R-27/ARC-5
(*) A (basic model) or B (1st revision). (#) No letter (basic model) or A (revision).
Differences between the ARC-5 Receiver Series:
(I will summarize the information below at the end of this section)
1. Front Panel Plug wiring:
Both the ARA series and the SCR-274-N series bring the headset audio output signal to pin 4 of the front-panel connector (audio also appears on the rear panel connector). The AN/ARC-5 series has no audio connection to the front panel! Audio for this series only appears on the rear panel connector.
2. Automatic Volume Control (AVC):
Of the three series, only the AN/ARC-5 series (i.e. R-23 through R-27 receivers) has Automatic Volume Control (AVC) circuitry. You should be able to quickly identify if a radio has this functionality (and thus is part of the AN/ARC-5 series) by looking for the following differences:
12SF7 replacing the 12SK7 tube in the Second IF Amplifier stage.
C26 (a capacitor connected between pin 2 of the 12SR7 (middle tube, last row) and the middle terminal of Z-4, i.e. L12) does not have a resistor attached in parallel across it (R14).
For comparison, here is an example of the wiring for non AN/ARC-5 receivers:
3. Filament Voltage:
Per my reading of the ARA, SCR-274-N, and AN/ARC-5 manuals, only one receiver was designed for 14 V operation, and that was the R-148/ARC-5X receiver, covering 0.19 to 0.55 MHz.
In this receiver, 12V filament tubes were used, and all filaments were wired in parallel.
All other receivers used 28 volts for their filament voltage. Given that the tubes are 12V filament tubes, the filaments of pairs of tubes were first connected in series (the RF Amp & Mixer pair, the 1st IF and 2nd IF pair, and the Det./Osc. & Audio Amp pair) and then each pair was fed with 28V.
Never assume that a receiver has been wired for 28V filament voltage! I have made this mistake! Always verify filament circuitry before applying power to a newly acquired receiver. Previous owners might have modified it.
For example, my R-27 AN/ARC-5 receiver should be wired for 28 V filament voltage, but all filaments are wired in parallel, while my CBY-46106 ARA receiver is also wired with all filaments in parallel and the original 12V tubes were replaced with 6V tubes!
4. Audio Output Transformer and Impedance:
Of the three receiver series, only the SCR-274-N series was designed for connection to high-impedance headphones (e.g. 8000 ohms). Of this series, the "-A" version was designed for use only with high impedance headphones, while the "-B" version could be used with either high-impedance or low-impedance (e.g. 600 ohm) headphones, depending upon how the Audio Output Transformer T1 was wired.
The other two receiver series (ARA and AN/ARC-5) were designed for use only with low impedance (e.g. 600 ohms) headphones.
Audio Transformer (T1) Summary:
ARA receiver: The Audio Transformer should have a "5613" part number. Its primary to secondary turns ratio is 8:1, and spec'd for low impedance headsets (e.g. 600 ohms). (See Table 19 of the Instruction Manual).
SCR-274-N receiver: Audio Transformers can have two different part numbers:
"6308": Used only with high impedance headsets (e.g. 8000 ohms). This transformer was used in the "-A" revision of the receiver.
"ES-691027": Provides connection to either a high impedance headset (e.g. 8000 ohms, via terminal 3) or a low impedance headset (e.g. 600 ohms, via terminal 6). This transformer was used in the "-B" revision of the receiver. Refer to Table 20 of the Maintenance Instructions. (Note, the turns listed for the secondary's tap were most likely counted from the "hot" end of the secondary (terminal 3), rather than its ground end, meaning that the tapped secondary would actually be 475 turns rather than 1325 turns).
The illustration below shows both the schematic and the wiring diagram for ES-691027 transformer in the "-B" revision of the SCR-274-N receivers:
AN/ARC-5 receiver: Audio Transformers support low impedance headsets (e.g. 600 ohms) and, per Table 20 of the Maintenance Instructions, can have one of two part numbers:
"5631"
"640268"
An example of the different T1 part numbers in different receivers:
(By the way, for an interesting discussion of military headsets circa the 1940's, go here).
Summary of the Differences between Command-set Receiver series:
ARA Series:
Audio Line at Front Connector (pin 4): Yes
AVC Circuitry: No
Dynamotor (Filament) Voltage: 28 VDC
Headset Impedance: Low Impedance (e.g. 600 ohms)
SCR-274-N Series, "A" Revision:
Audio Line at Front Connector (pin 4): Yes
AVC Circuitry: No
Dynamotor (Filament )Voltage: 28 VDC
Headset Impedance:High Impedance (e.g. 8000 ohms)
SCR-274-N Series, "B" Revision:
Audio Line at Front Connector (pin 4): Yes
AVC Circuitry: No
Dynamotor (Filament) Voltage: 28 VDC
Headset Impedance:Can be wired for High or Low Impedance
The receiver should have an attached metal tag stating its series, receiver identification, and sometimes its operating voltage (i.e. dynamotor voltage). The tag for the ARA and AN/ARC-5 series should be on the receiver's top cover, while the tag for the SCR-274-N series should be on the right side of the chassis, near the front panel.
Now, suppose you have purchased a receiver at a swap-meet. How do you know which receiver you actually have? Covers might have been swapped over the years, etc. Do not depend upon a cover tag being the correct one!
Here are the steps I would take:
1. Ignore the receiver tag on the top of the radio (if it exists); after all, someone might have put the wrong cover on the radio. Instead, look at the front dial and identify its frequency range. This narrows the selection down to one of four receivers (ARA, SCR-274-N ("-A" revision), SCR-274-N ("-B" revision), and AN/ARC-5).
2. Is the chassis bare aluminum or painted with black-wrinkle paint?
2.1. If bare aluminum, the receiver is either an SCR-274-N ("-A" revision) or an SCR-274-N ("-B" revision) receiver.
2.1.1. If the SCR-274-N receiver has no side-mounted ID tag, we will need to determine the receiver's revision by examining the audio transformer under the chassis. Remove the bottom cover. If the Audio Output transformer (T1) has an "ES-691027" part number, it is the "-B" version of an SCR-274-N receiver (e.g. BC-454-B). Otherwise, if the P/N is "6308", the receiver is an "-A" version.
If the radio is the "-B" version, now would be a good time to check if the audio transformer, T1, is wired for 8000 ohm or 600 ohm impedance.
2.2. Otherwise, if the chassis is not aluminum but instead painted with black-wrinkle paint, the receiver is either an ARA receiver or an AN/ARC-5 receiver, or it could even be an SCR-274-N (although I personally have not come across a "BC" prefixed receiver in black wrinkle, I have seen some pictures).
2.2.1. Is the black-wrinkle receiver of the SCR-274-N series? Check the side of the chassis for an identification tag. If there is one (and it starts with "BC"), then the receiver should be of the SCR-274-N series. You can double-check this conclusion by removing the bottom cover and examining the Part Number of the Audio Transformer, T1. If this P/N is either 6309 or ES-691027, then the receiver is of the SCR-274-N series.
2.2.2. The receiver will be in the AN/ARC-5 series if the second IF tube is a 12SF7 (either pull the tube out or check its filament wiring -- a 12SF7's filament pins are pins 7 and 8, while a 12SK7's filament pins are 2 and 7).
Alternatively, you can remove the bottom cover and check C26. If there is no resistor across C26, the receiver is an AN/ARC-5 receiver (with AVC).
2.2.3. If the black-wrinkle receiver isn't an SCR-274-N receiver nor an ARC-5 receiver, then it is an ARA receiver.
A caveat regarding these checks: modifications made by later owners might have substituted parts from other series, changed circuitry, or...? There are any number of reason why a receiver that appears to be of one series was actually born of another.
And finally, an important note regarding receiver identification!
The three IF transformer housings should each have a color dot in one corner on their topside. These three dots should all be the same color, and they should match the front-panel's tuning dial per the following table:
At least one published modification for an ARC-5 receiver changed its frequency range by swapping the three IF assemblies from one receiver for those of another. I would recommend this quick check to verify that the IF cans are indeed the correct ones for your receiver!
Powering the Receiver:
The diagram below shows the various ways by which power can be applied to an ARC-5 receiver.
Per the diagram's notes:
If a dynamotor is used, its Low Voltage (LV) power (e.g. +28V) must be applied via the Rear Panel connector (J3).
If an external power supply is used (supplying both LV and HV), this power can be applied either via the Rear Panel connector (J3) or via the Dynamotor connector (J2).
If the Rear Panel connector is used for power without the dynamotor installed, then the HV and LV pins on J2, the Dynamotor connector, should be covered (heat-shrink tubing?) to prevent accidentally shorts orshocks!
Also, if the Rear Panel connector is used for power (with or without the dynamotor), pins 6 and 7 on the Front Panel connector (J1) must be connected together to route +LV to the tube filaments.
The image below shows an FT-230-A adapter (for the SCR274-N receiver series) which will short pins 6 & 7 of the Front Panel connector. It can be used if no signals need to be accessed via the front panel, that is, if the radio's controls are connected to the Rear Panel connector, instead.
Note: The FT-230-A adapter is interchangeable with the MX-2/ARC-5 adapter (AN/ARC-5 receiver series) and the CBY-49107 adapter (ARA series).
Dynamotor Connector:
A dynamotor connects to the receiver via a 3-pin male connector on the receiver's rear "apron." Its pin assignments are:
Pin 1: Ground
Pin 2: +LV Input (Low Voltage, e.g. 28 VDC)
Pin 3: +HV Output (High Voltage, e.g. 250 VDC)
Front Panel Connector:
The Front Panel's connector (8 pins, male) has the following signal assignments:
Pin 1: Gain Control Line
Pin 2: Ground
Pin 3: (not used)
Pin 4: Headset Audio Output (Note: the AN/ARC-5 receivers do not have this signal).
Pin 5: CW Osc. (i.e. BFO) Shut-Off Line
Pin 6: +L.V. to receiver circuitry
Pin 7: +L.V. from rear-panel connector
Pin 8: (not used)
The following schematic shows how controls can be wired to this connector:
Front Panel Connector Notes:
ARA series and SCR274-N series of receivers provide audio to pin 4. Note that the load on the audio signal is expected to be either 8000 ohms or 600 ohms, depending upon the receiver.
AN/ARC-5 receivers have no internal connection to pin 4, and so audio cannot be accessed via the front of an AN/ARC-5 receiver.
If +LV (low voltage power, e.g. 28 V) is applied via J3 (the Rear Panel Connector), then pins 6 & 7 of the Front Panel connector must be connected together (either via a wire or a switch), to route +LV to the tube filaments (and Dynamotor +LV input).
Gain control is provided via a potentiometer (20K or 50K ohms), and should be wired so that, in its position of maximum gain, resistance is at its minimum.
The CW Oscillator (BFO) can be turned ON or OFF via pin 5. Note that the BFO is OFF when pin 5 is shorted to ground.
Connecting to the Front Panel Connector's pins:
I use 1/8" O.D. copper tubing to create homemade jacks to slip over the receiver's connector pins, a great idea from John Stanley's (K4ERO) Command Set article in the January 2016 issue of QST. I purchased a package containing a variety of tube diameters from Amazon (https://www.amazon.com/dp/B07VPWSJRC?psc=1&ref=ppx_yo2ov_dt_b_product_details)
Use a jeweler's saw (see below, available inexpensively from Amazon) to cut the 1/8" O.D. tubing to length!
Here is my control panel for my BC-454-B receiver:
The image below shows the short lengths of 1/8" O.D. copper tubing, at the end of each wire, that slip over the front panel connector's male pins.
Note that the Low-Voltage Power switch (that will connect to pins 6 & 7) is on the back of the potentiometer. If a dynamotor is used, the gauge of these two wires should be sized to handle an amp of current.
And to minimize confusion when installing the Control Panel, the wires are color-coded to identify to which pin they should attach (e.g. brown to pin 1, red to pin 2, etc.).
Rear Panel Connector:
There is a 7 pin female connector on the rear panel that has the following signal assignments:
Pin 1: Ground
Pin 2: Headset Audio Output
Pin 3: Gain Control Line
Pin 4: CW Osc. (BFO) Shut-Off Line
Pin 5: + Screen Grid Voltage (not used)
Pin 6: +L.V. (i.e. Low Voltage, e.g. 28 VDC or VAC)
Pin 7: +H.V. (i.e. High Voltage, e.g. 250 VDC)
Gain Control, BFO On/Off, and Audio Out signals can all be accessed via the Rear Panel connector. In this situation, their front-panel pins should not be used.
Rear Panel Connector Notes:
Pin 2 is the Audio Out signal. Note that the load on the audio signal is expected to be either 8000 ohms or 600 ohms, depending upon the receiver.
Pin 3, Receiver Gain can be controlled via an external potentiometer (20K or 50K ohms), and it should be wired so that, in its position of maximum gain, resistance is at its minimum.
Pin 4, the CW Oscillator (BFO) can be turned ON or OFF via pin 4. Note that the BFO is ON when pin 4 is OPEN.
Pin 6 is the +LV pin (low voltage power, e.g. 28 V). Note that if +LV is applied via this pin, then pins 6 & 7 of the Front Panel connector must be connected together (either via a wire or a switch), to route +LV to the tube filaments (and Dynamotor +LV input). The image below shows an FT-230-A Adapter, which can provide this function.
Pin 7 is the +HV pin. No connection from an external high-voltage source should be made to this pin if a Dynamotor is installed. If there is no Dynamotor, then +HV (e.g. 250 VDC) can be applied to the radio via this pin.
Connecting to the Rear Panel Connector:
Similar to an idea I saw mentioned in John Stanley's (K4ERO) Command Set article in the January 2016 issue of QST, I use 2.5 mm Banana Plugs (available through Amazon) to attach to the Rear Connector's female sockets (https://www.amazon.com/dp/B09S3VDSVK?psc=1&ref=ppx_yo2ov_dt_b_product_details). I chose these after first discovering that 2.0 mm plugs were too narrow and 3.0 mm plugs were too wide.
Attaching an 8-Ohm Speaker:
I prefer to use an 8-ohm speaker instead of a headset (even though I do have a 600-ohm headset) and I find that the receiver, connected to the 8-ohm speaker via an impedance transformer designed for the appropriate impedance ratio, can drive my 8-ohm speaker to quite loud levels.
With a 600:8 ohm Impedance Transformer between radio and speaker, and measuring voltage with an oscilloscope at the speaker, the speaker's signal starts clipping at at about 3 Vpp (i.e. about 1.07 Vrms if the signal were a sine wave), which equates to a speaker drive power of about 0.14 watts. With my speaker, this is VERY LOUD -- I need to wear ear plugs! A 1 to 1.5 Vpp signal is more comfortable.
The following Audio transformers, available from Mouser (if in stock), should be able to transform an 8 ohm speaker to 600 ohms (or 500 ohms, which should be close enough):
The image below shows the relative sizes of these three transformer families:
I chose the Xicon 42TM003-RC because physical size wasn't an issue and it could handle the maximum output from the receiver, if I accidentally left the receiver unattended, set to maximum volume, and tuned to a carrier.
Below is a schematic showing a possible wiring diagram. Note that the receiver is connected to the primary's center-tap (i.e. 600 ohms), not across the entire primary winding (i.e. 1200 ohms).
If using a 4-ohm speaker in lieu of 8 ohms, connect it to the secondary's center tap, rather than across the entire secondary.
Adapting an ARC-5 Receiver for Ham Use demonstrates how a small audio transformer (Xicon 42TL series) can be installed behind the front "adapter panel" and held in place with hot-melt glue.
Steps to Bring Up a Receiver:
1. Verify Filament Wiring:
As I mentioned earlier, always first verify receiver filament wiring before applying power to a receiver. A common modification was to rewire 28 VDC filament circuitry for 14 VDC (in practice, 12.6 VAC) by connecting all six filaments in parallel. You do not want to apply 28 VDC to a receiver that has had its filaments rewired!
Also check if the original 12V tubes were replaced with 6V tubes.
For 28 VDC operation the following pairs of tubes were connected in series:
12SK7 RF amp & 12K8 Mixer
12SK7 1st IF Amp & 12SK7 (or 12SF7) 2nd IF Amp
12SR7 Det./CW Osc. & 12A6 Audio Amp
The quickest way to verify if a receiver has all six filaments wired in parallel is to check the filament wiring of the following tubes:
12K8 Mixer, pins 2 and 7.
12A6 Audio Amp, pins 2 and 7.
12SK7 First IF Amp, pins 2 and 7.
If any of these pins are connected to ground, then chances are the receiver has been wired for 14 VDC operation (or 6 VDC operation, if the original 12 volt tubes have been swapped out for 6 volt filament tubes!).
Note: Of my six receivers, only one has retained its original 28 V wiring (the BC-454-B). The other five all have their filaments wired in parallel, and two of these even had 6V tubes substituted for the original 12V tubes.
2. Check Capacitors:
Over time capacitors can degrade and fail. All capacitors should be checked with a good capacitance meter (that measures loss, or dissipation factor, as well as capacitance -- I use a GenRad 1657 RLC Digibridge).
The most likely failures will be the electrolytic capacitors (C5, C30, and C32), so I would start with those. For the caps in the "tubs" screwed to the chassis, I simply unsolder the wires from their terminals (or terminal) and measure capacitance (and dissipation factor) from each terminal to chassis (ground).
Also look for capacitor "tubs" that might be leaking (these should be fairly obvious). Check these, too.
Some authors recommend replacing all of the capacitors. I prefer to just replace those that measure poorly. But replacing them all would be a suitable tactic if you don't have a good way to check the capacitors.
If you find faulty caps, replace them. For faulty capacitors contained in the metal tubs (for example, C6A, C6B, and C6C)), I would simply remove the tub and add a terminal strip with new capacitors wired to it (see: Adapting an ARC-5 Receiver for Ham Use). I'll show an example later in this post, when I describe my efforts with my BC-454-B.
3. Check the Receiver's Audio Impedance Requirement and add an Audio Transformer, if necessary.
The load required by the receiver's audio amplifier will be either High Impedance (e.g. 8000 ohms) or Low Impedance (e.g. 600 ohms). Refer to the discussion earlier in this post regarding how to determine your receiver's requirement. Note that a radio might have been modified by a previous owner. It's good to verify!
If your headset or speaker is not the correct impedance (if it's close to the correct impedance, it should be OK), you should use an impedance transformer to transform its impedance to the impedance that the receiver wants to see. For example, see discussion, above, regarding transformers that will transform an 8 ohm speaker to 600 ohms.
4. Should I Attach Control & Audio Signals to the Front or to the Rear Connector?
Connect the Control functions (Gain Control, BFO On/Off) to either the Front Panel connector or the Rear Panel connector, per your preference. Refer to the appropriate sections, above.
5. How will the Receiver be Powered?
Determine how the receiver will be powered (refer to the "Powering the Receiver" section, above). Here are three choices:
Dynamotor (installed on the receiver), with +LV applied from an external supply to the Rear Panel connector.
External power supply providing both +HV and +LV, attached to the Rear panel connector.
External power supply providing both +HV and +LV, attached to the 3-pin Dynamotor connector.
Refer to the "Powering the Receiver" section earlier in this post.
Only after I've gone through all these steps do I then apply power.
Example, BC-454-B Revivification:
Here are the steps I took to bring my BC-454-B back to life.
1. Verify Filament Wiring
The receiver is one of the SCR-274-N series of receivers and therefore should have its tube filaments wired for 28 volts. In other words, the filaments of each of three pairs of tubes should be wired in series.
I removed the bottom cover and visually checked to see if any tubes had no filament pins tied directly to ground. At least one tube fit this criteria, confirming that the radio was wired for 28 VDC.
2. Check capacitors
With the bottom cover off, I first did a visual inspection of all of the "tub" capacitors (note: anti-fungal coating can possibly mask leakage). The C7 capacitor tub (containing three caps: C7 A, B, C) looked like it had leaked a bit since the radio was first manufactured (in March, 1945), but unsoldering the wires to the three capacitor terminals and measuring their capacitance to ground revealed no problem. They all measured in the range of 60 nF (spec'd to be 50 nF (i.e. 0.05 MF). Close enough!
Without any other tubs looking like they had leaked, I decided to focus next on the electrolytic caps, assuming that the other non-electrolytic caps were probably good. One electrolytic cap, C32 (spec'd 5 uF) measured 6.7 nF (off by a factor of 1000 !!!), with a Dissipation Factor of 1.78.
The values of all other electrolytic caps measured OK, with DFs of sub 0.02.
Not willing to go through the hassle of trying to fit a new cap into the original tub, I removed C32 and replaced it with a 2-lug terminal strip. The cap filters the +HV line, so larger is better for this application. I dug up a 50 uF, 360 WVDC cap (from my junkbox), which I soldered in to replace the original 5 uF cap.
Note: the receiver manuals do not specify capacitor working-voltage, so I simply use parts that are rated (by a comfortable margin) above the receiver's +HV rating of 250 VDC.
3. Check Audio Transformer T1 for Impedance Requirement.
A quick verification of T1's Part Number (P/N ES-691027) and a check of its wiring, per the schematic notes in the SCR-274-N manual (see illustration earlier in this post) confirmed that the audio transformer's wiring had not been modified and that it was setup for a low impedance (e.g. 600 ohm) load.
Thus confirming the decal on the side of the chassis!
4. Fabricate and Connect the Front Control Panel
One of my six receivers had a filthy control panel with an old pot, BFO switch, and audio jack attached.
I cleaned up the panel, installed a new pot from the junkbox (with power switch on its back), new BFO switch, and new audio jack. Then I attached wires and home-made "pin-sockets" (from 1/8" O.D. copper tubing) to slip over the Front Panel connector's pins.
5. Rear Panel Connections
To use an 8-ohm speaker, I chose a Xicon 42TM003-RC transformer to transform an 8-ohm speaker to 600 ohms. The center-tap of the transformer's primary connects to pin 2 (Audio Out) of the Rear Panel connector. Pick one or the other of the remaining two primary pins (it doesn't matter which one) and attach it to pin 1 (ground) of the Rear Panel connector.
The '+' terminal of the 28 VDC supply connected to pin 6 (+LV) of the Rear Panel connector, and the supply's '-' terminal connected to pin 1 (ground).
6. I decided to first test the receiver my high-voltage "lab" supply (a Heathkit IP-32, set to 250 VDC), and so I connected its =HV to to pin 3 of the Dynamotor connector, with supply's ground attached to pin 1 of the same connector.
7. I attached my antenna to the Receiver's antenna connector.
8. And then the big moment -- I turned the Gain control on the front panel clockwise, turning on the power switch. +LV current (filament current) initially rose, but settled to under 0.5 amps (it might have been significantly under, I don't recall), and +HV current was on the order of 40 to 50 mA.
And as the tubes warmed up, I started hearing signals on the 80 meter band. It worked!
9. Check with Dynamotor
Next, after powering down the receiver ,I disconnected the Heathkit High Voltage supply and attached the Dynamotor to the back of the receiver. It spun up as soon as I turned on power and again, as the tubes warmed up, signals began appearing.
Final Configuration:
The diagram, below, shows my BC-454-B setup, including power supply (but not the antenna).
My BC-454-B, On the Air (YouTube Video):
Below is a video showing my BC-454-B in operation, powered by its dynamotor. Both AM and LSB reception are displayed.
Note that this receiver has no AVC, so some "riding" of the Gain control is necessary.
Also, note that the tiny impedance transformer (600 to the speaker's 8 ohms) is temporarily connected between the radio and the speaker using clip leads.
In summary, only one cap needed to be replace. Pretty amazing for a radio manufactured in March of 1945!
Thoughts on Modifications:
Personally, as an engineer, my feeling is that if modifications can improve the performance of a radio, why not make them? But I'm also sympathetic to those who want to keep their equipment in original condition -- à chacun son goût.
1. Bringing Output Audio to the Front Connector:
I haven't tried this yet, but if you have an AN/ARC-5 version of the receiver without the audio-out connection to pin 4 of the Front Panel connector J1, it should be possible to run a wire from the Rear Panel connector pin 2 (J3) to pin 4 of J1.
This will require removing the RF Coil Unit (Z5A, Z5B, and Z5C). Hopefully this is straightforward. I plan to find out when I start working on my R-27 receiver.
2. Adding AVC:
Lack of good AVC in the BC-454-B is certainly noticeable, and it might be more annoying if this were my only receiver.
There is a simple modification for adding AVC to receivers without it.
All the essential a.v.c. components are incorporated in all receivers, but there is no connection between them. The purpose of this conversion is to provide a.v.c. action to the r.f. and i.f. amplifier stages by completing the a.v.c. circuit. It required two additional resistors and a capacitor. Refer to figures 11A and 11B (not shown in this post, k6jca). First, unground pin 5 of the 12SR7 (VT-133). Connect the 100 uuf (i.e. 100 pF, k6jca) capacitor across pins 4 and 5 of the tube socket. Connect the 470K resistor from pin 5 to an adjacent ground lug. Connect the second 470K resistor between pin 5 and the junction of C-15A and R-11. Remove R-11 from the circuit to increase the effect of the a.v.c. action.
Note: I haven't tried this modification. I imagine it would be satisfactory for AM (where there is a carrier present), but perhaps not for CW or SSB signals. For those modes, you'll probably still need to keep one hand on the gain control.
Resources:
A Google search for various ARC-5 terms will return a plethora of hits. Below are some of the sites I found useful.
Command Set Summary. A good summary (from Australia) of the various ARC-5 receivers.
The Instruction Book (including schematics and wiring diagrams) for the ARA/ATA series can be downloaded as five PDFs via the following links. Note that this is the manual for the R.A.A.F. (which I assume is "Royal Australian Air Force").
List of component differences between the various receivers. Lists the C, L, and R component values for the different receivers in the ARA, SCR-274-N, and AN/ARC-5 series. (However, I don't believe this list identifies the differences in T1 between the series).
Adapting an ARC-5 Receiver for Ham Use. AD5X. Among other things, describes using 1/8" O.D. copper tubing for connectors and also a nice home-brew tuning knob.
Stanley, J., K4ERO, "Using Unmodified Command Set ARC-5 Radios on the Ham Bands," QST, January 2016
Surplus Radio Conversion Manual, Volumes 1, 2, and 3. Originally published as paperbacks by Editors and Engineers. Note that Volume 3 contains a simple AVC mod on page 15 (for the SCR-274N receivers, such as the BC-454 and BC-455, that lack AVC).
As always, I might have made a mistake in my equations, assumptions, drawings, or interpretations. If you see anything you believe to be in error or if anything is confusing, please feel free to contact me or comment below.
And so I should add -- this information is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Back when I was a freshman in high school (a very long time ago), a family friend gave me a BC-454 ARC-5 receiver with an AC power supply he had built for it.
It provided much listening pleasure for me, but in the fog of the intervening years it eventually disappeared to who knows where.
Over the past decade or so I've picked up a few ARC-5 receivers at various swap-meets, and now that I'm retired I thought I'd try to get them (or some of them) back on the air. With that goal in mind, I thought I'd try running them from dynamotors (as they were designed to be operated), rather than spending time to build an AC power supply.
The images below show one of my dynamotors (a little worse for wear!).
Top view:
Bottom view. Note that the dynamotor is labeled "DY-2/ARR-2", identifying it as a dynamotor for the AN/ARC-5 series of receivers (e.g. R-25, R-26, etc.).
(Note: The SCR-274 series of receivers (e.g. BC-454, BC-455, etc.) used the DM-32-A dynamotor (see table here: Wikipedia) -- the two dynamotors are essentially identical.)
The figure, below, shows how the dynamotor's jacks are defined for High Voltage (H.V.), Low Voltage (L.V.) and Ground:
The two dynamotors I had would not rotate when 28 volts was applied, implying that their bearing grease had solidified over the years. I would need to disassemble them and relubricate the dynamotor's bearings.
Disassembling the Dynamotor and Cleaning its Bearings:
To access the bearings the dynamotor's two end covers must first be removed.
Note that the screws holding these two covers to the dynamotor body might be held in place with a "safety wire" (to, I assume, keep the screws from vibrating loose). Cut this wire and remove it, and then unscrew the two screws holding a cover to the body. Remove the two screws and their associated washers and lift off the cover off the dynamotor. Repeat for the second end cover.
Below is a picture of the dynamotor with its end covers removed:
Because I will be cleaning and relubricating the bearings without removing the bearings from the dynamotor assembly, I first removed the four brushes (two on the L.V. side and two on the H.V. side) so that I would not get degreaser or grease/oil on them. When you remove your dynamotor's brushes, keep track of which brush goes into which brush holder, and note the orientation of each. Upon reassembly of the dynamotor you will need to return the brushes to their original brush-holders and in the proper orientation!
As an aid, note that the end brackets have embossed upon them "HV" or "LV" to identify which end of the dynamotor it is, along with "+" or "-" near each of the brush caps, as shown in the four images, below:
Remove the brushes. Note which one goes into which brush holder, and the orientation of each brush. (In my dynamotors the brushes each have either a "+" or a "-" engraved on one side of each brush, and they have always been oriented in their respective holders with the engraving facing up, not down. So all I really need to remember is which two brushes go into the L.V. side and which two go into the H.V. side.).
Next, remove the two screws of the end shield that covers the bearing, as shown in the image, below. Then remove the end shield itself and any washers (also known as shims) that were between it and the top of the bearing. (Note that on the L.V. side of my dynamotor there were four washers between the end-cap and the top of the bearing).
While work continues, let the end-cap and washers soak in a bit of the degreaser fluid.
Next, take a look at the bearing grease covering the bearing. The bearing-grease in my dynamotor was, hard, dried-out. I used a toothpick to remove the grease from the top of the bearing. (Note that this will NOT remove all of the dried grease!)
After I had removed what grease I could with a toothpick, I used a toothbrush and degreaser fluid (in this case paint thinner) to scrub away the remaining hardened grease. This process will take some time. Continually turn the the armature while you scrub -- much of the bearing (and its grease) is not easily accessible.
Note that while scrubbing I keep the bearing always pointed down to prevent the degreasing fluid from flowing onto the armature and possibly affecting the armature's lacquer, etc.
Also, I recommend doing this degreasing outside (to, hopefully, minimize inhaling of fumes), and wear protective gear (e.g. gloves).
Periodically use a clean cloth to examine at the degreasing fluid dripping from the bearing (I will set the bearing on top of a cloth to capture the dripping fluid, as shown below). The goal is fluid that is clear and not be dirty.
Below, after I finished the L.V. side, I started scrubbing away the grease on the H.V. side bearing. The piece of cloth helps prevent the degreaser from splattering on me, and the cloth is on top of a clay flower-pot base (to keep the degreaser-soaked cloth off of the table-top).
By the way, the image below shows the end-shield and single spring washer that covered the bearing on the high-voltage side of the dynamotor.
An Oops Moment!!!
During the cleaning process I accidentally dropped the dynamotor onto the ground, denting one of the pot metal end brackets and bending one of the clamp screws (that is, one of the two long screws that hold the two end-brackets together). To repair the damage, I further disassembled the dynamotor in an attempt to un-dent and unbend the parts I had mucked up...
With the dynamotor disassembled, here's a look at the dynamotor's armature:
Relubricating the Bearings:
To relubricate the bearings, the Maintenance Handbook recommends, "Apply three or four drops of a light machine oil to the balls and repack the outer side of the bearing with a small amount of AN-G-15 grease. Add only enough grease to cover the bearing. Do not pack the bearing full."
I used 3-in-One oil (that I had on hand) for the recommended "light machine oil. I did not have any AN-G-15 grease, so instead I searched Amazon and found a polyurea grease, manufactured by Gennel (China?), which claims to be applicable to high-speed fan bearings (which turn at high speeds like dynamotors, right?). Unfortunately, there is no technical info on this grease, so I'm not sure how well it will work. Time will tell if my choices for oil and grease were good!
After the bearings are relubricated with new oil and grease, the washers and end-caps are re-installed on both the H.V. and L.V. ends of the dynamotor
Finally, the end-cap screws are locked down with some nail polish.
(Note: If you look closely at the picture, above, you will see that the two end-cap screws are Philips-head screws, not slotted screws. I somehow lost the original two screws (probably when I was banging on the pot-metal end bracket trying to straighten it), and I had to dig through my junk box to find two other screws that would fit).
Other Notes:
1. SCR-274 vs. AN/ARC-5: Per Wikipedia, there were three separate series of Command-Set gear: 1) the ARA-ATA series, 2) the SCR-274-N series, and 3) the AN/ARC-5 series. These three series have now become known, informally, under the umbrella term: ARC-5.
Note that equipment designs can differ between series. For example, AN/ARC-5 receivers seem to use a 12SF7 tube for the second I.F. stage (this tube has an internal diode that is used to derive an AVC voltage), while the SCR-274-N series receivers seem to use a 12SK7 for the second I.F. (i.e. no diode, and thus no AVC).
2.14V vs 28V dynamotor: At least one version of the AN/ARC-5 receivers was set up to run from 14 volts in lieu of 28 volts. If your receiver came with a dynamotor, don't assume that it matches your receiver's voltage. Someone along the way might have rewired your receivers filaments, or replaced 12V tubes with 6V tubes, or added an unknown dynamotor.
To check filament wiring, remove the receiver's bottom plate and verify if the the 12 Volt tube filaments are all wired in parallel (e.g. to be used with a 14V dynamotor), or if pairs of tube filaments are wired in series (i.e. to be used with a 28 V dynamotor).
Similarly, if your dynamotor does not have any identifying information on it (as was the case for one of my three dynamotors), remove the L.V. end-cap and take a look at the voltage specification printed on the end of the armature:
Resources:
A Google search for various ARC-5 terms will return a plethora of hits. Below are some of the sites I found useful.
Stanley, J., K4ERO, "Using Unmodified Command Set ARC-5 Radios on the Ham Bands," QST, January 2016
Standard Caveat:
As always, I might have made a mistake in my equations, assumptions, drawings, or interpretations. If you see anything you believe to be in error or if anything is confusing, please feel free to contact me or comment below.
And so I should add -- this information is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
