Bringing an ARC-5 Receiver Back to Life, Part 2 (An AC Power Supply)

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.