Home Forums Show & Tell ‘Farm Radio’ Power Supply

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    Gerry O’Hara
    Keymaster

    In another post on this site, a ‘show-and-tell’ on the restoration of an RCA Model 97BK battery set, the photos show a small power supply I designed and built for this radio (and which is suitable other battery sets I have in my collection). I have some other power supplies designed for this type of service: one type is made by Grandad’s Electronics of Seattle, Washington (https://www.novatech-instr.com/grandadselectronics.html), but this is designed for smaller radios with 1.5v filaments and 90 volt plate supply. Others I have are very limited in either filament current or plate voltage. Using one of the ‘Grandad’ units on the RCA only made the radio ‘motorboat’. So I decided to make something more ‘meaty’ and fairly universal. The specification of the finished unit is:

    Filament Supply: switchable pre-set 1.5v, pre-set 2.0v and adjustable 1.2v to 5.5v regulated DC (ripple virtually undetectable) at up to 1.5A, current limited and thermal protected, plus 6.3v AC (unstabilized) at 1.5A. I may add a 6.0v DC output for 01A tube radios some day (though the adjustable output set to 5.5v would likely be ok for up to 5 x 01A tubes – though I have not tried the unit in this application);

    Plate Supply: choke/capacitor filter-smoothed at 175v un-stabilized (up to 140mA), plus nominal 135v, 90v, 45v and 22v outputs (can all be used simultaneously, eg. for those sets that need a lower plate supply to the detector tube) at up to 50mA on zener-stabilized outputs (actual voltages depends on zener diode voltages used). The total zener diode voltage should add up to around 135 volts for the series resistor used (820 ohm) and the zener diodes must be 5 Watt or higher dissipation. The zener string is mounted externally under the plate supply connector block so you can easily change the zener values to any combo you want up to 170v or so without any soldering (just a screwdriver). This arrangement also keeps some heat out of the plastic chassis box when no plate supply load is connected.

    Other Features: Double-pole power switch, indicator neon for line, LEDs for filament and plate voltage indication, line fuse and plate supply fuse, line noise suppression.

    The finished supply works well (not a trace of any hum on the RCA-Victor set, which draws 0.75A filament and only 13mA plate current). It was constructed from junk-box components apart from the box, transformer (which I bought years ago meaning to make a supply like this) and the four zener diodes.

    Component Details
    Power Transformer: Hammond Type 262F6, or similar with 115 volt primary, 120 volt (140mA) and 6.3 volt (1.5A) secondaries
    Choke: Hammond Type 157R, or similar ~2 Henry, >140mA, ~50ohm DC resistance (the one used in the prototype was from a scrap chassis)
    Bridge Rectifiers: Plate supply was a BY123, but 4 x 1N4007s in a bridge configuration would work well; Filament supply was a KBPC601, but four 1N5401s in a bridge configuration would work well
    0.22uf, 10uf, 33uf capacitors should be 250 volt (or higher) voltage working (I used 450 volt components)
    3300uf capacitor should be 25 (or higher) voltage working
    0.01uf capacitors must be 1000 (or higher) voltage working
    0.15, 4.7uf and 10uf capacitors are 35 (or higher) voltage working. Use a tantalum 4.7uf for best ripple rejection on the LM317T ‘adj’ tag
    1k pre-set, 2.2k ohm, 120 ohm, 75 ohm and 24 ohm resistors are 0.25 Watt rating
    820 ohm resistor is a 5 Watt rating wire-wound unit
    68k ohm resistor is 2 Watt rating
    LED’s are 5 to 10mA types

    The LM317T must be mounted on a suitable heatsink – the one shown on the photos was made up from two smaller ones scavenged from a defunct computer motherboard. Note: unless an insulating washer is used the heatsink will be at the output voltage (not a problem if shorting to another connection is unlikley – as for my form of construction – but would be if a metal chassis was used).

    I did not build any (negative) bias supply capability into the unit as it is fairly straightforward to use batteries for this application in radios that need it as they last almost the shelf life of the cells used. However this could be incorporated fairly easily either by using a transformer with an additional 6.3v winding or a (non-electrolytic) capacitor from the existing 6.3v winding, feeding a separate rectifier/smoothing arrangement for the (-ve) bias supply(s).

    The schematic and other details provided herein are for information only. Please bear in mind that line voltage and plate supply voltages can impart a very nasty shock and can be lethal in certain circumstances – extreme caution is therfore advised if anyone decides to construct this (or any other) power supply, taking care with wiring to avoid possible contact points with the operator and when testing/using the supply. All risk is with the constructor/operator. No liablity for any injury or damage to equipment is accepted by the author.

    (Click on the schematic/photos for a larger image. Better still, right click and ‘save picture as’ to your local drive before opening – this will prevent the images opening in a browser window and will give you more control over size and printing if opened in photo-viewing software)

    Download DSC00130 [1024×768].JPG. (Caution: This file may not be virus scanned.)

    Download DSC00133 [1024×768].JPG. (Caution: This file may not be virus scanned.)

    Download DSC00087 [1024×768].JPG. (Caution: This file may not be virus scanned.)

    Download DSC00122 [1024×768].JPG. (Caution: This file may not be virus scanned.)

    Download Farm PSU Schematic.jpg. (Caution: This file may not be virus scanned.)

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