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37-624 Flexible Resistor Replacement - Printable Version

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37-624 Flexible Resistor Replacement - davemc - 03-05-2008

I am restoring a Philco 37-624 6 volt DC radio. I wish to keep the underchassis appearance as original as possible. There are several flexible resistors used in the circuit for regulating the filament voltages. Several of these in the radio are OPEN, and I also have to change the value of one in order to replace a pilot lamp that is no longer available.

The values I need are 8.33 ohms, and 33.3 ohms. The resistors are currently inside red spaghetti tubing. The inside diameter of the tubing will just accomodate a 1/2 watt resistor.

The 8.33 ohm must carry 240ma (.48 watts). The 33.3 ohm resistors must carry 60ma (.12 watts).

I looked into using 2 volt Zeners, but cannot find any axial types at that voltage. I also considered using 1N400x diodes, which drop about 0.8 volts each and will carry one amp. But I am concerned about temperature stability! A short test indicated that forward voltage changed as they heat up while passing rated current (240 ma).

Can I use carbon comp resistors? For example, I picked three 2.7 ohm 1/2 watt resistors in series which adds up to 8.3 ohms, and will fit inside the original spaghetti tubing. Each 1/2 watt resistor must dissipate only .16 watts. But I have concerns about stability when carrying any significant current for any length of time.

For the 33.3 ohm resistors, I planned to use two or three 1/2 watt resistors in series. Same concerns.

Any suggestions for replacing these flexible resistors and keeping an original appearance? I don't know of any way to wind a replacement, even if the suitable resistance wire were available.

I suppose the only alternative is to try and find suitable small wire wound resistors and forego the spaghetti tubing.

- Chuck Schwark - 03-05-2008

See Flex resistor section in PARTS VALUES pages on my site. There is a link to a UK guy who makes repro flex-resistors for AK and Philco.

- davemc - 03-06-2008

Thanks, Chuck. I completely missed that link on your site, since my radio was not listed as using them! Perhaps you could include the 37-624 and its flexible resistors in your list (16.7 ohms and 33.3 ohms).

Since in my radio the resistors are covered by spaghetti tubing in the original radio, I will likely NOT go to the trouble and expense of having them reproduced. I will likely go with multiple 1/2 watt carbon comp resistors. But I would still like an answer on long term stability of these resistors, and also film type resistors, if anyone has the data or experience. I am aware that carbon comp resistors do drift over time.

I did do one test by running rated current (240ma) through my fabricated 8.3 ohm resistor (3 x 2.7ohm 1/2 watt, selected to equal 8.2 ohms) inside the original spaghetti tubing for 1 hour. It got slightly warm, but there was no change in resistance after one hour. I'm thinking this just might work if the radio is only used occasionally (I cannot imagine anyone actually using a 6 volt DC set for any length of time!)