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 Post subject: Flex resistors in 1930's radios.
PostPosted: Oct Sun 31, 2021 4:25 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1309
Location: Eagan, Minnesota, USA
I've run across flex resistors in many mid 1930's RCA and Philco radios. Right now I have to replace one that is 1K in an RCA C-15-3. Is there some special characteristic of these resistors that has to be considered when you replace them, and can a normal modern resistor be used? What was the reason they even used these? Is it a good idea to simply replace them during a chassis rebuild?

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 Post subject: Re: Flex resistors in 1930's radios.
PostPosted: Oct Sun 31, 2021 5:25 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 28025
Location: Detroit, MI USA
They are wirewound, and can be replaced with another wirewound resistor having the same resistance value and equal or higher wattage.

They existed at the time as an economical way to make lower wattage wirewound parts, most of the ones we commonly see are 1 or 2 watts.

They have no other special characteristics except that there were a few made which are intended to also be fusible and that would be mentioned on the schematic or the set's part list.

Two things happen to them, the cloth covering and asbestos core tend to deteriorate with age and that makes the resistor literally fall apart, although the wirewound element may remain intact. The other thing that frequently happens is they become intermittent where the ends are crimped onto the resistor.

While you don't automatically have to replace every single one of them, there are enough failures that it makes sense to.

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 Post subject: Re: Flex resistors in 1930's radios.
PostPosted: Nov Mon 01, 2021 1:49 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1309
Location: Eagan, Minnesota, USA
Dennis, thank you for your reply. I just replaced a bad one in my RCA C-15-3 that I'm rebuilding, then I replaced the remaining two flex resistors just to be sure. I used a 2 watt metal oxide resistor in each case, and the radio is working fine.

I just saw the other link with some additional info on these resistors. Good information.

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 Post subject: Re: Flex resistors in 1930's radios.
PostPosted: Nov Mon 01, 2021 4:25 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 11138
Location: Long Island NY
Normally it is good practice to replace like for like. Flexohms were wirewound resistors so it would be best to use modern low wattage wirewound resistors to replace them. Many values are available in 2 watt or 3 watt sizes which are easy to work with.

Wirewound resistors for electronics use are normally made with a relatively low temperature coefficient. Over the expected operating temperature range, their resistance does not change much. Some metal oxide film resistors have higher temperature coefficients, i.e. the resistance changes more with temperature. It might not make any difference in some circuits, but in others you could be faced with bias voltage shifts that could lead to distortion or loss of sensitivity after the radio plays a while.

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