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 Post subject: Is it a terminal strip, or what?
PostPosted: Sep Sun 05, 2021 2:20 am 
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Joined: Nov Sun 19, 2017 4:24 am
Posts: 25
Location: Rossland BC
That long metal strip below the tuning condensor, and rivetted to the chassis. What is it?
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There are three "pairs" of solder lugs along the strip. One of the centre lugs is part of the metal casing (thus grounded), while all the others are insulated from the casing. There is no continuity between any of the lugs...even within a pair, which is why I used quotes--there doesn't appear to be any pairing behaviour going on.

So what's perplexing is that each "pair" has one lug with two wires attached and one lug with a single wire attached. Single wires connecting to nothing...how useless is that?

This is a 1933 Hartco cathedral (24,35,47,80)--and an Audiola 517 doppelganger exists--that apparently wasn't significant enough to warrant a schematic...at least I haven't been able to find one for either radio. I'm a fair newbie to be attempting a restoration without a schematic and parts list, but hopefully if I go slowly and carefully I'll succeed and learn a lot along the way.

Cheers, Chuck


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 Post subject: Re: Is it a terminal strip, or what?
PostPosted: Sep Sun 05, 2021 2:27 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Detroit, MI USA
It's official name is a Candohm resistor, which is a multiple section wirewound unit housed in a metal jacket.

They are generally considered to be extremely unreliable and capable of causing catastrophic failure if the insulation fails and one of the terminal lugs contacts the metal case. They are also well known to intermittently open.

Most of us would choose to replace it with terminal strips and individual wirewound resistors of the correct value for each section. The only way to know what those values are, is to get them off the manufacturer's schematic since they are almost never marked on the part itself.

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 Post subject: Re: Is it a terminal strip, or what?
PostPosted: Sep Mon 06, 2021 12:23 am 
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Joined: Nov Sun 19, 2017 4:24 am
Posts: 25
Location: Rossland BC
Thanks so much! I said there was no continuity between the lugs because readings were outlandish (20M-ish) and I figured I was just getting backwash readings from the rest of the circuit, but knowing that it is a set of resistors then perhaps they're valid readings after all.
Chuck


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 Post subject: Re: Is it a terminal strip, or what?
PostPosted: Sep Mon 06, 2021 12:33 am 
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Joined: Feb Wed 07, 2018 6:52 pm
Posts: 1097
Location: Stone Mountain, GA
It would normally have resistances of a few k ohms down to less than 100 ohms, so it is open.

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 Post subject: Re: Is it a terminal strip, or what?
PostPosted: Sep Mon 06, 2021 2:26 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 27851
Location: Detroit, MI USA
You will somehow have to find the schematic. as we can't guess at the values, or the connections.

Most Candohms are one resistor, with multiple taps. But some of them have one or more sections that were completely isolated from the other section(s).

As already mentioned, it would be very unusual to find one with any section higher than 20K. You do find some having one or more sections well under 100Ω though.

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Dennis

Experience is what you gain when the results aren't what you were expecting.


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 Post subject: Re: Is it a terminal strip, or what?
PostPosted: Sep Mon 06, 2021 2:59 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 10990
Location: Long Island NY
It's worth noting the construction of these resistors. There is a fiber or paper core around which the resistance wire is wound. The terminals were riveted to this core. The finished core was then covered with a fish paper insulator and crimped into the metal holder. They were a lot less expensive than ceramic wirewound resistors, and being riveted to the chassis, there was little chance they'd get hot enough to damage the paper under normal conditions. They were not rated for high voltage use, and were mostly used in the return legs of AC radio power supplies where they only had a few volts across them.

Unfortunately as paper ages it loses both mechanical and electrical strength. Hot spots developed which carbonized the paper, then they'd short to ground. After a few years the resistors developed a reputation for being troublesome. The manufacturer tried to improve the design by using resin impregnated cloth to replace the fish paper insulator, and in later versions they encased the resistance element completely in phenolic, held in a metal bracket. But if you see one with the paper insulation (the one pictured is that type) shorts between the resistance element and the case are always a concern even if it is still working when you find it.

What this means is that you must resist the temptation to use the old resistor as a terminal strip and bridge the open sections with new resistors. The resistor may be open but its sections could still short to ground anyway. The new resistors have to be put on insulated terminal strips or connected by other means and the resistor has to be taken out of the circuit completely.

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