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 Post subject: Candohm Wattage Question
PostPosted: Feb Thu 08, 2018 6:01 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 842
Location: Southwestern,Ontario Canada
Hi guys, I'm working on a Canadian version of an RCA Victor 262 (1934) radio. It is a Canadian General Electric M106. Schematic can be found here.

http://pacifictv.ca/schematics/rca262schematic.pdf

https://www.radiomuseum.org/r/rca_262_1.html

This set uses a single 5Z3 rectifier, instead of the dual 80's used in the American RCA. The original Candohm is 3 sections total resistance of 8820 ohms, divided as follows R27, 220 ohm, R28 3,900 ohms, R29, 4,700 ohms. The original was bad, and had earlier repairs done. 10 watt wire wound resistors replaced for R28 (3,900 ohms) and R29 (4,700 ohms). The 220 ohm R27 had been left alone. I measured it too, and it was open as well. I drilled out the rivets securing the original wire wound resistor and put in a phenolic "stand off" . I'm plan on reusing the replacement 4,700 & 3,900. The only wire wound resistor I have is a 5 watt 240 ohm (that measures 230 ohm) for the R27. My question is, Is 5 watt's enough for this resistor ? Or should I get a 10 watt ? It is paralleled with C56 a 10 mfd @ 25 volt electrolytic. I pried apart the original Candohm and can clearly see that R29 (4,700) was generating the most heat, as the heat sink area of the metal and (fish paper ?) was black to dark brown, the R27, and R28 had hardly no discoloration..

Tony


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 Post subject: Re: Candohm Wattage Question
PostPosted: Feb Thu 08, 2018 7:19 pm 
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Joined: Feb Wed 07, 2018 6:52 pm
Posts: 318
Location: Stone Mountain, GA
5 watts should be fine. If you figure it can have 20 volts on it max, then that is 2 watts dissipated. Even at 25V (Max for cap) it is 3W.

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 Post subject: Re: Candohm Wattage Question
PostPosted: Feb Thu 08, 2018 7:39 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 4425
Location: Near Brandon, Iowa
All of the B+ current in the radio flows through the 220 ohm resistor on its way to ground. For a fairly high-tube-count radio, this current would typically be in the 100mA range. By the equation P= I*I*R, this means that the 220 ohm resistor has to dissipate around 2.2 watts. A 5-watt resistor should be adequate if the 100mA guessestimate is accurate. You might want to install the resistor that you have and measure the voltage drop across it so that you can calculate the actual current flowing through it. If the current is very much above the 100mA estimated value you will need to go to a resistor with a higher power dissipation rating.

It is pointless to do this until you have recapped the set, since leaky bypass or filter caps can load the B+ bus and give an inflated representation of what the set's B+ current consumption is.

The value of the 220 ohm resistor sets the negative bias voltage applied to the grids of the type 42 output tubes. Increasing to 240 (measured 230) ohms will cause the bias voltage to be a bit more negative, but probably not enough to affect the radio's audio output quality.

Evidence of heat damage in Candohm resistors can be misleading, since one of their common failure modes is for the resistance wire to burn through the fish paper insulation and then shorting to the metal (grounded) case. This will cause overheating and probable destruction of the damaged segment, but isn't necessarily indicative of an excess current situation with the part IF it had not already been in a failure condition.


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 Post subject: Re: Candohm Wattage Question
PostPosted: Feb Thu 08, 2018 8:51 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 842
Location: Southwestern,Ontario Canada
Thank-you both for the feedback; and lorenz200w for the detailed explanation of the function of the 220 ohm resistor. That was kind of my game plan, to do a total recap of the set, and measuring the voltage across the 220 ohm once it is up and running. I can then see if it is running anywhere near the limit for a 5 watt resistor. I have everything I need, I just didn't want to put in an order for 1 resistor if I didn't need to. I'll make a note of it, and add it to the next time i put an order together.
Thanks again guys !
Tony


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