Resistor Watts?
 

Hi Everyone,

So I replaced a selenium rectifier with a diode and dropping resistor on a Zenith C845 (thanks everyone for your help with that), and I flipped the thing on.

Everything was fine, until after about three seconds I noticed smoke from the chassis. Inspection revealed the smoke was coming from a resistor-- a 680 ohm resistor. I guess that sucker is fried.

My question is, how do I determine the watt rating that I need to replace it with? The schematic just says, "680". The resistor leads into a 6BJ6 R.F. tube, and says "98v" at the input.

I suspect I should be able to figure this out from the schematic, but I'm not very good at reading these things.

Usually its a 1/2 watt resistor unless otherwise noted on the schematic. Many schematics actually say that somewhere on them. What you'll need to look for is why the resistor failed. Often its a shorted capacitor or something, resistors don't fail on their own like that.

Thanks, Gadget. There must be a short somewhere because I replaced it with a 1/2 watt and the thing fried instantly. I guess in that case I should proceed with recapping and see if that solves it.

I have read that it's good practice to check that the radio works after each capacitor is replaced, but if something is frying that doesn't seem wise.

I would start with the capacitors on each end of that resistor that failed, and go from there. I can't locate a schematic for that model, but if you've got one I may be able to suggest what caps to start with.

I think he may be talking about the resistor (R3 on some schematics, R2 on others) that feeds the screen grid on the RF tube. I wonder if the B+ voltage might be too high after the rectifier change out?

Hi Gadget,

The schematic is online at http://techpreservation.dyndns.org/s...ics/Zenith.htm -- it is chassis # 8C02 in the drop down (if you have the djvu viewer-- if you don't I can scan my paper copy and send that).

The resistor is the 680 ohm heading into the 6BJ6 (into the screen grid I think, as eufaula1 suggests).

I was also wondering if the silicon diode is providing too much juice. I put a 50 ohm, 10 watt dropping resistor before the diode, but maybe this isn't enough? I had some people say 50-100 ohms would suffice, and others say no resistor at all was needed.