Hot chasis

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Hot chasis

Post by Patrick on Tue Nov 09, 2010 6:32 pm

I am not an expert at radio repair but have restored several radios. My current Philco 42-PT-321 has me stumped because I measure 117 volts from the chasis to a ground. I have replaced all caps and put on a new polarized cord with the hot wire to the switch. The radio works OK but is too dangerous. Any ideas please?

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Re: Hot chasis

Post by Bill Cahill on Tue Nov 09, 2010 6:47 pm

The switch is usually the cold side. Reverse power cord wires.
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Hot Chassis

Post by Alfred Corbin on Tue Nov 09, 2010 7:15 pm

Patrick:

I could not find a schematic for your Philco, but....

If it's a transformerless set, there's a 50-50 chance your chassis will be deadly hot depending on how it's plugged in. You may need to reverse the wires on the plug.

If it has a transformer, the chassis may have a high impedance 115 volts on it due to the line bypass capacitor. If you are using an old fashioned analog voltmeter (the best kind, I think), that voltage will show as only a few harmless volts. If you are using a high-impedance modern digital meter, it will read 115 volts.

You are wise to check it out to avoid a nasty shock.

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Re: Hot chasis

Post by Patrick on Tue Nov 09, 2010 10:04 pm

It does not have a transformer. I thought you always wired the hot wire to the switch to avoid a hot chassis even with the set turned off. Guess I was wrong.
Schematic at: http://www.nostalgiaair.org/Resources/512/M0013512.htm

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Re: Hot chasis

Post by exray on Tue Nov 09, 2010 10:50 pm

I'm not meaning to be snarky but don't bother measuring the chassis to a separate ground.

You have a cap (schematic is blurry) across the AC line. That will always result in a measurable voltage between AC line 'neutral ground' and a real ground. You have to view the caps impedance at 60 Hz AC to realize that its only a "resistance" at 60 cycles and does indeed bleed one side to the other. (The tech purists will kill me for that simple explanation).

Betcha a six-pack that you're using a DMM and with a cheap VOM you wouldn't see 115-120 volts.

The idea nowadays with across the line capacitors is that they need to be less than .015mf because somewhere in the annals of the UL the effective series resistance of a .015 and your getting a tingle from the chassis is below the threshold of heart fibrillation.

This isn't some odd thing. Its just to say that "ground" rarely means zero volts. If you measure between two "grounds" you'll always have voltage since no ground is ever perfect.

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