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AC on Chassis - Printable Version

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AC on Chassis - Raleigh - 07-02-2007

I've worked on 2 1941 Philcos now, a 41-250 and a 41-280. I rebuilt the 39-03 Bakelite AC bypass caps using .01 Mf X1/Y2 safety caps from JustRadios. Both of these chasses have about 65 volts AC on them when I measure chassis to AC neutral or earth, anytime they are plugged in, even when turned off. When I measure with a micro ammeter, the current from chassis to neutral or earth is less than 1 micro amp. Grounding the chassis to Earth doesn't hurt reception as far as I can tell.

I did an experiment by disconnecting the hot lead from the Bakelite cap, and voila, the AC voltage went away. Disconnecting the neutral lead made no difference. (By the way, I put polarized plugs on these radios.) I thought I'd made a mistake wiring the Bakelite assembly, but when
I connected the hot lead to the chassis through a different X1/Y2 cap the result is the same.

It looks like it is natural to this design to pass a little bit of AC current through the cap to ground on the hot side. Since the switch is between the bypass caps and the transformer, there is low-level AC going through that cap to the chassis all the time. I'd like to reconnect the switch so it is between the power cord and the bypass cap on the hot side. Is there any reason NOT to do this, other than tradition?

- Chuck Schwark - 07-02-2007

Perfectly acceptable to re-wire ahead of the AC line bypass caps. Good that you used X/Y safety caps besides.

- Poston Drake - 07-02-2007

I agree with Chuck, no problem in re-wiring switch ahead of the AC line bypass caps. This is a matter of personal choice, and if you feel more secure by doing it by all means do so. But it should be realized that the 65 VAC you are measuring on the chassis is not an unsafe condition, as it is at very low amperage. Less than 1 micro amp in your case, hardly enough to be a safety concern.

Poston