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Philco 41-280 60 cycle hum problem Need Help - Printable Version

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Philco 41-280 60 cycle hum problem Need Help - W5ALS - 06-30-2010

I have a 41-280 philco I am working on. I hook it to a varac brought the power up slowly everything look good, after 30 sec. a loud hum started with the volume all the way off. First thoughts Electrolytic cap. bad. Look at the cap. someone had change them out before they used a 30mf 450v and 2 16uf 450v. My riders print show they should have used 8uf 12uf and a 16uf 450v cap, I put what the print show in the radio, power it up slowly hum came back again. So I change out all the old cap. in the radio, power it up again hum still there. This time I change all the tubes out to rule that out, hum still there. Buy this time i was asking myself what am i doing wrong. I than put the larger cap. in the radio, I though the last person new something I didn't, hum still there. I know it is something i am doing with these 3 electrolytic cap. I am attaching 3 pictures, I hope some one can see something i am missing. Please help!!!!!!
thanks
alton

Re: Philco 41-280 60 cycle hum problem Need Help - exray - 07-01-2010

I don't see a wire coming back from the speaker field coil coming back to the 2nd filter cap? C77A in the following diagram.

http://www.nostalgiaair.org/PagesByModel/415/M0013415.pdf

Hard to tell in the photo. I'd recommend tracing out the PS circuit against the diagram to see if anything has been connected incorrectly.

Re: Philco 41-280 60 cycle hum problem Need Help - jeremyfries - 07-01-2010

I am only just learning myself and am by no means an expert, so please take what I say with a grain of salt....

From what I understand, the rule of thumb is that any electrolytic condenser that meets or exceeds the capacitance (uf) and voltage (v) rating is fine, and for other condensers the ratings should be as called for in the schematics or exceed by less than 20%. (Someone please correct me if I'm wrong!) In other words, I don't believe that the problem is with the existing condenser ratings.

I'd suspect that there is an open in one of the condensers (which would be odd considering that they all look new in the pics as far as I can see on my screen) or maybe that there is a ground issue?

I'm interested to hear what others have to say so I can learn from this one myself! Icon_biggrin

Re: Philco 41-280 60 cycle hum problem Need Help - W5ALS - 07-01-2010

Alright i found were the hum is coming from, this is the first on me in 25 years. If you look at the pictures the top tube has 2 yellow caps hook to it. to the left the is a small metal plate covering the 7B7 tube, that plate was the problem take plate
out hum go's away put plate back in hum comes back and the plate was touching nothing. Strange!!!!!!
alton w5als

Re: Philco 41-280 60 cycle hum problem Need Help - exray - 07-01-2010

Weird!

Re: Philco 41-280 60 cycle hum problem Need Help - Dave - 07-01-2010

That's an odd one, it seems like the plate is there as a noise shield. Another thing to consider... has anything been moved around (wires or components) or has the length of any leads changed? Even a small change can make a big difference. I noticed something else in you pictures that could be an issue in the future. The exposed leads of the electrolytic that's strapped to the chassis look like they could easily short if the radio gets jostled around. You may want to dress those a little shorter or cover them with some heat shrink.