The PHILCO Phorum
Filter Cap Humming and Vol Control? - Printable Version

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Filter Cap Humming and Vol Control? - regel_1999 - 03-11-2011

I was looking at my schematics for this RCA 56x5 and Philco 42-327. The Philco hummed loudly after I had replaced all the caps, filters included. I was looking at the schematic and a question occurred:

If the filters are bad and not filtering, a steady (or near steady) current is sent to the primary coil of the speaker's transformer, thus, causing the speaker to get a steady signal and hum. Looking at the diagrams it appears that the current would come straight from the wall, no step down, no filter, no half-wave rectifier. Just straight AC. Am I right to assume that if there is a hum and it is the filter caps that the radio would have no volume control, the tone would be steady, and the reading at the output of the speaker would be proportional to the turn-ratio for primary-secondary windings?

It was mentioned in another post that the 50L6 tube (Beam Power Amp) which feeds to the speaker transformer could cause a problem with loud humming if the internals were shorted. A resistance check on a broken 50L6 should then show a short for two pins... excluding the heater element, of course, right?

Ron, and all, I think I'm actually beginning to figure the components out, how they work, that sort of thing. Please, though, tell me if my logic is incorrect.

Thanks!
-Brandon

Re: Filter Cap Humming and Vol Control? - Radioroslyn - 03-12-2011

>Am I right to assume that if there is a hum and it is the filter caps that the radio would have no volume control, the tone >would be steady, and the reading at the output of the speaker would be proportional to the turn-ratio for primary-secondary >windings?
Yes.
>It was mentioned in another post that the 50L6 tube (Beam Power Amp) which feeds to the speaker transformer could cause a >problem with loud humming if the internals were shorted. A resistance check on a broken 50L6 should then show a short for >two pins... excluding the heater element, of course, right?
Maybe... Sometimes the short will only show up when the tube is on, hot, and filament lit.
Terry

Re: Filter Cap Humming and Vol Control? - codefox1 - 03-13-2011

First of all, the set with the 50L6 does not have a power transformer. The best way to test this tube is substitute a known good one. If there is leakage or a short involving the filament to any other element of the tube you will indeed get a lot of hum. Be careful, these chassis are always shock hazards.

You should use an isolation transformer when working on transformerless AA5 sets, especially if you want to measure things with other instruments that are AC Powered.

Minimal filtering is usually fine for the smaller sets since the output transformers and speakers will not pass much of anything audible for 120 HZ ripple, and are even less capable of reporting 60 HZ from a half wave rectifier. Anyway you can measure the AC element remaining after each stage of the B+ supply with a digital meter or VTVM, or you can have a look see with your scope. If properly wired and componnents are good, there should not be any objectional hum coming from the stock speaker and cabinet under normal operating conditions.