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Philco PT-30, only hums - Printable Version

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Philco PT-30, only hums - Specter - 11-23-2010

right, hello, my name is Peter. brief introduction, i'm an auto mechanic / enthusiast with a pension for mid 70's to late 80's Japanese classics. recently got into the vintage electronics game and came across a really well kept Philco PT-30. only thing is, it only hums, with no volume adjustment. now, i have done a little research [here and elsewhere] and based off a few other topics here, it seems that the capacitors for the transformer are blown/gone/otherwise fubar. now i know my way around electricity,in fact that's mostly my job at the shop, is fixing automotive electrical issues, but that's just it, automotive issues. along with the fact that i can't read all the cap. ratings off of the schematics posted on this site [good help though, just not complete enough for me at least]

so, few questions; one, are the caps on the transformer the only problem that would cause a radio to hum with no volume change, or just the most likely [ i do plan on changing them all, ALL, out of the chassis just due to their age]

and two, could someone clarify some of the ratings needed [well aware again that i am going to need to sub. out ratings due to them not widely manufactured anymore]

thanks in advance, i hope to radiate my home with the analog sounds of awesome soon

-Peter

Re: Philco PT-30, only hums - Raleigh - 12-01-2010

Peter, I couldn't find a parts list for the PT-30, but Nostalgia Air has schematics with parts lists for some of the similar PT-xx models. Check here: http://www.nostalgiaair.org/
It looks like your electrolytic capacitors are in a dual-section can (might be cardboard.) If they are like the similar models, they are probably rated at 150 volts. You can buy 160 or 450 volt electrolytics in the original pre-war capacitance values from Just Radios. http://www.justradios.com/ I'd probably get the 450 volt types unless space is at a premium. If you re-stuff the original capacitor can, you might need to use the smaller 160 volt types, just so they fit.

The tubular capacitors are mostly 200 or 400 volt, but I doubt you can buy any less than 630 volt. Just Radios has those, too, in the pre-war capacitance values. As long as the ones you buy are rated higher voltage than the original, you'll be OK. You can use modern capacitance values as long as you stay within about 10% or so of the originals, but I always use the pre-war values.

Resistors are all 1/2 or 1/4 watt, unless I missed one. I'd use 1/2 watt for all of them. You can buy them from Just Radios, too. Radio Shack has a decent selection of values in 1/2 watt, but they don't have all values. Remember, I'm looking at a parts list for a similar AC/DC radio, and your resistor wattages might be different.

There are a few things that can cause hum, but dried up electrolytic filter capacitors is the most likely. I don't think your radio has a power transformer, by the way. It is the AC/DC type with a half wave diode tube. All of the filaments are in series and are heated with AC. A bad tube of this type, especially the final amplifier tube (50L6GT) can cause a real bad hum.

I wouldn't replace any tubes until you've changed out the caps and out-of-spec resistors. If then it works except for hum, try swapping out the 50L6. If you don't have a tube tester, you might want to test the tubes with a meter for filament continuity. I sometimes check to make sure none of the internal tube elements are shorted together, by testing each pin against each other pin with the meter. It is a little tedious, but I've found several shorted tubes this way. Nostalgia Air also has tube diagrams so you can see which pins are supposed to be shorted and which are not.

If you want the best documentation, buy the service package from Chuck Schwark. It'll have the complete parts list and schematics in good clear copies of original Philco service documents. http://www.philcorepairbench.com/schematics.htm