Cap Problem with philco 49-1401

[gto127]

I just re tubed & recapped a philco 49-1401 but one of the caps keeps blowing after about 15 minutes. It is cap C101. I did measure reverse voltage across it but it is connected exactly as schematic describes. I did use a normal wet electrolytic instead of a dry electrolitic that was in there originally. Would this make any difference? I will be glad to email schematic to anyone that would like to look at circuit. Thanks in advance.

[nigelwright7557]

Are you sure you orientated the capacitor the same way as the old one ?

I would need to see a schematic to say which way around it should go.

Could be it was a unpolarized electroyltic in there.

[gto127]

Yes the old one was polarized I put in like the original. [IMG]C:\Documents and Settings\user.USER-44C8A002F3\My Documents\My Pictures[/IMG]

[kevinkr]

Originally Posted by gto127

Yes the old one was polarized I put in like the original. [IMG]C:\Documents and Settings\user.USER-44C8A002F3\My Documents\My Pictures[/IMG]

That didn't work, need to use "advanced edit mode/manage attachments" and then be sure to actually upload the photograph.

Note that if you measured reverse voltage across that cap it is more than possible that the schematic symbol as drawn in your schematic is backwards. Or did you install (new) solid state rectifier diodes and put them in backwards?

[gto127]

Thanks for tip on attachments. The rectifier is a tube 50Y6GT . Since my last post I got a dry paper electrolitic & put in circuit & radio has played for an hour so far without cap blowing. I am concearned though because the reverse voltage has risen from -20 to -65 volts. I sure hope these type caps aren't flamable.

[DigitalJunkie]

Eek! Sounds like that cap is in backwards to me. As Kevin said,the schematic may have been incorrect with respect to the polarity of that cap.

[gto127]

Just uploaded schematic.

[Mr. Zenith]

Make sure the plates of your rectifier tube aren't glowing. I once put the electrolytics in backwards in an old 1930's era Stewart-Warner radio set. The "tubes all lit up" (as they say on eBay) for about 15 minutes, but the radio still didn't play. It turned out I almost succeeded in melting the 5Y3! I have no idea why the electrolytics didn't blow...

[gto127]

I did reverse the cap but I get no voltage on C102A & no sound.

[kevinkr]

This appears to be a half wave voltage doubler and the cap appears to be oriented correctly in the schematic.

The voltage across that capacitor has a substantial AC component across it and a lot of cheap digital meters don't work properly under such conditions. The current through this cap is also significant in that not only does it have to cope with the ripple current, but also the entire load current of the radio it is powering on a continuous basis - a very demanding situation. A film capacitor (>= 400V rating) with a high continuous current rating might be a safer long term solution here.

Measure the voltage at the capacitor connected to the cathode of the other half of the rectifier that feeds the rest of the PSU. If this voltage is correct then the supply is working correctly.

Something else to note is that this radio/phonograph is line operated and is potentially extremely dangerous to work on. I strongly advised the use of an isolation transformer when troubleshooting this radio..