My two newly aquired sets, a predicta and an zenith SC 300 both share a common problem. Leave the set on for five minutes, and the picture shrinks slowly, and then fades to black.

The weired thing is that the philco came back a few seconds later, very quickly and light crackling came with it. Needless to say I pulled the plug. The zenith stays black.

I checked both rectifiers and replaced then with others i had, but the same problem persists. The plates aren't glowing and no fuses blowing, so there probably isn't some horrible overload in either.

Is this a sign of a dying power supply? Leaky filters perhaps? I don't troubleshoot power supplies often, and I've never come across this problem in any other set I've owned, transformer or transformerless. Any input and advice on what to look for is much appreciated.

Jordan

Seems like a thermally intermittent component...all paper caps and electrolytics should be replaced. Sounds like the horizontal drive signal is getting weak when a component like a cap or maybe a resistor heats up and then is disappearing, meaning the oscillator is stopping or is getting really weak...there fore you have no HV and no picture. Do not operate the sets like this as it is hard on the HO tube and flyback and can destroy them. If the sound stays normal it is likely not in the main power supply but in the horizontal/HV supply.

Please make sure you are familiar with HV safety procedures before commencing repairs.