(Please note I am using the Sam's schematic here. I got lucky and had the schematic for this radio!)

I replaced two 0.01 uF capacitors; one is between the 50C5 plate and ground, the other was between each IF transformer secondary to ground. Still no luck; in fact, there was no intermittent reception this time. No stations could be picked up at all.

Then I tackled the 0.05 uF that separates the electrical from the chassis ground. (I replaced it with a 0.047 uF.) Before trying it out, I sprayed some cleaner inside the IF cans; I managed to do this without disassembling the cans. The radio plays consistently now! I will do some further testing however to make sure it is fixed.

While the radio was playing, I made some voltage measurements. Only the plate voltage on the 12AV6 reads below the rated value by more than 10V. (I'm getting 43V instead of 55V.) All negative voltages are actually lower than the schematic values, except for pin 1 on the 12AV6, which is reading -0.72V.

Many thanks to Tom and Doug for your help.

Mark: I suppose it's possible that the line-to-chassis cap could have been sputtering, but I doubt it. If the problem was in one of the IF xfmrs, it'll likely be back. But meanwhile, enjoy the radio.

Unfortunately, the problem did return after an hour of playing; however, I can still hear a station. I just have to turn the volume up a lot more. It sounds a little distorted.

I made a quick check of pin 5 of the 12AV6; it is reading over +1V when it should be reading -0.55V. This reading doesn't remain steady. I just wonder if there is a cold ground connection somewhere.

I made a slight adjustment in the IF's, but it didn't make any difference.

:Mark: I suppose it's possible that the line-to-chassis cap could have been sputtering, but I doubt it. If the problem was in one of the IF xfmrs, it'll likely be back. But meanwhile, enjoy the radio.

Hi,

check any capacitors feeding the grid of the 12AV6 - what you describe is typical of a leaky capacitor. If you have a valve tester, it is worth checking the valve for gas as well.

Jim.

Schematic: http://techpreservation.dyndns.org/schematics/53/53_Westinghouse_H-378T5_167.djvu By all means, check the caps for leakage per Jim's suggestion. Even better, replace all the film caps that you haven't replaced.

To me, things are pointing to primary-to-secondary leakage thru the 2nd IF xfmr, i.e., silver-mica disease.

Transformer leakage sounds likely, as pin 5 is not the grid, but rather one of the detector plates.

Removal of the transformer, with a successive continuity check in all directions (shorts across and between coils) will reveal this.

Thomas

I checked the second IF transformer for continutity; I get around 17 Ohms across the primary and 17 Ohms across the secondary. I checked continuity between primary and secondary; there is an open reading (as there should be).

The last items replaced were the 2000 pF capacitor on the 12BA6 grid, the 150 Ohm cathode resistor on the 50C5, and the 1W power resistor between the filter capacitors. While replacing the grid capacitor, I ended up replacing the 680 Ohm resistor that connects between the bottom primary lead of the second IF transformer, and the second filter capacitor. (In order to get the capacitor out, I had to remove this resistor too.)

If the radio dies out again, at least I know where the problem is. Meanwhile, I'm going to line it up. Thanks!

:Transformer leakage sounds likely, as pin 5 is not the grid, but rather one of the detector plates.
:
:Removal of the transformer, with a successive continuity check in all directions (shorts across and between coils) will reveal this.
:
:Thomas

So you got the radio working?

Thomas

It is working, although the problem may still occur after it works for a while. Will check further.

I hope I don't end up opening the IF's. I'll have to figure out a way to do it. This radio is different from all others I've worked on. The underside of the chassis is plastic with bare conductors running everywhere.

Today during the alignment procedure, I noticed a strange oscillation while aligning the antenna section. I tried it out for a few minutes after taking it home, and it works.

:So you got the radio working?
:
:Thomas

Congratulations. Hope the problem doesn't re-occur, though if it does, it is likely a shorting condenser, whether under the chassis or inside the IF transformer. It could also be a resistor opening up in the power supply, but unless you notice any cracked or burned resistors, I doubt this.

I had a radio in which the 50C5 tube screen grid shorted. It was only a momentary kind of short, but angered me since it was a new tube. I put the tube in my EICO 625 tester and connected the offending elements to the up and down positions. I then put the selector on #4 (maximum current and voltage). I pressed the merit button and tapped at the tube. Sparks flew inside the tube for a moment and then the short went away. I banged on the tube after this and it would no longer short (I melted the short). I don't recommend this for all tube testers. Mine is capable of momentary overloads.

Good luck,

Thomas