When I received this radio, it had been recapped, but the rubber covered wiring had not been changed.
After changing the wire, I powered it up and resistor 27A&B got hot. I found a clipped component lead from the previous recap had shorted out to the adjacent tube socket. Also the volume control went bad. All other components seem to be within tolerances.
Now when I power it up, I measure 99V across cap 26A and only 40V across cap 26B.
The plate voltages are: 7A8 - 36V, 7B7 - 39V, 7C6 - 36V and 50L6 - 37V
Only the 7C6 voltage is close to the schematic:
http://www.nostalgiaair.org/PagesByModel...014179.pdf
I've double checked everything that I can think of, but definetly missed something.
Any suggestions?
The first thing I'd check is the 35Z3. If you don't have a tester just solder a silicon diode across the plate to cathode of the 35Z3 with the proper polarity. If everything else is good that should boost it right up. Give that a whirl and see what happens.
Terry
ps #27 should get hot. It's dropping 120vac down to 93a@150ma
I've already tested the 35Z3 and even tried another new one. Same results.
Try swapping out the 50L6. That's where over 70% of the B+ current is consumed under normal conditions. Also the little mica cap C17 might be bad, I'll bet that was not replaced. Also check field coil and output transformer for any partial short or arcing. You can insert an analog (old school) 250 ma DC meter to trace out how much current is being drawn on the B+ to any stage, but don't leave the set running for very long if you find you are drawing more than, lets say 90 ma. And make sure the B- is not shorted to the chassis at any point. Good luck.
I tried Terry's suggestion, placed a silicon diode across the 35Z3. Monitoring the plate voltage on the 7B7, it went up to 149V, but after a few seconds dropped back down to 39V.
Subbing a new 50L6 didn't change anything.
Subbed a 500 ohm resistor for the field, no change.
Changed C17, no change.
While touching my meter probe to the center tap of the volume control, speaker hummed, so I would say that the output transformer is good.
Not finding a short anywhere from B- to chassis.
I'm getting cross-eyed double checking the schematic. Would have been so much easier if somebody else hadn't started recapping this thing.
Ok so this is where the rubber meets the road. I would make an adapter that would only connect the filament pins. In other words a socket and a tube base with jumpers connecting pin 2 from the socket to the base and pin 7 from socket to base.For the 50L6. This will remove any HV current that the tube would draw. With the loctals it's a little trickier but with a few clip leads and some heavy bare wire you should be able make it work. Go stage by stage till you find one that the HV doesn't drop after the tube heat. It sounds like one of the stages is hauling down a bunch of current once the filament gets hot rather than a short somewhere that would cause the volt to be low as soon as you turn it on.
Terry
Next disconnect the field coil from the B+. That will rule out the entire output stage. I'm afraid you may have a bad output transformer.
Terry,
Taking each tube out of the circuit produced the following B+ voltages
50L6 - 121V, 7C6 - 109, 7B7 - 121 and 7A8 - 107V
Hmmmm. Check voltage from pin 8 (neg) to pin 5 (pos) of the 50L6 with all tube in place. Should see - 10v or so. If positive voltage cap on pin 5 is leaky.
Terry
Was also thinking if you are the adventurist type and you had a small audio amp you could see if the rest of the set is working. The amp could be something like the amplified speakers for your computer. Two important things 1# You must use an isolation transformer or make sure that the chaiss doesn't have 120VAC between it and earth ground. If you don't have an isolation transformer you can connect your volt meter to the chaiss and earth ground with it set on AC volts plug the set in. With it plugged in one way it will show 120AC turn the plug around and plug it back in and it will read 0V AC. You want it to read 0 VOLTS. If you try it the other way you can get a helluva shock and cause other thing hooked to to burn up. Don't ask me how I know! #2 Must use a cap to couple the audio signal from the set to the amp. Size isn't vary important anything from 500mmfd to .1mfd is OK 200v or better. Solder it to pin 2 of the 7C6 other end goes to amp input. Double check voltage from amp chaiss to radio chaiss . Should be 0 VAC. Hookup just the filament pins on the 50L6 like before.
Terry
Well, this looks like it's gonna take some time and patience. It's rare to have a new capacitor defective; but it happens now and again. You might want to take the coupling capacitor to the 50L6 out and sub a new one. The other thing that happens now and again is then there is a lot of crud inside the IF cans and a short occurs, that will bring the B+ down, but usually with some clues like heat, smoke, and drippings. If you have blasted these out with deoxit and allowed to dry overnight, rule this out.
Measure the current consumed in each stage by inserting a dc ammeter and you will find where the problem is. If you have any doubts about the field coil/speaker/output transformer, sub a resistor for the field coil, and run a known good output transformer and PM speaker to test it out. Don't worry about values so much, 500 ohms for the field coil sub, and just about any generic output transformer and speaker should work OK. Just keep an eye open for the load on the B+, and shut down if it gets too high. Again, the current draw against B+ will be your guide.
If you have a variac, you can proceed at a more liesurely pace.
Just one thing more, sometimes the crappy tube sockets on some of these sets develop an arc trace. Cleaning them will not cure the problem. You might have to replace the tube sockets for the 50L6 and/or the rectifier tube. They both get real hot, and the unmentionable pollutants over the years within the set ultimately create "current sinks."
You're getting close to solving this one. Let us know!