Anyone know what might be causing this condition with my 1959 Zenith? Set has been fully recapped. Tubes test fine and swapped audio tubes as well. Out of tolerance resistors all replaced too.

When I turn the set on it is fine for the first 5 minutes. Runs beautifully. Then all of a sudden, the audio fades out slowly to nothing.

If I leave the set alone, after a few minutes the audio will come back to normal all by itself. Just pops back in. Does not fade back in. I sometimes can make the audio come back faster by changing the channel. Only does it once each time I turn it on. After that one time, I can leave the set on the rest of the day and it is fine. Any ideas???

Cold solder joint?

A possibility: the 'gradual fade to nothing' could be caused by a 'floating grid' condition of the output tube. That's where an open (or misconnected) grid resistor going to G1 of the tube allows the tube to gradually self-bias to cutoff.
Does the audio become distorted during the fade, or remain undistorted?

Could be a tube, as it heats up an internal short occurs and as it continues to heat to full operating condition the short clears. I just had a 6AU6 that was fine for a minute then the filament would go open then after it cooled it would make contact again and would keep doing this as long as the set was on.

Gregb

Intermittent grid connection sounds likely, could also be another tube upstream of the audio output. Try cleaning the tube pins and sockets first, the look for poor solder joints. A grid can develop quite a potential by picking up electrons. Sometimes it build up enough to discharge, then the audio will come back. You can also test by momentarily grounding the grid when the audio fades, it will then come back. I had that issue on a 30s radio with the grid cap type tubes, touching the grid cap brought back the audio. Turned out to be a cold solder joint.

Ive had intermittent filaments like that too, usually on high voltage heater tubes like 35C5. Set would go off, then turn back on in 30 min.

A possibility: the 'gradual fade to nothing' could be caused by a 'floating grid' condition of the output tube. That's where an open (or misconnected) grid resistor going to G1 of the tube allows the tube to gradually self-bias to cutoff.
Does the audio become distorted during the fade, or remain undistorted?

I agree 100% Look at this first.... Bad resistor, possible coupling cap bleeding
some DC to the grid....

.

Chassis or model number, please? Hard to pull a Sams with just the year.


http://videokarma.org/showthread.php?t=259601

Sorry guys, this is the SAMS for my set:

http://www.earlytelevision.org/pdf/Zenith-A2221-Sams-393-4.pdf

Chassis 17A20

Originally Posted by old_coot88
A possibility: the 'gradual fade to nothing' could be caused by a 'floating grid' condition of the output tube. That's where an open (or misconnected) grid resistor going to G1 of the tube allows the tube to gradually self-bias to cutoff.
Does the audio become distorted during the fade, or remain undistorted?

I agree 100% Look at this first.... Bad resistor, possible coupling cap bleeding
some DC to the grid....

.

The audio fades from normal over about 10 seconds to nothing. As it is fading it seems to get muffled as well. When audio is gone there is a slight muffled humming. When the audio comes back, it suddenly pops back in at normal. Not distorted coming back. Just bam! Back. It only does it once each time I turn on the set. Last night I turned on the set around 7 PM. For the first 5 minutes or so that the set was on, everything was normal. Then the audio faded out. Was gone for around 3-4 minutes, then popped back in. The set remained on until 9:30 PM and operated fine. Does this every time I turn on the set.

Really sounds like a bad connection, such as a corroded tube pin or socket, or bad solder joint allowing the grid to float and buld up a negative charge, cutting off the tube. Once the voltage get high enough, it arcs across and clears away enough oxide or solder flux to re-establish a connection.

Thanks guys. I will check out the audio tubes once again to make sure all pins are clean, all sockets are good and check for any bad solder connections. Will update on if I find anything and if condition improves.

Looks like the audio output tube's control grid gets its biasing via the wiper on teh volume control. I've had that go open circuit. I've added a 1meg resistor to the grid to ground to keep the grid fron going open circuit if the wiper contact goes bad. The audio will go missing, but the tube won't "run away" with excessive current.

Cold solder joint?

That's where my money would go on a bet. Especially after the poster said that he can sometimes speed up the process of regaining the audio by changing the channel. I'm not sure about whatever model that is, but the 59 Zenith that I parted out a couple of years ago had quite a KA THUNK to the channel changer. The vibration is probably correcting the intermittent connection.

Now as to which connection; I leave that to the experts. I wouldn't say it is the heater connection as the audio just comes back instantly as opposed to a "ramping up" as would a cold tube.

Well, if you think it's a bad connection, and it can be. Run it with the back off, get a
short stick and start hitting things ! Hit the audio tube, and hit parts of the chassis,
hit the boards if it's got a PC board, hit along any path that carries audio first.....
Hitting is an art - Use a nice 1/2" wood dowel semi-sharpened, and hit stuff vertically
so the point focus of the 'hit' is localized. Be easy at first, then harder, if you find a sensitive
spot, ease off on the hit intensity, light tap to zero in on the bad spot....

Good luck.

.

"No production shortcuts here" as the old ad says.
100% hand wired.
It sounds like a filament is dying. The set is built on terminal strips & tube sockets. Zenith mounted them 2 ways. Either by soldering 2
extra pins on the sockets & strips to the chassis or rivets. When
soldered in they use that as a GND & they can break loose.
For now run the set & watch all the audio tube filaments for
going out or dimming when the set acts up. If thats not it
follow the squirrels sage advice. Beond that there are a few
crude tests u can do without equiptment.

73 Zeno:smoke:

I'd suspect the 6BN6 tube.