Zenith Video Issues

[DaveWM]

a picture (or video better still) is worth a thousand words. The hardest part of diagnosing sets with out being there is trying to visualize the problem. Next biggest problem is not being able to tap around and adj controls.

All that being said if you think you have an agc issue you should look on the schematic for a small value eletro, like 5uf or so. they are often singles small round with red painted on on end. This generally manifest as overly critical agc (from blanked out signal to loss of sync with very little adj in between. I don't know if I agree with the prob as described as AGC.

likely issues are tuner (dirty) tube sockets (dirty) baluns (open or not connected right). I look to this for problems with poor video signals.

[Tubejunke]

Thanks Dave! I agree whole hardheartedly on the tips that you gave me. But I have had more sound than video issues with all of my sets when it comes to baluns connections and the use of stand alone signal sources like digital converter boxes (free TV) and VCRs ect. I had two sets that I thought had sound circuit problems (low volume) that turned out to not be happy with the signal device configuration.

Specifically with this Zenith set, I may still have something like that going on, but I have swapped TV out cables. I haven't tried a different baluns. The only reason I mentioned AGC was the darkening picture. I forget if that would effect the sound, but the sound again is perfect.

Hopefully soon I can get a picture of the picture. That will be a big help.

[zeno]

A screen shot would be nice but... just because a tube checks good
it dont mean its good. The worms make me think an IF tube.
AGC usually messes up the sound if overloading or washes out
(no contrast) the pix.
Anyhows always start with cleaning & adjusting then tubes next.
Its the cheap & easy way to go. Two of the IF tubes & several of the
other low level tubes are the same. Try swapping them & see if
you get different symptoms.
Dont forget the lost art of tapping. Light tapping of tubes often
show a bad tube or dirty socket. Give light taps one tube at a time
while watching the pix in a mirror.

nuf fer now
73 Zeno

[Tubejunke]

Originally Posted by zeno

A screen shot would be nice but... just because a tube checks good
it dont mean its good. The worms make me think an IF tube.
AGC usually messes up the sound if overloading or washes out the pix.

Dont forget the lost art of tapping. Light tapping of tubes often
show a bad tube or dirty socket. Give light taps one tube at a time
while watching the pix in a mirror.
73 Zeno

Very true on all statements and methods. These resurrections or restoration projects are unique as we are often working with something with years of use in a smokey room and then dormancy by the decade. With that being said, we don't really need the laboratory equipment that only some of us may own. You need these rudimentary, common sense skills like tapping tubes. cleaning their pins and sockets, and feeling capacitors to see if they are getting hot.

I just talked to the guy who gave me this Zenith yesterday and he told me that he got it in 1997 and that it had been in a basement before that since the late 70s. I feel that I am lucky to have gotten the set to where it is right now. I must look at all that is functioning which is most everything. Just the few issues that I am mentioning isn't that bad in the grand scheme of things. Also, I have seen the set play almost perfectly a time or two, but it's momentary. I am crippled by lack of space to move the thing around a lot or pull the chassis. At least it has that nice access panel on the bottom that gets me to much but not all of the components.

You mentioned IF tubes. I have tested and swapped all the video IFs. No change. Like I have mentioned, I need a screen shot......

[Tubejunke]

Originally Posted by zeno

A screen shot would be nice but... just because a tube checks good
it dont mean its good. The worms make me think an IF tube.
:

I have found at least one problem that I sort of suspected, but failed to mention. The 6GJ7 tube in the tuner has a dead short. I remember when I first applied power to the set, now a long time ago, this tube lit up blue like a Christmas tree light for a few minutes, not seconds, minutes; then it went to a normal operating appearance. I figured that it was just really gassy. I hoped that no permanent damage was done, but such is not the case.

I also figured that I would have another tube since I am up to my butt in vacuum tubes, but nooooooooo. Anyway, if anyone here has one or two of these (6GJ7) to spare I would gladly pay them the value of a good tube and of course shipping OR if anyone has any tube needs I would gladly give them back more than they would be giving. Especially if you need Compactron types. If you need those I will give you two (maybe more) to one for any tube needed for a 1964 Zenith model 5111 with a 25LC30 chassis. I know that there are a few in my set. The only one that I don't want more of is a 6BA11. They seem to be everywhere; in my hoard at least. Other than this set I have NO use for the later style tubes or "Compactron" types as I refer to them.

[old_coot88]

Does it display the same symptoms with a different signal source? Just wanta be 100% certain the problem is in the set and not the signal source.

[Tubejunke]

I have definitely gone down that road as that or similar issues have been an issue with several of my old sets, but none (or few) of my newer, everyday sets. Basically, each set has it's own properties as to how it will accept say a digital TV converter, or a VCR as a signal source. The Zenith we speak of doesn't seem to be as persnickety as some of the older black and white sets I have and on the Zenith I have swapped baluns, cabling, and tried different sources and they all bear the same result.

My beloved Philco had a tough time when analog signal left our world for the most part. There was never a problem with a set of rabbit ears, but when I got a digital converter or other things that had to be cabled in it simply didn't like to perform. Oddly I get good performance with one lead of the balun connected to the tuner and the other connected to one side of the built in antenna. It would seem that it wouldn't work, but that's how it does work. I have a 1950 RCA that had very low sound until I tweaked the signal process somehow. The picture was perfect with almost inaudible sound.

So, I feel or dare I say I know that I must replace any shorted tube. It may not make the set play perfectly, but I strongly feel that it will help a lot.

[Zenith26kc20]

My RCA CTC9C and my Zenith 25MC33 both have problems with the converter boxes. The RCA had terrible herringbone and the peoples faces would turn black. The Zenith had bad times with any graphics on the screen and loud audio buzz. I mentioned in a earlier thread about using a agile modulator. It cured both tv sets. The Channel Master box was the best one, the Digital Stream the worst (except for DXing, where it is the best).