Got a Zenith H2438 porthole with some sweep issues

[AmigaMan]

Hello all. I recently picked up this beauty for free from a video artist's basement. It's a 1951 Zenith H2438 with a 24H20 chassis, which uses the 16GP4 metal cone CRT. The tube is in beautiful shape, with amazing brightness and sharpness. It's labeled Sheldon, so I assume it was a replacement or rebuild.

I managed to work out most of its problems. I replaced the electrolytic and paper capacitors, replaced the weak vertical integrator, cleaned the tube sockets and pots, etc., but it has a slight blip in the horizontal sweep, visible in the image as a vertical light line, around which the image is distorted, as if the sweep slows at that point in its horizontal travel. The problem is intermittent, and it changes in severity abruptly. It can be seen clearly in the second photo.

One of the two 6BQ6 horizontal output tubes has a slight intermittent leak to the cathode as determined with my basic tube tester, and I believe that to be the fault. Any ideas on that? I'm off to a local electronic component store to search through their vast collection of unorganized tubes to find a replacement, so any simpler ideas would be wonderful.

Is there anything else I should know about these sets in general? I'm very knowledgeable about television broadcast/studio gear, but this is my oldest consumer TV.

Photos are attached.

Thanks!
–Tom

[CoogarXR]

I don't have any help to offer, but I just wanted to say the cat in your 2nd picture made me giggle...

[compucat]

I would definitely try substituting horizontal output and damper tubes.

[jeyurkon]

Nice set! Hopefully the cat knows to stay away from the metal CRT.

[Microtrol]

That line looks like the horizontal drive is turned up to much but being that it is intermittent, I would change out the horizontal output tubes as well. Sometimes the tube tester doesnt always check completely as I have had slightly shorted tubes that checked out initially but started to fail as they warmed up more

[John Marinello]

[QUOTE=AmigaMan;3007650 ... replaced the weak vertical integrator...
–Tom[/QUOTE]

That's an "Aldrich".
On an unrelated note, what did you replace the weak integrator with?

[Zenith26kc20]

If the set has an adjustable horizontal drive capacitor (trimmer capacitor), it may be intermittant. Monitor the drive waveform to the horizontal output tubes and see if it increases significantly when the problem occurs. The trimmers can get intermittant from corrosion or dirt causing leakage on the mica insulator

[miniman82]

X3 on the H-drive adjustment, that's classic horizontal folding.

[old_coot88]

Since it's intermittent, some judicious tapping around with a well insulated tool often reveals the faulty component. As others said, the horiz. tubes are the first suspect. oc

[earlyfilm]

Originally Posted by AmigaMan

... 1951 Zenith H2438 with a 24H20 chassis, which uses the 16GP4 metal cone CRT. . . . . it has a slight blip in the horizontal sweep, visible in the image as a vertical light line, around which the image is distorted, as if the sweep slows at that point in its horizontal travel.

Tom,

This was a very common problem with the early 1950's sets.

You either need to, or already have replaced the capacitor across your horizontal osc. coil and the new one is within 20 percent of the other one, but off just enough to throw the osc. wave form alignment out.

Get a Sams and adjust osc. coil to match the waveform pictures. (If you use a modern scope, adjust the time so you have at least 6 or 8 cycles on screen instead of the two shown in Sams.)

If you post a schematic of the sync and horiz circuits, I can verify this. (To the best of my memory from 50 years ago, these early Zenith sets use a sync/osc circuit similar to the same era RCA one.)

James

[AmigaMan]

Thanks for the suggestions!

I forgot to mention some things earlier. First, the waveform coming into the control grids of the horizontal output tube pair is stable, which leads me to believe it's the outputs tubes, not the oscillator or hold controls at fault. The service manual says that the horizontal drive control adjusts the amplitude of the sweep applied to the output tubes' control grids, so I think it's stable too, but I'll clean it to be safe.

I also did some tapping around as per OC's suggestion, and the line seems to jump in severity when I get to V24, which is the same horizontal output tube which read a little leaky on the tube tester when I tapped on it during test.

So I guess I'm off to search for a new 6BQ6 or two. I'm sure I'll find one.

And John, I replaced the old integrator with an almost equally old one I found in my big random tube gear parts box. The old one had an output of about 1.2mV, and the new one is a little higher, which helped out the vertical hold a little bit. My bigger problem there is an AC hum ripple in the signal causing it to fall in and out of vertical sync, but I'm replacing that faulty cap today.

And oh yes, the cat's name is Cookie. Better pic posted.

–Tom

[Electronic M]

I got that same model (albeit more beat up and missing the pencilbox door) for 20$ at the ETF auction this year. The darn thing works with only tube replacements, and about 3 paper caps changed(all these repairs were done in the 70's)!

Tom C.

[AmigaMan]

So I found a couple new 6BQ6 horizontal output tubes, and a few 6BN6 tubes used as a sync clipper and audio detector (one of them read weak and the other read leaky, so I got replacements). Also replaced one electrolytic cap I missed in the IF amp stage (10µF @ 450V).

The swapping out the leaky 6BQ6 totally fixed the sweep issue. The other tubes and the capacitor didn't change much, but at least I cut down on future issues it could have.

I posted some "after" pics of the CRT displaying some clean test patterns, and I also shot some quick videos before:
http://www.youtube.com/watch?v=h12ETWSWnxY
and after:
http://www.youtube.com/watch?v=Jb5SELQNgHQ
for a better idea of what video is looking like on it. In the second video I switch between the fullscreen vertical stretch and standard aspect ratio modes, both of which look great after a quick amplitude and linearity adjustment.

Thanks for the input everyone, and I'll definitely be posting some pics of my progress with the cosmetic restoration very soon.

–Tom