I might be getting a 1968 Zenith color!

[TUD1]

The CRT in this thing is trash. It tested decent, but the picture quality is very poor. Hardly any contrast and very dim. I put my brightener on it, and my dad is watching it now. It has a little more brightness with the brightener on it. I think it's an RCA Colorama tube. I know it has a green halo, but so does my other Colorama.

[old_tv_nut]

If the CRT tested well but the picture is dim, I would hesitate putting a brightener on it until you do some further testing to see if there's something wrong with the chassis. No sense burning out a good CRT if the problem is really somewhere else.

Have you checked the DC voltages on the CRT? Do you have a scope and a schematic with waveforms to check the video output amplitude?

[DavGoodlin]

Also make sure your video output tube stage is not the issue. I forget if that was the 12GN7.

[TUD1]

Checked the 12HL7, it was fine. I'm going to try adjusting the AGC as soon as I find a screwdriver long enough. Ran a brightener on the CRT for about 30 minutes, and it's wide awake now. It's way brighter, but the picture is still very washed out and lacking contrast. Here is a picture of it next to my CTC-25, which has a perfect picture.

[Electronic M]

I'd turn the color off, and adjust fine tuning and AGC for best picture...If color don't work at that fine tuning setting then that is a separate issue that should be addressed. If you can't get good monochrome picture contrast I'd recommend checking DC voltages along the video chain. On Zenith sets you can often un-solder the video det diode or unplug the tuner and directly inject composite video at the detector output...If you see a noticable improvement in contrast with vid injection then your IF or tuner most likely have issues (check tubes, and change tubular film/paper caps first)...Though AGC could also affect RF/IF gain, and should be looked at.

Normal AGC control action should have a range of no-signal at bottom, low to high contrast, overload with sync loss and or negative picture. Adjust up till you hit overload then back down just a tiny bit below/after the point where sync returns.

[dieseljeep]

Originally Posted by TUD1

Checked the 12HL7, it was fine. I'm going to try adjusting the AGC as soon as I find a screwdriver long enough. Ran a brightener on the CRT for about 30 minutes, and it's wide awake now. It's way brighter, but the picture is still very washed out and lacking contrast. Here is a picture of it next to my CTC-25, which has a perfect picture.

Make one out of a wire coat hanger, like we used to do.
Cut the hanger apart, hammer the end flat and if no file or grinder is handy, form the end on the concrete steps or sidewalk. Bend a loop on the other end for a handle.

[TUD1]

I hadn't thought of that. Thanks! I've got wire coat hangers everywhere. I checked every single tube when I got it. It still needs a 6U10.

[BigDavesTV]

Looks quite nice, good job, U.D.1! CRT looks a little tired, but not too bad.

[DavGoodlin]

The Zenith looks a bit brighter but I agree your issue may be in the video output stage, which affects the contrast. If you start by replacing paper coupling caps and any electrolytics through the 1st and second video stages, it can only improve. Need a schematic?

Years ago I worked on enough Zenith consoles from this era to know they can have a moderate issue and still look close to normal. Ill check my notes in the "TAB" color TV service manual for Zenith.

After all, Zenith used to advertise extra components were used that other manufacturers (think GE here) did not include. The downside of all those parts is that when one would fail, it was not any thing resembling a common symptom. Open electrolytics could present in some very strange ways.

[TUD1]

Adjusted AGC using a homemade screwdriver that I made blacksmith style. Contrast is still pretty poor, and focus isn't so great either. Also looks kind of fuzzy and unclear next to the CTC-25.

[TUD1]

I think I found the source of all the fuzziness. A shorted 6GJ7 in the VHF tuner. And I'm sure the shorted 6U10 horizontal oscillator can't be helping much either. This thing was FULL of dead tubes!

Replaced 6GJ7 and 6U10. No improvement, but I did find the source of the fuzziness. This TV is incredibly picky with its input source. I have it on a splitter with the CTC-25, and it does not like it one bit. If I plug in the source directly into the VHF tuner, it's fine, but with the splitter, it has lots of squiggly interference. The RCA does not. Nuvistors anyone?

[TUD1]

The VHF tuner is really starting to act up. Here are some pictures of what it's doing now. The only way I can describe it would be like watching TV through a burlap sack.

[DavGoodlin]

You're getting real close here but a tube is not causing what we see. That tuner may need to have its channel 3 and 4 strips cleaned off with alcohol. Clean plugs on the IF cable from the tuner also.

Then flip your $ervice $aver $et on its side, try a new 4mf-450 volt cap from pin 8 of the 12HL7 to ground. Contrast and video sharpness in general can be affected by this screen bypass cap AND tack in another 50 mf cap from the center post of the CONTRAST control to chassis. This has to do something positive here. Next try the electrolytic capacitor around the 6BA11's AGC section (pins 2 thru 8).

[old_tv_nut]

Originally Posted by TUD1

I think I found the source of all the fuzziness. A shorted 6GJ7 in the VHF tuner. And I'm sure the shorted 6U10 horizontal oscillator can't be helping much either. This thing was FULL of dead tubes!

Replaced 6GJ7 and 6U10. No improvement, but I did find the source of the fuzziness. This TV is incredibly picky with its input source. I have it on a splitter with the CTC-25, and it does not like it one bit. If I plug in the source directly into the VHF tuner, it's fine, but with the splitter, it has lots of squiggly interference. The RCA does not. Nuvistors anyone?

If the interference occurs when you have both sets turned on, turn off the RCA to make sure it's not radiating back into the Zenith. Probably not, but doesn't hurt to check. Also check for broken RF cables.

[TUD1]

The Zenith has interference regardless of whether the RCA is on or not. When I went through the Zenith, I cleaned the tuner with tuner cleaner and a rag, cleaned the IF wire going to the chassis, and the tube sockets.