1968 Zenith 20Y1C50 chassis - advice needed

[JohnCT]

Originally Posted by joe111671

Thanks John. It's 168 ohms.

If Chester's in Kenosha has one I'll grab it Monday, otherwise I found a NOS one online. Can that regulator tube make the HV excessive and unadjustable if it's bad? I might as well get one of those too if it's a possibility.

I think the control is OK. In a control that high in value, there's a bit of resistance from where the wiper finishes on the carbon track and the terminal crimp is.

To be sure, unsolder the other end and sweep the control through it's range to make sure there's not an open spot in the middle.

John

[zeno]

When you have a stage that quickly burned up parts its good policy
to just change the tube involved. Odds are its shorted or int shorts.
IIRC with a bad VDR you would get a bend in the pix at the bottom
& poor / no regulation.

Tube type Zeniths are known for bad spots on the vert. size, HV adj , &
G-2 ( screen) controls. The bad G-2's went through the CC 2 sets.
They were so common I still remember many of the part numbers.

73 Zeno
LFOD !

[joe111671]

There's a 1.5 meg resistor across the pot that's not in the schematic. So if the pot was 1 meg like it's supposed to be, then the resistance across it couldn't be higher than .6 megs but it measured 1.1. So I ordered the NOS one and a NOS 6HS5.

Originally Posted by JohnCT

To be sure, unsolder the other end and sweep the control through it's range to make sure there's not an open spot in the middle.

John

So I did that, and didn't see any open spots, but what I did see is that the pot is 3.5 megs when the schematic has it listed as a 1 meg. So with that 1.5 meg resistor across it, it would measure 1.05 megs, and it was close enough.

So my dilemma is: do I use the NOS 1 meg pot and leave the 1.5 meg resistor out, or hook the original back up? I'd prefer to use the new one since I'm going to have it anyway & they're known to go bad, but don't want to modify the HV circuit because I don't know why they did it like that. Thoughts on this are appreciated!

[joe111671]

The 6HS5 regulator tube finally came. I installed it, and the new HV adjust pot (minus that resistor that was across the old one). The HV is now adjustable and I was able to set it to 25KV so I'm happy with that. Only thing now is the 750v boost is still too high at 903 volts. Not sure why, but it still has those white elemenco caps on the 750v, so I'll change them since I have them and see what happens.

The tube that was in there was a 6JD5, didn't notice that at first and have no idea if that was contributing to the unadjustable HV, or if it was just bad.

So far this set needed a good cleaning, a cap in the contrast circuit, a new focus stick, new resistor off the HV reg tube that was burnt, and HV reg tube. The HV cage was dirty, and cap on the rectifier tube had green corrosion and was causing sizzling in there.

The HV adjust pot and VDR may or may not have been bad but they're new, so all good there.

The tint is worse than before. I have the control all the way to one end and the flesh tones are still too red. It still needs the degaussing thermistor and the convergence is pretty bad, but it's working!

[Jeffhs]

Originally Posted by joe111671

The 6HS5 regulator tube finally came. I installed it, and the new HV adjust pot (minus that resistor that was across the old one). The HV is now adjustable and I was able to set it to 25KV so I'm happy with that. Only thing now is the 750v boost is still too high at 903 volts. Not sure why, but it still has those white elemenco caps on the 750v, so I'll change them since I have them and see what happens.

The tube that was in there was a 6JD5, didn't notice that at first and have no idea if that was contributing to the unadjustable HV, or if it was just bad.

So far this set needed a good cleaning, a cap in the contrast circuit, a new focus stick, new resistor off the HV reg tube that was burnt, and HV reg tube. The HV cage was dirty, and cap on the rectifier tube had green corrosion and was causing sizzling in there.

The HV adjust pot and VDR may or may not have been bad but they're new, so all good there.

The tint is worse than before. I have the control all the way to one end and the flesh tones are still too red. It still needs the degaussing thermistor and the convergence is pretty bad, but it's working!

The convergence doesn't look too bad on the thumbnail image you posted. The tint control should be at mid-range to produce normal flesh tones, with said tones being red and green at either end of the control's rotation. The 3.58-MHz color burst oscillator tube may be weak; I would replace it before doing anything drastic. The chances are this will center the tint control, but if not, set the control at midrange, then tweak the color burst phase adjustment until the flesh tones are normal.

BTW, your set needed a good cleaning in the worst way, especially if you found green corrosion on the plate cap of the HV rectifier; that would explain the sizzling sound within the HV cage. It was a good thing you replaced the HV adjustment control, as it may have been dirty and/or had bad spots on the carbon track; if the latter, you could have had intermittent high voltage and/or moderate to severe arcing. Since color TVs using CRTs generated extremely high (by b&w TV standards) second-anode (ultor) voltages, it is entirely possible, even likely, your set's HV adjust control was arcing internally. If so, I'm surprised the HV fuse (or even the TV's own line fuse) didn't blow immediately, as soon as the arcing started.

As for the VDR, however, I'm not sure. Your Zenith TV was made in the late 1960s; it surprises me that Zenith was designing their color TVs with VDRs at that time. I would not have expected to find a VDR in a color TV from that particular time frame; in fact, I'm all but amazed any make of TV even had VDRs, in the HV plant or elsewhere in or on the chassis, at that time. Zenith must have been well ahead of its time when the company designed and built your set. I had relatives, now long since deceased, who owned a Zenith color set similar to yours; they liked it, and it worked very well, as long as they had it. The only major problem they ever had with that set was when the CRT screen cracked, diagonally, probably due to a design flaw (this occurred about six months or so after they had purchased the TV); once the tube was replaced, under warranty of course, the set worked very well for them for years, until it was eventually replaced by another Zenith 25" console.

[zeno]

Pix looks good. Purity is off so when you fix the DGS you may
need to do a CRT set-up before going further.
Boost is a reflection of HV so if you have good HV boost will follow.
The Sams could be wrong also, wouldnt be the first time. Maybe
someone can look in the Zenith manual for you.

Zeno

[joe111671]

I found the correct thermistor and installed it. The one I ordered from Mouser was so tiny, it probably would've vaporized in less than a second! I had a good laugh when I saw it, that made it worth buying.

The tint problem wasn't actually a problem after all, the purity was just way out of whack from the set being on its side, not having a degausser didn't help.

The picture came out pretty good, but something still isn't right. It's like a slight ghost image to the right, it's very noticeable with a crosshatch pattern applied. If I can get rid of that, I think it will be really close to how it's supposed to look.

I took all the tubes to Chester's to test them, minus the sweep ones that I can see are working. 5 of them were weak or had shorts, and they had all in stock except the 6KT8 Sound IF tube. Not sure if I even need to change that, because the sound is fine now. Before, I could turn the volume all the way up and it wasn't all that loud. The 6Z10 was so weak, it barely moved the needle on the tester.

Then there was the 6HA5 RF amp tube on the tuner that I did not replace. The socket on Chester's tester is worn out, so it took some patience to hold it just right to get a reading, and it was showing good emissions, but shorted. The needle still showed shorted when I pulled the tube, so I assume it's something in their tester.

But to be sure that tube wasn't causing that ghost image, I took a gamble and pulled the IF cable from the tuner and plugged it into my BT modulator's IF output. I wouldn't have tried it if the set had a hot chassis, but it doesn't, so I crossed my fingers and did it.

The picture quality improved somewhat, but that ghosting is still there so I think I'm safe to rule out the tuner. I'm stuck again, but hopefully almost there. What do you guys think is making that ghost image?

[DaveWM]

you could try some diff IF tubes.