1968 Zenith 20Y1C50 chassis - advice needed

[joe111671]

Originally Posted by JohnCT

If you don't want to unsolder the pot to check it, put a jumper across R163 and measure the center tap of the pot to ground. You should be able to read to 50 ohms or less when you rotate the control.

John

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.

[Jeffhs]

Originally Posted by joe111671

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 would replace the HV regulator tube on general principles, especially if the tube now in the TV is the original or hasn't been replaced in some time. It wouldn't surprise me if a new tube cured your problem. Your set dates back to the late 1960s-early '70s; if few or no tubes have been replaced in it since then the originals are all but certainly weak, so it would be a good idea to test all tubes in the set, replacing any showing low or no emission.

Being a Zenith, your TV is one of the best CRT sets made in the analog TV era, so when you get this one working at peak performance you won't be disappointed. I have owned several Zenith televisions and still have one, a 19-inch SMS1917SG table model, which still works as well as it did when it was new (it is now sitting, unused, in my bedroom since being replaced by a flat screen HDTV some years ago).

Good luck. As I said, your Zenith TV is one of the best CRT sets there is, IMHO; you won't be diappointed when you get it working at peak performance. This set should make a darned excellent picture with a cable box or satellite dish (or even with an antenna, if a suitable converter box is used between the set and the antenna). It was a darn shame Zenith went out of business 20+ years ago, as the TVs this company made were, again, some of the best analog CRT sets ever manufactured.

BTW, I like the styling of the cabinet on your Zenith TV. I have a coffee table and two end tables in my apartment with the same style of legs (I believe this style is known as cabriole) as your TV has, and I like them quite a bit.

I hope you will enjoy your Zenith TV for many years to come once you get it working. As I said, these were some of the best televisions ever made; it was a shame Zenith left Chicago and moved to Korea in the 1980s-'90s. We will, unfortunately, never again see this level of quality in TVs or in anything else.

[old_coot88]

Originally Posted by joe111671

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?

Here's a re-posting from post# 19:

[QUOTE]Even back in the day, that regulator tube frequently went bad, causing low HV and bad focus, even tho the tube tested 'Good' on a tester. The only fix was to replace it.

[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]

Thanks for the info everyone. I'll replace the HV regulator tube on principle. I'll also do the test on the HV control that John suggested and replace it if needed.

Earlier today I decided to check the voltages on everything in the HV regulator circuit against the schematic. Everything looked good, except the 750v boost voltage is 981 volts. That has to indicate something but I don't know what it is yet.

[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.

[old_tv_nut]

Don't try to diagnose IF/Video problems with the crosshatch image. It will be way too sensitive to fine tuning, and will show normally negligible stuff strongly.

Looking at the strong overshoots on the program image - do they vary strongly with fine tuning or is a lot of it independent of fine tuning? If it's mostly varying, suspect IF tubes / alignment. If more constant with fine tuning, suspect video circuits.

If you have a way to inject baseband video, see what that looks like, to separate video circuit issues from IF.

[old_tv_nut]

Originally Posted by DaveWM

you could try some diff IF tubes.

worth a try

[old_tv_nut]

By the way, what is the program source? Beware of VHS, which can have its own overshoots.