I got the parts today. I replaced the burnt 1000 ohm resistor off pin 3 of the HV regulator, and the new MOV to replace the VDR that I was hoping was causing the excessive HV. I can't tell if it made any difference. The picture looks the same, the HV is 27-28kv with the bright/cont all the way up, and still buries the needle (32+ kv) on my probe with bright/cont all the way down.

I got the HV adjustment pot to turn, and it only varies by 1kv from one end to the other with bright/cont all the way up. No change when the probe is maxed out at 32kv.

When I first powered it up, as soon as the HV came up, there was a SNAP and a flash of light from behind, but I didn't see where it came from since I was looking at the screen. I shut it off, got the camera rolling and turned it back on but it didn't do it again. Maybe something was crabby in the HV cage, the cover is still off.

So I'm stuck again.

I'm gonna ask the dumb question:

Have you tried replacing the pulse regulator?

Got back at it today and I think the HV control is bad. I studied the schematic for a while and decided to check all the resistors in the regulator circuit, then check voltages. All the resistances came out fine except the HV adjust pot.

Schematic shows that it's a 1 meg. It measures 1.1 meg end to end, so I think that if I center the control, it should be half that from the center to either end, but it measures 1.1 meg from center to either end. If I vary the control, resistance starts rising to 1.3 meg, then drops to about 130 ohms from center to either end.

I have a good feeling replacing this will finally take care of the excessive HV. Now to try and find a replacement.

Is that the 6HS5 tube listed as the shunt regulator? I haven't replaced it, but I will if need be.

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.

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.

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.

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

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:smoke:
LFOD !

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.

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.

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!

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.

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

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?

you could try some diff IF tubes.

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.

:thmbsp: worth a try

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

Will do. I swapped the 1st & 2nd back & forth (both the originals) and didn't notice any change. The 3rd IF 6EJ7 was one of the bad ones that I replaced. I just now swapped it with the old one with the set still connected to the IF of the modulator and there are now two ghost images on the test pattern. That's a good clue!

It varies strongly with fine tuning. With the tuner connected, the "ghosting" goes away completely when I fine tune it into B&W, and I can exaggerate it by fine tuning it clockwise out of tune. That's what made me decide to try hooking the modulator directly to the IF cable and try to rule out the tuner. Maybe I'm using the wrong term when I say "ghosting" - if it's called overshoot, I want to make sure I get it right.

I don't have a way to inject baseband video.

I'm using a DTV converter box for the OTA content, and it has USB for the jpg images of the test patterns.

Thanks for the replies guys, more food for thought.

Any image that follows the main image on the right could loosely be called a ghost.

An overshoot is a following transient in the signal the opposite polarity of the main signal. It is possible for some circuits to produce a transient before the main signal, which is called a preshoot. Generally, if the transient is the same polarity as the main signal, it is called a smear.

TV sets generally use narrow overshoots and preshoots to "sharpen" or "peak" the picture. The one you are seeing is way too wide and too big to be pleasant.
Deliberate overshoots are relatively easy to generate in the video circuits. Peaking coils (in various places) and RC circuits involving small capacitors in the video amplifier cathode circuit may be used for this. A simple sharpness control may use a small capacitor and variable resistor in the video output cathode circuit.

Preshoots are harder to generate, because you can't generate negative time delay. You need a "non-minimum phase" circuit that delays the main signal a little bit. The IF alignment can do this, but it will not be adjustable by the user. TVs for many years used a fixed pre-shoot produced by the IF with a (possibly adjustable) overshoot generated in the video circuits.

Towards the end of analog design, video ICs became complex enough to generate symmetrical preshoots and overshoots in the best sets. Before that, only studio monitors had this capability. This was called "aperture correction" if it was in a studio camera, because it compensated for the shape of the scanning spot. In this case, the signals were produced by a tapped video delay line rather than an IF phase response. The aperture correction name stuck when it was incorporated in video monitors.

Here's Zenith's 1981 patent for a symmetrical peaking circuit (analog bipolar IC) that also included a noise-coring capability. Transient amplitude was detected from the first derivative signal across the resistor, and low level random transients would not be amplified.

https://pdfpiw.uspto.gov/.piw?PageNu...D4%2C296%2C435

That's over my head, yet fascinating. Thanks for the explanation, I hope it all makes sense at some point. At least now I understand what the overshoot looks like, and why I shouldn't use a crosshatch. I can see the overshoot now, even on a program fine tuned into B&W where it looked fine with the crosshatch. I should have some NOS 1st & 2nd IF 6EH7 tubes tomorrow and I'll see if they do the trick.

This is a good place for an analyst like a Sencore VA62 etc.

You can try lifting one end of the video detector diode & driving it with
your video source (the RCA plug). Remove the 3rd IF also ( 6EJ7 IIRC ). One volt P-P is pretty much standard. This will divide the chain in half & tell you
whether to look to the left or right of the diode. Divide & conquer :thmbsp:

BTW analysts are almost dirt cheap now. The sencore will do a LOT
more than most. If you get one be SURE it works, has all the cables,
& both the quick set up guide and full manual.

73 Zeno:smoke:
LFOD !

Thanks Zeno. I got the NOS 6EF7's and they didn't make any difference.

So do I pull the 6EJ7, and lift the anode side of the diode and drive the diode with my source? Or am I just lifting the diode and connecting my source to the IF cable of the set? Or is your suggestion in the context of me having an analyst? I'll search for one and get it if that's what I need to do. Sorry for not understanding 100% but I don't want to make a big mistake.

I'm pretty sure what he means is disconnect the detector diode on whichever side is easiest, hook composite video to the grid of the 1st video amp and see what happens...it's a good signal injection troubleshooting approach. If it gives you video then everything between the ant terminals and detector diode is suspect, but if it doesn't give you video then the video amp stages are suspect.
BTW good idea to not crank brightness too high when looking for video if video is weak you'll see it best with the raster slightly brighter than the minimum brightness needed to perceive that the screen is lit...many properly working sets with strong CRTs will totally wash out the video if brightness is set to max.
If you have video, and good IF voltages but no reception AGC might be improperly adjusted.

Correct on both counts. Keep in mind that it was common with cross hatch
& dot patterns to get a ghost to the right. Turning down the brite
eliminates most of & & FT can also. Also if the set has AFT or any other
auto color buttons turn them OFF til the set is repaired.

To inject to the 1st IF input you can make a tuner subber.
Take almost any solid state click type VHF tuner. Get Sams for the
set to get the B+ & AGC voltages. Make a supply for B+ & an
adjustable divider for AGC. Ground the AFT input.

73 Zeno:smoke:
LFOD !

Another thing to watch out for on late 60s Zeniths is an unsoldered tuner lead terminal...I've seen that factory miss on 2 Zeniths so far...

This has to be a safety feature. It prevents the HV from coming up if the set is turned on with the regulator tube removed.

I put this set away for a while and finally got back to it. After letting it run for hours and making some adjustments, it looks pretty good, so I think I might have been too picky about it. The convergence needs some work, but other than that, I think it's pretty good.