The TV was a daily driver for about 6 months before it had a failure. I dozed off while it was on and woke to it having a dim and shrunken screen. No arcing or smoke. Flyback seems fine. A quick power off and on returned the brightness but I noticed as the brightness changed the pincushion on the lower right of the screen would bend and straighten.

I've replaced the trio of HOT/damper and the 6JH5 but it still seems to not regulate. HV is set to 25k but as I vary the brightness you can see the meter fluctuate and the screen size changes.

This pulse type regulation is not as simple as the shunt style and I dont know how it works. I'd like to understand why it's not regulating.
I'd like to check R177 off pin 3 of the 6jh5.
I'll probably shotgun replace C118.
Theres two diodes, x28 amd x26. connects to the cathode of the regulator. whats that for?

Anything else I should check or just replace?

Thanks,
Matt Davala

Classic symptom of a bad VDR in the HV reg. Search drh4683 here.
He did a study of it & found a modern sub for it. The original
part was color striped. They subbed to a new p## a very nice blue color
with a Zenith 63-#### on it.
Trouble is the new OEM parts are going bad on the shelf.

If this helps pls let doug know & thank him for his work.
BTW always be sure the damper & HV reg tubes are per the layout chart !
There are 2 pairs that work together but subbing one screws things up.
Usually there it a caution sticker on the HV cage about it.

73 Zeno:smoke:
LFOD !
73 Zeno

Thanks for your reply. I did make sure to use the correct combo of tubes and I did find Doug's post on the VDR replacement. Seems he couldnt get the HV down to 25. Mine will get to 25kv but lacks any kind of regulation with change in brightness.

I still have a question on how these sets regulate HV.

Thanks!
Matt Davala

Classic symptom of a bad VDR in the HV reg. Search drh4683 here.
He did a study of it & found a modern sub for it. The original
part was color striped. They subbed to a new p## a very nice blue color
with a Zenith 63-#### on it.
Trouble is the new OEM parts are going bad on the shelf.

If this helps pls let doug know & thank him for his work.
BTW always be sure the damper & HV reg tubes are per the layout chart !
There are 2 pairs that work together but subbing one screws things up.
Usually there it a caution sticker on the HV cage about it.

73 Zeno:smoke:
LFOD !
73 Zeno
The set in question seems to be one of the odd-ball models that Zenith built in the B&C model years, 1971,72.
I don't think that set has 18 tubes either.

The set in question seems to be one of the odd-ball models that Zenith built in the B&C model years, 1971,72.
I don't think that set has 18 tubes either.

The 18 does not refer to the number of tubes the set used; that number is, rather, the approximate size of the picture tube screen. Every Zenith television I have ever seen or owned has used this numbering system. If the set in question did have 18 tubes, however, it wouldn't be surprising to me as it probably did not use many (or any) multi-section receiving tubes, except in the RF tuner, which ordinarily used a 2-section tube as the converter/oscillator.

BTW: I had to leave my former lifelong home and my TV service area (the latter in the basement of the house) in 1999 to move into a very small apartment, my current residence, in a small town (long story and OT). I miss my shop and have very fond memories of it (probably not unlike VK member oldtvnut in Arizona), but I had no choice but to give it up due to circumstances (a death in my family) which forced me to leave my former lifelong home, forever and for good.

The 18 does not refer to the number of tubes the set used; that number is, rather, the approximate size of the picture tube screen. Every Zenith television I have ever seen or owned has used this numbering system. If the set in question did have 18 tubes, however, it wouldn't be surprising to me as it probably did not use many (or any) multi-section receiving tubes, except in the RF tuner, which ordinarily used a 2-section tube as the converter/oscillator.

BTW: I had to leave my former lifelong home and my TV service area (the latter in the basement of the house) in 1999 to move into a very small apartment, my current residence, in a small town (long story and OT). I miss my shop and have very fond memories of it (probably not unlike VK member oldtvnut in Arizona), but I had no choice but to give it up due to circumstances (a death in my family) which forced me to leave my former lifelong home, forever and for good.
Zenith changed their numbering system in about 1973, when they were building more solid state sets.
12A12C52, 12 tubes 14A9C52, 12 tubes 16Z8C52, 16 tubes. All I can think of now! There's many more. :scratch2:

The set in question seems to be one of the odd-ball models that Zenith built in the B&C model years, 1971,72.
I don't think that set has 18 tubes either.

SRY missed the conv. No power, phone, water, internet @ generator out 4 days.
Thank God for blankets & the Big Green Egg !

Any hows only other probs I had with the HS5 / HV5 where high ohms
resistors & the control. Never studied the theory behind it.

They were oddballs. At one point you could get a Zenith in 14" 16"
18" 19" 20 & iirc 21 0R 22. I will post a list just for laff sometime.

Well the power blinked a few times so hope to be lucky !

73 Zeno:smoke:

Maybe some of the resident gurus could post a brief synopsis on how the feedback/pulse regulator system works. Design aim musta been to eliminate the inefficiency of shunt-regulating the HV itself. Since Zenith wasn't our house brand, we never got much into the details and minutiae of their HV system.

Maybe some of the resident gurus could post a brief synopsis on how the feedback/pulse regulator system works. Design aim musta been to eliminate the inefficiency of shunt-regulating the HV itself. Since Zenith wasn't our house brand, we never got much into the details and minutiae of their HV system.

Love to see that. In the olden days I could just call Zenith or the local
Zenith service rep. May help on the problem too. Just never needed to learn it at the time. The regs were quite reliable & usually just the tube.


Zeno

The regs were quite reliable & usually just the tube.
Yep, always. Set would present with out-of-focus, shrunken raster, and it was always the reg tube (with matching damper when indicated).

As I understand it the HV reg monitors the B+ boost voltage (boost voltage is a good indication of HV level, loading and sweep width) and changes bias on the H output tube to increase conduction if boost is low and decrease conduction if boost is high. Basically if boost indicates more energy is needed it tells the H output to give more energy to the flyback, and vice versa.
It's gentler on flybacks because instead of giving them constant high energy then loading the output down as needed the fly can loaf along with less drive when it doesn't need max drive levels to work best.

Been a while since I last read up on operation of this and it's late so take that with a grain of salt.
Last time I had to troubleshoot one of these sets for an HV issue the yoke had corroded and it's phenolic was disintegrating and it took me a LONG time to trace the low HV problem to that especially since the set would be okay for 5-15 minutes before going dark.