View Full Version : My new Zenith 20YIC50

09-21-2017, 09:25 PM
Got this set this weekend along with the CTC-15, but just now got around to putting it back together. The picture tube is marginal, but it will make a decent picture. The high voltage however, is out of control. When I measured it, it was at 41 KV! I tried adjusting it, but the stupid control would not move. It's the kind that you have to stick a pencil eraser or something in to adjust. I don't like it a bit. I'm just going to keep the brightness cranked, so hopefully it will keep the HV down around 30 KV, and it won't destroy the tube. I think this set uses those darn varistors, so that's probably what's wrong with it.

09-21-2017, 10:40 PM
Decent looking set.

Jon A.
09-21-2017, 10:54 PM

09-21-2017, 11:14 PM
I was able to adjust the control and get the HV down to about 35 KV with a dark screen.

09-21-2017, 11:34 PM
Those color bars look great. Are you absolutely sure your high voltage probe is accurate? I know mine always reads low.

09-21-2017, 11:35 PM
Still very high. Could be the 6HV5.

09-21-2017, 11:37 PM
You sure that HV probe is calibrated right? 40KV is awful high for a tube type HV supply. No-load, unregulated shouldn't go much over 32-33 KV if that.

Jon A.
09-21-2017, 11:42 PM

09-22-2017, 01:11 AM
I toggle between saying "twenty-why-one-see-fifty" and "twenty-why-eye-see-fifty." Saying the latter is quick and easy. This is the 21st century after all. Quick and easy is king nowadays.

I'll get the schematic out later and see if we can find that stupid varistor.

Jon A.
09-22-2017, 01:29 AM

09-22-2017, 07:43 AM
Doug did testing & found a modern sub for the thermistor. It here
somewheres. Trouble has been the OEM's are going bad in the

Keep in mind always check the tube chart for the right damper / regulator
tubes & do not sub a wrong pair.

73 Zeno:smoke:

Electronic M
09-22-2017, 08:29 AM
Here is some good reading on trouble shooting the Zenith Pulse regulator circuit:

I believe this is doug's thread on VDR replacement.

09-22-2017, 11:20 AM
Doug showed me agood replacement for the varistor on the FB page.

09-22-2017, 11:57 AM
Here's the offending part on the schematic.

09-22-2017, 01:06 PM
Save your old Varistor if possible - I'd like to have it, to investigate the failure curve.

When Doug did his experiment(s), I took to my own Zenith Varistor stash to see if I had any that had failed. Of 9 unused/NOS, all 9 were good. Of 7 pulls from various chassis, and some identified only by comparing the body color markings to NOS ones, I was able to see only one failure - a beige/purple striped Varistor that was open - no response all the way to 1KV.

Silicon Carbide Varistors (what Zenith and other TVs used) have a different V/I curve than the semiconductor MOVs Doug used. The SiC varistors all have a linear V/I curve, and like MOVs, they are rated by "some voltage at some current" with the most prevalent current being 1mA. So if a SiC varistor is rated at 400V @1mA, applying 400V DC to the varistor in a closed circuit will yield 1mA of current flowing. Some Zenith varistors are rated to 950V, not a problem for me to measure with a Fluke 332D 0-1000VDC (okay, only 999.999V) voltage standard with a current compliance to 20mA, plenty of room for varistor testing.

About 4 years ago, I got a Zenith 800-600 Zenith Varistor/Thermistor kit with 90% of the varistors being new, unused. All had good V/I measurements. My workman stash got raided as well, with only one (used, solder on leads) varistor being w-a-a-y out, topping at 560V for a 410V varistor.

If you wanna try an original, lemme know what you have, and I'll send you a NOS one, free, if I have one - and verified good.

09-27-2017, 10:22 PM
I put the new component in the Zenith today, and I got perfect HV regulation right off the bat. Put in a new picture tube while I was at it, and the picture it has now is nothing short of amazing. Only thing that needs attention now is the horizontal centering.

09-28-2017, 12:43 PM
Found this while looking for something else,info on VDRs. Here is the link. hope this helps. Seems the web not being nice so go go to the ARH site then go to the PF-Porter page and download 1971-11, Electronic-Servicing. All the best, Tom. J

09-28-2017, 06:51 PM
Well i'm going to try that link for the vdr story again. Tom.J