Zenith HV regulation issue

[drh4683]

I'm currently working on a '71 Zenith color set that uses the 14B9C50 chassis. It had a HV regulation issue where I couldn't get the HV to adjust below 29KV. It turns out that the VDR (voltage dependent resistor, aka "varistor") in the regulation circuit was defective. The schematic calls for p/n 63-8161 which I had on hand as NOS. I subbed it in and noticed that the regulation was not adjustable even in the slightest, it was actually more adjustable with the old VDR in place. I figured maybe I just had an oddball defective NOS part. It seems possible... So I went ahead and did a search and discovered that Talon electronics had three NOS 63-8161's in stock. Since I work on these sets frequently, I bought all three. They just arrived today and was ready to put one in. These varistors were made by different manufacturers for Zenith, typically slight physical differences, but electronically the same of course. That was very typical. When I clipped in the three different NOS varistors, I noticed that I had a wide variety of results when subbing each of them. With the first one, I could get the HV down to 25KV but that was at the very bottom end of the HV adjustment range. That's no good... Then I tried the other two which were physically the same construction as my defective NOS one that I had from before. Interestingly, both of those failed to work as well, not even responding to HV adjustment, almost as though they were wide open. So I had an NOS Workman brand substitute, an FS1220. With that clipped in, I got a perfect HV range adjustable between 20-30KV like it should be, so I know for sure the trouble is indeed with these VDR's and not some other problem in the regulator circuit. Apparently these blue colored Zenith VDR's become defective with age regardless if they were in a circuit or not.
Since I have results all over the board, I would like to install a modern MOV (metal oxide varistor) in place of the original. I know NTE makes a bunch of different types that I can pick up locally at Fry's. The problem is, I'm unsure which would be the correct substitution. The Workman one worked, but who knows how reliable it would be in the long run. The NTE cross reference comes up with nothing when I put in the either the Zenith or Workman part numbers.
According to my 1983 Zenith parts manual, the original 63-8161 was replaced by p/n 863-1060. It appears that numerous other varistors were replaced by the 863-1060 as well, but unfortunately I have no technical data on this in which to find a good substitute. Maybe someone has other cross reference materials and can help me out, or has even run into the varistor problem before and knows of the correct NTE part to use. It would be much appreciated!

[DaveWM]

I have run into it and did the same tried different ones, even went so far as to mod the circuit to a later style (20xc138 to a 20Yc136 or something like that) which worked MUCH better. I was reading the zenith tab book and there was a lot of changes made at least in the early chassis. I did not realize the modern MOV's could sub for a varisitor. Maybe you could try a test circuit, feeding in different voltages to the varistors and monitor the current (this is what I did at one point and it was all over the place) IIRC the books stated that it was not that precise but just that it did vari with voltage. Maybe some of the other parts are just not enough to make up for the variance.

[Gunslinger]

That VDR was changed to the purple one. Number is 63-9068. I probably still have a few. Also, there was a chassis that had a vdr in the horiz output grid ckt if memeory serves me. It is hell getting old.

[rrrhre2s]

My memory is in the dumpster was it Zenith or someone else that used 2 neon lamps in the High Voltage Regulator circuit that were marked with black dots for polarity (?) Never really got that a neon had a polarity though.

I liked Zenith until the 2nd generation System 3 with the 9-160 series of modules, Picture was still good but reliability went out the window.

By the 3rd generation Advanced System 3 could you even get a new set with a good CRT ?

I quit servicing Zenith sets around 1997 or 1998.

Had to start working at something you could make a living at.

rrrhre2s

[BigDavesTV]

Glad to see this post, I have a Zenith 25Y1C50 chassis with the same HVREG problem. I believe the VDR is similar, I know it's original, and it's a light blue color. I have put the set aside, figuring I'd address the issue at a later time! It has a Sylvania Color Screen 85 CRT, That checks good all 3 guns, was replaced in early 80s. The HV in my set was reading over 30kv, and wouldn't adjust much lower at all, tubes are good.

[BigDavesTV]

I will soon revisit this chassis, and see what Zenith p/n the VDR is supposed to be. Looking forward to a solution, eventually!

[DaveWM]

aside for the obvious (regulator tube)) I would start by checking the bias voltages at the tube and work from there.

[rca2000]

Originally Posted by rrrhre2s

My memory is in the dumpster was it Zenith or someone else that used 2 neon lamps in the High Voltage Regulator circuit that were marked with black dots for polarity (?) Never really got that a neon had a polarity though.

I liked Zenith until the 2nd generation System 3 with the 9-160 series of modules, Picture was still good but reliability went out the window.

By the 3rd generation Advanced System 3 could you even get a new set with a good CRT ?

I quit servicing Zenith sets around 1997 or 1998.

Had to start working at something you could make a living at.

rrrhre2s

If you are speaking of the period in which LG was making Zenith tubes (late 90-98), It was NOT too easy to get a good tube in a set. Maybe a 1 in 4 chance--it wold last maybe 2-3 years..before it would die and mess up the chassis. Sad...because i actually LIKED the chassis in those later Zenith sets--the split chassis type particularly--a LOT better than the 9-160 or certainly the 9-181/214 style...they were a cold set, not hard to fix and semi-reliable...UNTIL...the tube took a shit. NOTHING--I mean NOTHING Zenith EVER made..was better than the 1st gen flat chassis CC set, such as the 25DC56. !! Not even the ROUNDES--from a reliability standpoint in particular. They may have had a better pix--but otherwise--no contest...

[drh4683]

Originally Posted by rrrhre2s

was it Zenith or someone else that used 2 neon lamps in the High Voltage Regulator circuit that were marked with black dots for polarity (?) Never really got that a neon had a polarity though.

rrrhre2s

Zenith indeed used dual neon lamps in series as part of the HV regulator circuit. They were in series with the varistor off the HV adjustment pot. They did this only in 1972 on four different chassis', the 18CC29, 18CC30, 20CC50, and 25CC50.

BigDave,
The varistor in the regulator is almost without a doubt the cause of your unadjustable, excessive, HV.

Zenith started using varistors in the regulator circuit starting with the "X" series chassis models in 1967. These were the first to use the 6HS5 pulse regulator tube that replaced the old 6BK4 shunt regulator used on previous chassis'. Zenith used the same basic circuit in the '68 and '69 models using the "Y" and "Z" series chassis'. The regulator varistor used on all X, Y, and Z chassis was the same part number, the 63-6485, except for the 16Z8C50 which used the 63-8161.

When the "A" line chassis was introduced in 1970, depending on the specific chassis, Zenith used either the 63-8161 or a 63-8214 varistor, same with most of the "B" line chassis' in '71 as well.

"C" line chassis used the 63-9068

The small 14" portable color sets between '68 and '72 all used the 63-7658.

The 63-6485, 63-8161 and the 63-8214 were all replaced by a common substitute part, the 863-1060. However, as previously mentioned, I have discovered that most all of these Zenith varistors, NOS or original, "spoil" with age. Just yesterday, I received (5) NOS 863-1060's which were newer (made in the mid 80's) and found even these to be faulty with 3 of the 5 being defective. This becomes very misleading when troubleshooting as subbing in these unknowingly "defective" NOS varistors one after another would lead a tech to believe the problem is elsewhere in the circuit!

Fortunately, I have two good working original part number varistors and I have been conducting a series of tests on them (before they go bad!), such as voltage drop across the terminals and current draw throughout the adjustable range when in a HV regulator circuit. With this information, I can compare the operating characteristics to those of modern MOV varistors in which to find an acceptable substitute. I have ordered a few NTE MOV's which fall within the operating characteristics of the regulator circuit and hope to report back with a reliable sub that can once and for all cure late 60's and early 70's Zenith sets of the unadjustable, excessive high voltage symptom.

[DaveWM]

that would be great to know Doug. looking forward to how those MOV's test/work out.

[kx250rider]

That's good info! I've been stymied by those last all-tube Zeniths with excessive HV a few times, and never figured it out. Thanks for sharing, and I'm going to follow this thread.

Charles

[BigDavesTV]

Thanks, Doug for the information on the Zenith Varistors! It will be great to see what you discover someday, about a sufficient "modern day" replacement. I'll "stay tuned" in the meantime! :-)

[drh4683]

I have come up with a modern replacement MOV that works excellent in Zenith HV regulator circuits while operating within the original design characteristics of the original part.
After taking some voltage readings of known good varistors in the circuit, I had enough information to order numerous varistor types from Mouser. I have attached a list showing the results of my test. The chart is arranged from lowest max VDC ratings of the varistor, then by maximum clamping voltage. As can be seen from the chart, a rather directly proportional trend in HV output can be noted as the voltage rating of the varistor increases. So the lower the voltage rating of the varistor under test, the lower the HV range is. This is because the lower voltage varistors allow more voltage to pass through them, thus, biasing off the HV regulator tube which in turn limits HV output. Then as the voltage rating of the varistor increases, less current passes through, thus, less bias on the regulator tube which in turn lets the HV circuit run closer toward maximum potential. With the varistor completely disconnected from the circuit, HV runs up to a maximum of 32 KV.
With these tests, I was able to find a sweet spot where the current draw through the varistor was almost identical to a good working original while at the same time permitting me to have a near perfect adjustable HV range.

An original Zenith 863-1060 in good working order permits me to achieve and adjustable HV range between 21-30kv. A voltage drop across the varistor at 25kV was 399.7 volts and a current draw of 100 uA.

The best performing modern MOV varistor was made by KOA Speer, type NVD14UCD430. This varistor is rated at 350 VDC with a maximum clamping voltage of 710 volts. This yielded an adjustable HV output between 21-28 KV, a voltage drop of 433.8 volts with a current draw identical to that of the original, 100 uA @ 25 kV. The varistor operated without any rise in temperature and voltage output remained stable at exactly at 25kV from a cold start and after prolonged operation of over an hour. The HV adjustment pot was centered exactly within it's mechanical range at 25kV.

See the attached photos showing the results and also a chart I made showing the different varistors. Except for a few chassis, for which I wasn't able to test since they don't sub using the 863-1060, I can't claim that the aforementioned varistor will work (but I am sure one of these will probably work). However, the good news is, the bulk of chassis that are known to experience excessive, or out of range HV adjustment are the 1967-69 models using the X, Y, and Z line chassis' which used the 63-6485 varistor. The KOA Speer NVD14UCD430 is the new cure all to this age-old problem!

[DaveWM]

thank you for taking the time to post your results. this will be a huge help to the many of us that have had issues getting pulse regulation zeniths to work properly. I like you used NOS replacements with less that correct results. this lead to many false turns and head aches. Looking forward to getting some of these modern replacements for problematic zenths.

[rrrhre2s]

Originally Posted by drh4683

I have come up with a modern replacement MOV that works excellent in Zenith HV regulator circuits while operating within the original design characteristics of the original part. --Snip--

Just curious have you verified with more than (1) sample ?

Wondering about tolerances in manufacturing ?

Thanks for your time !

rrrhre2s