Zenith HV regulation issue

[rca2000]

VERY good job doug !! I often wonderd myself if those old VDR"s were failing from age--even still sealed in the bag. A new, MOV_type is MUCH less likely to fail--and MUCH easier to get too.

We do have some SMART people here on this site, don't we? Some if us may "Macgyver" things out of necessity--but we GET THE JOB DONE--and SOLVE problems !!

[rca2000]

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.
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 VCD 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!

Ok Doug--how does the sub MOV act n these other 14" sets? does it attempt to regulate at all? High or low? If so, perhaps a different value one with a different clamp level, or perhaps a combination of more than one of the ones you found AND another in series--might work.

[Gunslinger]

A lot of work done. All you needed to do is use 63-9068

http://www.moyerelectronics.com/BVMo...3-ec580bb1f45f

[Gunslinger]

here is more. look at the middle of the article



http://www.boxcarcabin.com/rcacathode.htm

[rca2000]

Originally Posted by Gunslinger

A lot of work done. All you needed to do is use 63-9068

http://www.moyerelectronics.com/BVMo...3-ec580bb1f45f

I think the whole point of the experiment is that the 63-9068 and ALL of the other Zenith VDR's are NOT likely to be any good after all of this time. It seems that they all fail whether used or not, just sitting around. A brand new one in its package is evidently NOT in the same realm as a brand new 10BP4 in its package.

[drh4683]

Originally Posted by rca2000

I think the whole point of the experiment is that the 63-9068 and ALL of the other Zenith VDR's are NOT likely to be any good after all of this time. It seems that they all fail whether used or not, just sitting around. A brand new one in its package is evidently NOT in the same realm as a brand new 10BP4 in its package.

Yes, exactly. Well said.

Many of these original VDR's have failed with age. It was important to test a wide range of varistors so that a near exact substitute could be established. Three different KOA Speer NVD14UCD430 varistors were tested for purposes of analyzing manufacturing tolerances. I am happy to report that all three worked excellent with essentially identical operating characteristics.

The only reason I was not able to verify a modern substitution of a particular MOV in the 14" color sets was because the 63-7658 and 63-8688 as used in those sets do not cross reference to the 863-1060. The bulk of sets other than those noted can sub using the 863-1060 which is what I based all of my testing on.
The only reason the 14" sets would have had a different varistor was only because those sets work at a lower HV output of 21.5 kV, while larger screen sets were all typically 25 kV. With the lower HV on the 14" sets, I can only assume that one of the smaller value varistors that I previously tested would work in that application, but I didn't bother taking a known good working 14" set apart just to find out. In fact, of the few 14" sets I have, all of them have good working regulator circuits.
Same goes for the 1972 models which also used a specific varistor not crossing to the 863-1060. Those HV regulator circuits were slightly modified over previous generations in that those use the dual neon bulbs as part of the regulator. I've yet to run into regulation issues on those sets as well.
Again, most of the regulation issues can be found in any varistor that cross references to the 863-1060, and that would be '67-'71 models which basically used the same regulator circuit design.

[BigDavesTV]

Thanks very much for your Engineering work and your time and money spent on this Zenith HV Reg. Issue, Doug. I and many others do appreciate and will benefit from your research. I'm now anxious to dig out my Zenith that has that very same problem and make it one of my next projects! I've got to place a parts order soon!

[DavGoodlin]

Thank You Doug, my (later) childhood 1971 Zenith 23" color (B4223/12B14C52) console has a hope of working again.
BTW, I have a 16Z8C50 swivel base console and may be getting another today.
This helps eliminate one of the most confounding issues with "golden age" Zenith sets.

Once again, Doug has earned the most appropriate avatar.

[drh4683]

Thank you for the nice words gentleman. It is always a pleasure to share the information among fellow friends in the hobby!