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!
