Working voltage for MOX HV resistors

[ohohyodafarted]

I am in the process of rebuilding the entire HV cage of a 15" Westinghouse early color set. It was worked on and botched up real bad by a "would be" restorer. In fact the entire chassis was a complete mess.

Rebuilding the HV cage on this Westy involves the replacement of many MOX (metal oxide) high voltage resistors. MOX resistors are difficult to obtain in the specific values and wattage's that you need, because distributors do not stock them, and they are expensive. I found a good source in Switzerland. This is a link to their web site, and they have an online store. http://www.high-voltage-resistors.co...RoCPzMQAvD_BwE

A MOX resistor is the type that is made from a ceramic body, and has a "candy cane" metal oxide resistance stripe spiral around the ceramic body. On modern MOX resistors the body is coated in either a ceramic fired glaze, or epoxy coated. The higher quality MOX resistors have the fired ceramic glaze coating.

I thought some of you might be interested in a formula that will tell you how to determine the maximum working voltage of a high voltage MOX style resister.

Where E is the maximum working voltage of the resistor; and P is the power rating in watts of the resistor, and R is the resistance in ohms of the resistor.

E is equal to the SQUARE ROOT of (P*R)

As an example, if you have a 2 watt 80 megohm MOX resistor E (maximum working voltage) would be the square root of (2 * 80,000,000) = 12,649 volts.

So in a HV circuit a 2 watt 80 megohm MOX resistor will withstand up to 12,649 volts across it before it would break down . If you are going to exceed that calculated voltage drop, you would need to use a resistor with a greater power rating.

Hope this is of some value to somebody.

[Electronic M]

That supplier will probably be helpful to me in the future (already know how to derive the math). When I did my 21CT55 I had to change HV resistors to stabilize focus and get the HV regulator in range and I had to go with pairs and a trio of series parts based on what mouser and digikey stocked.

[zeno]

Many ( mid 60s- mid 70's) sets used high ohm resistors for bleeders & focus dividers. Even some ceramic focus controls had a hi ohm built in resistor.
Often a bleeder is housed in plastic & inside was the resistor. they
were barber poled yellow & black, I think IRC made them. So thats another
source to consider.
Goood luck with the resto.

73 Zeno
LFOD !

[etype2]

I’ll take all the information I can get. Thank you. Will be watching the process.

[dtvmcdonald]

Mouser sells modern Ohmite high voltage resistors. These don't look like the old ones,
but I've used them and they seem to work fine. One caveat is that the flat ones are quite fragile: I broke an 8 dollar one dropping it 10 inches.

[trinescope]

I got resistors from Mouser as well for my Westinghouse, but they are rather expensive like $20 for one of them. I also had to go with an 85MEG part in place of the 80MEG. It worked out OK, however. The problem I was having was the front panel convergence control being at the very end of its travel and not quite getting the convergence right. Now I can go either side of correct with the control. I still need to replace the 8MEG part on the ground side of the focus control for similar reasons ( can't get focus voltage low enough for good focus).

[ohohyodafarted]

The high megohm high voltage resistors on either side of the convergence and focus pots on 15GP22 sets are of critical importance. These resistors are in a voltage divider arrangement and if the values of the resistors drifts too low, it will create excessive current draw through the resistance element of the focus or convergence pot. Excessive current draw through either one of these very high megohm pots will cause them to BURN UP, and these pots are pure unobtanium.

[dtvmcdonald]

Originally Posted by ohohyodafarted

The high megohm high voltage resistors on either side of the convergence and focus pots on 15GP22 sets are of critical importance. These resistors are in a voltage divider arrangement and if the values of the resistors drifts too low, it will create excessive current draw through the resistance element of the focus or convergence pot. Excessive current draw through either one of these very high megohm pots will cause them to BURN UP, and these pots are pure unobtanium.

My CT100 has those spiral IRC resistors. I measured them, as well as
the same kind of resistors in all my HV probes. ALL were still
within 1% of the rated value. I useed large numbers back in the 60's and 70s
for science stuff, and they never failed or drifted.

Obviously, too much voltage could overheat them, or
one could arc to the chassis ... and this could destroy them.

[ohohyodafarted]

Originally Posted by dtvmcdonald

My CT100 has those spiral IRC resistors. I measured them, as well as
the same kind of resistors in all my HV probes. ALL were still
within 1% of the rated value. I useed large numbers back in the 60's and 70s
for science stuff, and they never failed or drifted.

Obviously, too much voltage could overheat them, or
one could arc to the chassis ... and this could destroy them.

I agree, and in fact I found evidence of where there was indeed arc over to the chassis on the set I am working on.

Additionally one also needs to consider how old these resistors are, and what kind of environmental factors they may been subjected to over 60 plus years. Just because these types of HV resistors were reliable when new, does not necessarily hold after many years of potential adverse environmental factors.

There is also a fair amount of surface corrosion on the chassis of this set, indicating that it was in a humid or damp environment at some point.