HV coupling and filter capacitors in VT-71 or other 7" sets

[ChrisW6ATV]

Hi-

Besides working on a Motorola VT-71 that I recently did restoration work on, I was making a list of high-voltage capacitors I will need for several electrostatic TV sets that I have. One thing I found out is that some of them (Hallicrafters and National, I think) appear to use ceramic capacitors for the horizontal sweep coupling, and maybe for high-voltage filtering in some cases as well. These are 500pF or .001uF capacitors, 6000 volt rated. Tom Albrecht did some excellent tests of several capacitor types in this discussion, starting in Post #19:

http://www.videokarma.org/showthread.php?t=260749

While Tom's tests were in the vertical deflection circuit, I may have already read that ceramic capacitors are OK in these sets in the horizontal circuits, but I want to know from any of you who has had experience with them. The main reason is, the ASC .001/6kV capacitors are now around $5 each, but there are Vishay ceramics that are available for maybe $1.00 or even less.

Any comments or suggestions? Thanks.

[Kamakiri]

It's been my experience that disc caps work fine in the horizontal. I feel your pain on those ASC units, I bought like 8 of them a couple months back

[ChuckA]

Originally Posted by ChrisW6ATV

Hi-

The main reason is, the ASC .001/6kV capacitors are now around $5 each, but there are Vishay ceramics that are available for maybe $1.00 or even less.

Any comments or suggestions? Thanks.

Try finding .01 to .03 @ 6KV or 8KV caps from ASC or Illinois for prewar 7KV supplies, they are around $20 each now. And I complained about paying $6 a few years ago.



Chuck

[Steve McVoy]

I also have found that ceramics work fine in the horizontal and HV.

Here is a $3 cap that will probably work in the vertical:

http://www.alliedelec.com/search/pro...x?SKU=70189472

[Tom Albrecht]

I haven't tried ceramics recently for the horizontal, although I think I have in the past and not found a problem. Even for the vertical, my previous work showed that a ceramic can be used; you simply have to use a much larger capacitance than would be needed with tubular. It would be worthwhile to try .05 uF 6 kV ceramic caps if they can be had for less.

I'm not sure the 3000 V capacitor recommended above is a great idea, since the set will continuously apply 5+ kV DC across the caps.

[ChrisW6ATV]

I just had a chance to get back on the forum now. Thank you all for your comments and suggestions. I will try some ceramics when I order capacitors soon. Seeing them as original parts in some of the electrostatic sets was a surprise to me, actually.

Years ago, I did some repair work (not a full restoration) on my Pilot TV-37, and I may have used ceramic capacitors in its sweep circuits. When I get to work on that set again, I will find out what is in there.

[bandersen]

Originally Posted by Steve McVoy

I also have found that ceramics work fine in the horizontal and HV.

Here is a $3 cap that will probably work in the vertical:

http://www.alliedelec.com/search/pro...x?SKU=70189472

That's only rated for 3,000 volts and 10x the capacitance needed.
Instead grab these while you can! 0.0047 rated for 10,000 volts and on sale for only $2 !
VT71s originally used 0.005 in the vertical and these are plenty close enough.

http://www.alliedelec.com/search/pro...x?SKU=70006900

While you're at it, they also have Vishay 0.001 ceramic @ 6 kV for $0.77 http://www.alliedelec.com/search/pro...x?SKU=70079541

[Steve McVoy]

You guys are right. I must have been thinking of the capacitors in prewar 5BP4 sets, which use 2500v on the anode.

[dtvmcdonald]

Allied (probably) has a few ASC 0.0047 uF 10,000 volt caps
for a very good price. For 0.01 just parallel a couple. You can
see a picture of this in my CT-100 thread in the repair forum.