Bad Micas?
 

I've been working on this Tele-Tone Porthole for the last couple months, finally got it recapped and replaced some out of spec resistors.

It plays pretty well but I'm having one last problem and I'm thinking it could be some bad Micas in the Horiz circuit.

It'll play fine for a while then it starts to tear at the left edge, sometimes it flops over completely or tears so bad it's unwatchable.

This is basically a 621/721 chassis, it has three trimmers on the back of the chassis for Horiz Drive, Frequency and Locking Range.

There are Micas connected to the trimmers so I'm suspecting they could be the problem?

Pic 1 is working normally, pic 2 shows the tearing.

I would certainly suspect the Micas. I'm doing a 1948 Philco 48-1001, and there are 7 Micas, thankfully, only seven, to worry about. I tested them all and they were fine, except one.

However, I haven't fired up the Chassis with CRT yet, as I'm waiting on a Focus Pot, and a few odds and ends. If I get the symptoms your reporting, I'll replace what Micas are suspect.

That's all there's left, as I have completely recapped the entire Set, except the Micas.

I'll know Tuesday. If I were You, I'd shotgun the Micas in the affected circuit.

LJB

Yes, some old micas are showing their age. I replaced a few in my RCA 630TS on the advice of another restorer.

Phil Nelson

A lot of those "micas" aren't really mica caps at all... Especially the reddish-brown ones in the RCA chasses. They're nothing but paper caps in a flat bakelite casing. I usually find the ones on the horizontal oscillator transformer way out of spec, along with the ones coupling the tuner output to the 1st IF, and the IF interstage coupling caps. Replace with true Mica caps, if you can find them.

Charles

Horizontal circuits work their caps a lot harder than RF and IF circuits do. So the horizontal micas (if in fact they are micas) could be bad.

I agree with the posts but for troubleshooting purposes, take a little freeze spray to the suspects one at a time and see what happens.

Those epoxy dipped micas are still available, 500 volt rated.

LJB:smoke:

Can these be replaced with Ceramic Discs? Those I can get locally otherwise I'll have to order them and wait.

At least a couple of them are rated at 1000v by the way.

I don't believe so, but, someone else who's in authority should chime in.

CE Distribution carries those nice Epoxy dipped Silver Micas, in values ranging from 10 to 1000 pF, rated @ 500 watts. Mucho better than the old dominoe micas, I'm sure. I'm using those.

website:

www.cedist.com

LJB:smoke:

I'm not the world's expert, but based on experience and on the knowledge of engineers with whom I've discussed this in the past, the answer is No. The ceramic discs are too temperature-unstable, and you'd be adjusting the horizontal hold every 5 seconds. I know that All Electronics, Inc has micas, but they're here in the LA area. However, for troubleshooting purposes, you could parallel two paper caps, and that would be much better than a disc, and certainly better than leaky/open/shorted original micamolds. (if it's a .05, use two paper .022 @1600v). That way, you could order the micas knowing whether it's going to solve the problem.

Charles

I have been told more than once that ceramic discs are not suitable for horizontal circuits. Something about not being good with high frequencies, although I'm sure that temperature instability wouldn't help, either.

Phil Nelson

I had to order a few replacements for mica caps, got them from Moyers . Silver mica caps i think they were @ 600 v . Worked just fine in the Philco i finished and they were in the horizontal deflection circuit .

There are various ceramics used. NPO, Z5U etc. They all have different characteristics. Z5U changes capacitance with voltage. It also changes with frequency.

I have no experience using them in horizontal circuits, but I agree with the others that they are likely to be problematic.

If you have film capacitors of the proper values they might work though. I don't think the frequency is high enough to be an issue with them.

John

I think I'll just make up a list tonight and order the Micas, there are only a half dozen or so.

They are all in the Pico range, 5-500 Pf I think.

BTW, this seems to be a Phasing problem rather than Frequency, the picture doesn't flop sideways so much but mostly just shifts sideways so the picture's split in the middle.

Now I'm waffling. In the range of 5-270pf you could get COG disc ceramics that are very stable and have low dissipation. It would be hard to find values above that though in COG. You're probably better off getting the micas, but now I'm curious.

John