Bad Micas?

[M3-SRT8]

In the Philco 48-1001 I'm working on, I just replaced all the "Domino" micas with Epoxy-dipped 500v micas. They all tested fine at rest, but, to quote Remo Gagji in "Casino:"

"...Why take a chance? Anyways...that's how I see it..."

LJB

PS: These "Domino" Micas are 60 years old. Plus, they control some pretty picture quality-related circuits. Time for Retirement...

[Eric H]

I could get a few discs and see what happens, they're well under a buck each locally.

The set won't sync at all now, it's pretty much stuck with the bar down the center, that might make it a little easier to fix!
It was acting up only now and then which is a major PITA to find.

I'd like to get this thing off the bench (i.e. kitchen table) so I can start on the monster Philco 49-1280

[Eric H]

I spent a couple hours on this last night, up till 3:00am actually and went to bed frustrated.

The set worked good after the initial recap but would tear and pull sideways occasionally, the last couple days it crapped out completely and the picture was a jittery mess.

I found a 100pf cap on the sync sep tube that responded to heat, I replaced it with a .001 mylar, it worked and the picture was rock solid but now it was split down the middle with two pictures side by side.

I fiddled for two hours replacing resistors and subbing caps to no avail.

This morning I turned it on again and it works perfectly, go figure.

I still think it has some bad Micas or whatever these square flat caps are, they are Micamolds so who knows but I'm going to replace them (one at a time of course so I don't get myself into more trouble)

[jeyurkon]

Pretty nice picture when it's working! There's a lot of detail in that tree.

John

[bgadow]

Word on the street is to never trust a Micamold, it's just a paper cap in disguise.

[Eric H]

I let it play for two hours this afternoon, not a hiccup, shut it off and back on, still perfect, on again now several hours later and still fine!

I'm still going to replace the Mica(molds) in the Horiz section though.

[kx250rider]

My fingers are crossed for you! I know that feeling of knowing there COULD be an issue, but it isn't showing up. When the picture is split and solid, it's usually a Horizontal Phase problem. If that set uses a 6AL5 as the horizontal phase tube, try changing it. Those are ultra-sensitive, and can do that type of thing even if the tube has good emission.

Charles

[Phil Nelson]

In my limited experience, it's the rectangular Micamold caps that are paper and should be shotgunned without thinking. Square flat ones are usually real micas, regardless of brand name. But as mentioned, they can go bad, although much less frequently than papers.

I love it when problems cure themselves, though!

Phil Nelson

[jeyurkon]

Originally Posted by Eric H

I let it play for two hours this afternoon, not a hiccup, shut it off and back on, still perfect, on again now several hours later and still fine!

I'm still going to replace the Mica(molds) in the Horiz section though.

Eric, at some point could you post a photo of the color code side of the Micamold capacitors in your Horiz section? Either before or after you remove them.

John

[Eric H]

Sure, here's a pic of the 100Pf that was causing problems with the Sync Sep section.

Here's a Site that explains how to read them: http://www.tpub.com/neets/book2/3g.htm

The first (yellow) dot indicates the type the first two dots are brown and black so that equals 10, the multiplier on the second row right is brown so 10X10 =100Pf. It doesn't say how you determine voltage but I would guess it's the last dot on the lower left, black which should be 100 volts?
The Schematic lists it as a 500 volt cap.

[Eric H]

Here's a couple more pics of that cap.
I did an Autopsy on it

Pic 1 shows the construction, the mica and foil are squeezed between the end connectors, a bad connection here could cause problems.

Pic 2 shows the Mica, notice the black mark on the edge of the Mica sheets, I think this was either Silver migration or some arcing that was happening, either way it doesn't look good and seems to confirm that this one was bad.

[marty59]

Wow, thanks for sharing the dissection of that cap..interesting observation!!

[jeyurkon]

Originally Posted by Eric H

Here's a couple more pics of that cap.
I did an Autopsy on it

Pic 1 shows the construction, the mica and foil are squeezed between the end connectors, a bad connection here could cause problems.

Pic 2 shows the Mica, notice the black mark on the edge of the Mica sheets, I think this was either Silver migration or some arcing that was happening, either way it doesn't look good and seems to confirm that this one was bad.

Thanks, those are interesting photos! O.K., Now I know I really do need to avoid Micamold even if they are real mica. Those mica sheets don't look very healthy either. I wonder if they were like that before splitting it open.

I would have guessed that they were paper due to the white dot in the upper left corner. But, it was actually the first significant figure. I forget and mix up silver and white sometimes. It it had been silver, it would have been a paper cap and if it had been black, then definitely mica.

John

[John Folsom]

Dr Eric, thanks for those interesting post-mortem photos. From what I have seen, the Micamold caps in the flat square package which are .001uF and larger are generally just paper capacitors masquerading as silver micas. There may be exceptions, of course, and when in doubt use a silver mica. You can always replace paper with silver mica, but not always the other way around.

[gadget73]

The paper micamolds are a fair bit bigger. My Philco had a few of those inside. I replaced them, all the bumblebees, and finally most of the silver mica caps to fix it's problems. Of course most of the instability issues were the silver mica caps in the horizontal stage, and those were the last to get swapped. I have a couple of the big flat paper caps that had simply fallen apart, no autopsy needed to see they were paper.