Capacitor Question for my Predicta

[leadlike]

Thanks everyone for the tips. I'll leave it on for now. I did find some caps with temp coefficients at Mouser.com, but nothing close. I'm just doing a general shotgun replacement of parts right now. I kinda like the idea of a board with all new parts on it.

I've left the ladyfinger modules and couplates on for right now. Should I breadboard some replacements for these as well or leave them in situ?

On removing the crt shroud, I found that the picture tube is a low miles rebuild. That coupled with the new flyback should hopefully give this set a nice long life!

[Phil Nelson]

If you're up to making couplate replacements, and you want to use the set over the long term, why not do the work now? This article shows one of my caveman couplate replacements.

http://www.antiqueradio.org/philc12.htm

Regards,

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

[Dave S]

I totally redid one of my Predictas a while back. Everything was either in spec or it was replaced. Electronically it was like new. Everything except the couplates that is. Left those alone. Wouldn't you know it, after putting the set back together, it ran for about two minutes and then died. Guess what failed? Those assemblies seem to be problematic. Not sure why, since they don't contain the type of parts that tend to fail.

--Dave

[Phil Nelson]

Somebody told me that couplates may contain paper capacitors, just as failure-prone as other paper caps.

It's hard enough to keep a Predicta working under the best of circumstances. I thought I had done a thorough job on mine, and it worked beautifully, but eventually the horizontal went on the fritz -- Grrr!

Phil Nelson

[Ampico-kid]

Why should it be any easier to keep a Predicta up and running now than it was when they were new?

They apparently were quite problematic way back when, and they haven't gotten any better with age.

Their only saving grace is that incredible "Jetsons" looking 1950's Space Age design.

[Old1625]

Those blasted couplates can be a royal PITA. And they do fail. And finding out what's inside one of them, or finding a replacement, can be a challenge at times.

Perhaps unrelated to television, but I was working on a Thomas organ in a music store where I worked 25 years back. There was a series of frequency divider couplate networks for each of the 12 chromatic notes to derive lower octaves. One couplate was dead... and all attempts at finding a replacement lead to dead ends--strictly unobtainium. I had no service manual to the organ, meaning no info on the wiring and component values inside these packs.

Two "warts" at the top edge of the package indicated a couple of bipolar junction transistors that completed the Eccles-Jordan multivibrator circuit commonly used in divide-by-two stages in organs.

After taking some measurements and some educated guesses I felt certain that one of the pair of transistors was bad. Rather than try to reconstruct the circuit, I unsoldered the couplate from its PC card, marched over to the bench grinder, and ground both transistors down to lead level, as revealed by six oval metal spots in the ceramic coating. I surface soldered a matched set of transistors to the spots, epoxied everything in place, and reinstalled the couplate on the card.

Worked.

[Phil Nelson]

Originally Posted by Old1625

finding out what's inside one of them, or finding a replacement, can be a challenge at times.

On my schematic, the couplate components were shown as discrete items within a dotted-line box, although it was a little confusing since the components for a single couplate were shown in two separate boxes. I suppose some schematics may show the couplate as a "black box" with unknown components, since the assumption was that you would simply install a new couplate.

Phil

[Tubejunke]

Originally Posted by Phil Nelson

Somebody told me that couplates may contain paper capacitors, just as failure-prone as other paper caps.

Paper capacitors are the most failure-prone as far as I know...