Capacitor Question for my Predicta

[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