Paramount soft start problems (retrofit)

[vetmed]

Due to time and space constraints I am attempting to do one amp at a time, and not very successfully! Finished the installation of one board and all the resistance checks were ok, as were the voltage checks. Adjusted the plate voltage for the driver, shut down and installed the amp. On turn-on heard a loud crack, but the heaters were glowing, so the fuse is ok. No sound, double check that everything is connected ok. Still nothing so turned it off and visual inspection revealed a loose connection of the b+ supply at terminal 16. Reasoning that the loose connection probably caused the noise and that everything else measured ok I fired it up and was greeted by 4 or 5 bright white flashes inside the driver tube, and get a whiff of a burning smell. This time I see that one of the LEDs on the A side is actually melted and the other is reading low resistance in either direction. All the zener string terminals read 156K or lower instead of the very large values you would expect, terminal b6 reads 156K. The connection of the zener to terminal B6 was loose! So as you can see big problems and not sure what to do next. I have not done the other board yet, I could build the other one and install the other set of zeners but robbing Peter to pay Paul? Oh, terminal B8 reads less than 2 ohms.
   In my defense I have successfully built 5 or 6 Bottlehead kits with only minor problems, most have worked correctly the first time they were plugged in.

[Doc B.]

We hoped that the great big zeners would help preserve the circuit if things were miswired. In this case they were unable to do their job because of the intermittent connection at B6. You're gonna need another setup for that channel, or at least all new semiconductors - LEDs, transistors, TL431 and zener diodes.

[vetmed]

I was afraid you would say that. I've stuffed the other board and installed the extra set of Zeners and now listening to a mono recording of Strauss's Alpine Symphony! Hopefully I will soon be able to listen to stereo recordings,  thanks Doc.

[vetmed]

A couple of questions. Is the original zener string robust enough or am I better off waiting for the new ones? Any problem trying to salvage semiconductors from the old boards? Thanks for any and all opinions

[Paul Joppa]

It is in theory plenty robust, once the solder joints are repaired. However, I would not count on it; semiconductors are so much more fragile than tubes!

The problems in the past have had to do with cascading failures. It's hard to pin down the sequence, but we've seen and participated in too many cases where no matter what part is replaced, the board still fails and the failure takes out more parts. The only thing that works is to replace ALL the semiconductors; sometimes R2 must also be replaced. That much unsoldering and resoldering often damages the PC board, lifting or breaking traces. It may be time-consuming, but it hardly ever works.  :^)

So, while the zeners may be good, I would not trust them, or any other semiconductor on the board.

[vetmed]

Several years ago I attempted to re-use a pcb, you're right it was a royal pain in the nether region. Guess I will need a replacement board, parts to stuff it and a new set of zener diodes.