Strange issue after speedball upgrade [resolved]

[Chappyaire]

I installed the speedball mod with no problems, resistance and voltage checks were fine. After about 5 minutes of listening there was a pop on the right channel and it stopped working.

The voltages on the speedball
Small board
0A 166.3V
1A 185V
B-A/B 0V
1B 185V
0B 85V

Large board
0A 166.3 V
0B 106 V
G 0V
B+ 185V

All other voltages on the amp test good except those connected to 0A
U7
U1
A6
B3
All test 166V


Now when I was testing the voltages when I touched A7 I did not touch another terminals but there was a small discharge and then the LEDs on the input tube dimmed and come back on after that I had normal voltages

Small board
0A 85V
Large board
0A 113V

I tested and everything worked again, I disconnected and looked for a possible short or anything touching in that area there was nothing, so I thought maybe something was loose and it was sorted. After listening another 5 min the same pop happened, I tested A7 and got the discharge and it worked again but after few more minutes in stopped.

So is my input tube just bad and was exposed by the slightly higher voltage of th speedball or perhaps something I did wrong during the install.   Or should I be searching for a different problem?

[Tom-s]

It might be a bad connection somewhere on the A board or around your led bias on that side. Do a chopstick test.

[Chappyaire]

I have a new 12au7 coming just in case, I was thinking maybe a bad connection or something wrong with socket and it's not grounded properly that's why I get the discharge when I touch with the tester.

[Doc B.]

Sounds like you might have an intermittent connection due to a wire that has broken under the insulation where you can't see it.

[Chappyaire]

I was considering replacing all the small wires I had to move to get the nylon fastener in place. Any good test for this other than just wiggling and or replacing.

[Doc B.]

Wiggling is good with the amp running. A continuity test while wiggling is good with the amp off.

[Chappyaire]

I have done lots of tapping wiggling and replaced the wires in that region, checked and ref lowed any joints on the pcb that may have been suspect and still the same issue that can be temp fixed by touching A7 with the tester lead. I did notice something while testing that may or may not be of significance, while listening I shutdown the amp and as the amp powered down the right channel worked for a short time until it goes silent.

[Chappyaire]

Ok, I'm not sure if this happened because of all the tapping or if the tapping exposed the real problem. On of the hlmp6000 basing the tube now does not light and I get 13v at A8 instead of 1.55 and my voltages at 0A are now 177v also one of the hlmp on the A side of the large board is about 10x brighter than the others. So I'm hoping replacing those 2 diodes may be the answer.

[Paul Birkeland]

Yes, the socket LED that's not lighting is now damaged. You can replace it or run a jumper from A8 to A3.

You do not need to replace the LED's on the PC board.

[Chappyaire]

I'm getting 2.2 v at the really bright led on the board. I was short one hlmp 6000 somehow in the kit or maybe I dropped it and I was sure I had some here but now I'm thinking it was not an hlmp 6000 I'm going to swap it with some new ones I ordered but would the higher voltage on the board be causing me any additional issues.

[Paul Birkeland]

Please replace the damaged LED on the 9 pin socket, or install the jumper I described, then re-evaluate your situation.

Seeing 2.2V across an HLMP-6000 and having extreme brightness would indicate that potentially two transistor legs on the bottom of the board are touching, a transistor is in backwards, or a transistor is shorted.

[Chappyaire]

Ok, took me a little while to get the hlmp' ordered and get around to this. I replaced the 2 diodes and it worked like a charm for about 5 minutes then a pop and the right channel went out as before. Chopstick test yielded nothing and only grounding a7 brings it back for a few minutes. I checked everything on the pcb and no legs are touching. I thought maybe the socket could be a problem because a8 is pretty loose even with the tube in. But a continuity test would suggest the socket is good. Anyone have more suggestions.

[Paul Birkeland]

Recheck your voltages.  I don't understand the bit about grounding A7, as there is a path to ground for A7 through the volume pot (it's grounded if the pot is turned all the way down).

-PB

[Chappyaire]

Ok so I have run over everything again
0A 187
1A 188
1B 188
0B 86


I took the small board out and spun it around and the problem follows the board so then I ref lowed everything on the A side. Still nothing, q2a the mje350 has no resistance from e to b. I switched the mje350 from side b into side a and it still did not work. and side B stopped working confirming the bad mje350

When the good mje350 was in the a side the voltage droppped to
0A 165

The pn2907 on the A side after a diode test also seems to be no good, there is 0V voltage drop on the collector.

[Paul Birkeland]

Now would be a good time to post photos.

For what it's worth, I've literally installed hundreds of C4S boards over the years, and I've never come across a defective transistor.

I have, however, performed defective soldering.

-PB