Driver Testing error (RESOLVED)

[Urbansandz]

I made the mistake of using alligator clips testing the IA and OA on the A socket board. These two spots are small and close by and I think what happened is that I accidentally bridged the two resulting in some damage to the board components (see pic). The light is not good, but the R1, D1, and D2 were obliterated. The A side Q1, R2, Q2, and potentiometer appear to have some discoloration, but I'm not sure if usable. Any thoughts on where I should go from here? Get new board/parts? only replace the clearly damaged parts?

[Doc B.]

If you look at the schematic you can see that R1 D1 and D2 are in series with Q1, Q2. Sometimes with this kind of damage it will turn out that one or more elements of the zener diode string are blown too. Those should all be checked. Resistance across them should be near infinite in one direction and above 500 ohms or so in the other. If any zener or any pair of transistor legs reads low like 50 ohms or less it is toast.

[Urbansandz]

Thank you for the advice.

[Urbansandz]

An the A side I replaced the R1, Q1, Q2, D1, D2, and R2 (tested fine, but replaced anyway), . On testing I got 523VDC on IA, but wasn't getting anything on OA. After a short time ~30 seconds R1 burned out, resulting in fried D1 and D2. The burnout happened while I did not have my testing lead on the board. My guess right now is that my initial driver testing mishap burned the board resulting in current flowing from OA to IA, but I'm not sure that makes sense. Otherwise I can't make sense of why R1 would burn out when the IA is testing correctly. Anyway, my plan is to request a new board and new board parts and start over on this board.

[Paul Birkeland]

Did you replace the zener diodes that Doc B recommended? 

[Urbansandz]

Focusing on the board components I neglected to pay proper heed to DOC's advice re: the zeners, which I've checked and confirmed that they're all toast on the A side (0 reading going either direction), which would explain the issue with R1.  The D side zeners are all ~826 one way and infinite on the other. Thanks for your help. I completed the Moreplay and never had to troubleshoot, so this has been a good learning experience for a me.

[Paul Birkeland]

A question that will linger for a bit is how the zener diodes were destroyed.  Were they all properly oriented?  It's possible to mix up the wires going into IA and OA and put the whole power supply across the zener stack, which would blow them right up, but even that is a bit inconsistent with what's gone on here...

[Doc B.]

If I am understanding this correctly (50/50 chance) I think that is what the OP posted - that IA and OA were accidentally shorted. So the Zeners got zapped. When we were developing the Paramount circuit I recall doing that a few times myself. The LEDs blow in a most spectacular fashion.

[Urbansandz]

I rebuilt the board on the A side and it's working well (516 IA, 296 OA), but then I encountered the same blow-up on the D side, so I'm thinking it wasn't due to bridging OA and IA. Anyway, I rebuilt the D side board and now my IA is looking good at 515, but the OA on the D side board is measuring ~146VDC, so not quite half of what I should be getting. Any thoughts on what could cause such a low DC voltage on OA on the D board?

One more note. If I measure the D board OA right when I power up, the OA is higher, maybe 250VDC or even higher and it seems to slowly bleed down to 145VDC, where it remains stable.

[Paul Birkeland]

Check the orientation of the zener diodes in the string. 

[Urbansandz]

I attached a photo. The zenners appear to be aligned correctly and they're measuring fine.

[Paul Birkeland]

Can you post some photos of the top and bottom of the new board?

[Urbansandz]

I've attached photos of the bottom and top of the D side board. I kept the standoff attached because I neglected to provide sufficient working room under the capacitor, so just easier to keep it on.

A few notes regarding the D side board:

1. the LED's on the B side are not lit.
2. R2 on the B side is measuring 69VDC on one leg and 140VDC on the other. A board is ~145/293.
3. R4 on the B side is measuring 69/002 VDC, A board is 142/002
4. bB is 69, but on the A board it's 147.

[Paul Birkeland]

If you run the amp with no 5670 on that side, what is your OA voltage under those conditions? (you can just touch your meter probe to the metal tab on the MJE5731A to grab that voltage)

[Urbansandz]

When I remove the 5670 on the problem (D side) I get 349 VDC on the OA.