Shrapnel: Prevent a Brother's Hara-kiri

genXslacker

genXslacker

New member
I've exhausted the limits of my knowledge and fortitude, and I'm begging for help now. I'll try to be brief.

I boxed and then attempted to rock. No joy. Bypass mode, however, worked perfectly.

I pulled the lid and noticed the D2 clipping LED was solid on... and bright at that. D3 was solid off. Brief inspection and DMMing. No problems obvious to me.

Unboxed and re-flowed all solder joints. Tried it again, before boxing this time, and everything appeared and sounded great. D2 and D3 were varying dimly on when passing signal.

Re-boxed and was right back to the condition where D2 is solid on, D3 is off, and it's only passing sound in bypass.

Unboxed again, but it didn't change anything.


An audio probe indicates signal going in on IC1 pin 3, but not coming out of pins 1 or 2.

Voltages:

D100
A) 9.22
K) 8.92

IC1
1) 3.624
2) 0.486 - 0.542
3) 4.168
4) 0.0
5) 3.741
6) 3.742
7) 3.745
8) 8.92

BS170
D) 7.23
G) 2.492
S) 1.62

2N3904
E) 2.933
B) 3.354
C) 8.92

D1
A) 0.0
K) 2.495

I've swapped out ICs and BS170s. I can't remember if I've swapped out the 2N3904, but it seems pretty far removed schematically from the IC and the clipping diodes in the feedback circuit, and anyway, things did work briefly after I re-flowed the joints. I did check the 2N3904 on a DCA75, and it indicated that it's functional.

It's a tight squeeze into a 1590B, and I'm thinking I might be knocking something loose when I box it up. But does anyone have a guess on what it might be based on the brain dump above? IC1 Pin 2 voltage seems suspect to me, but I know just enough about dual opamps to embarrass myself.

The next thing I can think of is to de-solder the gain pot to verify its resistance out of circuit, but I hate de-soldering PCB-mounted pots and I'm hoping some kind soul will spare me that agony by pointing out something that I've overlooked.
 
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You need pics, my dude.

Both sides, including the off-board wiring. Also in box.

If it's working out of box, then most likely it's shorting on the enclosure, or there is a stray unwanted contact.

You have pot dust covers?
 
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What kinda dc jack you rockin?

I notice you got a bare wire for the hot side and insulated on the ground side - nothing wrong with that, but it would be more conventional to do the opposite. Just want to make sure you’re not hooking up backwards.

Are those two 22n caps tantalum? Is the polarity right?
 
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genXslacker said:
Again, it's not working out of the box. And it's not currently in a box.

The pots all have dust covers.
Click to expand...
Ah, but it did work, you said. If it did, and it sounded good as you say, your circuit is okay, the fault is in a connection or a short.

For me, those generally happen at the 3pdt or at the jacks.

I do kind of look askance at your blobby soldering between the pots, there, but probably it's not the problem.

If you want to be sure and you can't find the issue just by poking things, run an audio probe from the jacks into the board along the schematic order. I bet you can find the dead point within 10 minutes.
 
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