Shrapnel: Prevent a Brother's Hara-kiri

genXslacker

genXslacker

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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:

VREF (added in edit to post)
at positive leg of C101: 4.28V

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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Update:

  • Thanks to all for the help thus far.
  • I verified that the tantalum caps are in the right way around.
  • I put the IC in a known-good circuit, and it functioned perfectly... better than the JRC4558D that it replaced, actually. If you happen to have a Greer Black Mountain Crunch pedal, I recommend trying an OPA2134 in it.
  • After some tooth-gnashing, hair-pulling, and eventual loin-girding, I sucked it up and took the gain pot off the board. It tested nearly dead-on at 1.01M.
 
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falco_femoralis said:
It seems that pin 2 of the IC, or something connected to it, is contacting ground. This would make it so one LED is constant on while the other is off. I would check underneath your pots and trim the leads that have been left long. Check out your IC socket with a meter for continuity and see if you can find a short to ground between pins 2 and 4
Click to expand...
Update:

No continuity to ground from: IC1 pin 2, D2, D3, C7, R5, or Gain pot

No inter-pin shorts in DIP8 socket.

On to check all traces, I guess.
 
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Your issue is definitely at pin 2, most likely a mechanical short if the effect works out of the box and not mounted. Given the readings of pin 2, it is likely a very weak short, since the readings are indicative of some of the current and voltage still wanting to flow through your meter, giving you a value > 0. This could be a fragment of wire, a solder drip or the back of a pot.
 
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Verify whether you are getting something closer to VCC or VREF at pin 1 of the IC

Reading thru your volt findings, if you compare pins 1-3 of the IC to pins 5-7, you can see there's a voltage diff between pins 3 and 2 and this is why the LED is constant on. Check to see if VREF is present at R11, and go back and check your IC socket. Try pressing on it to see if it changes the continuity/resistance thru various pins
 
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andrewsrea said:
Your issue is definitely at pin 2, most likely a mechanical short if the effect works out of the box and not mounted. Given the readings of pin 2, it is likely a very weak short, since the readings are indicative of some of the current and voltage still wanting to flow through your meter, giving you a value > 0. This could be a fragment of wire, a solder drip or the back of a pot.
Click to expand...
Thanks, I'll focus my visual inspection in related areas. It's all solid-core wire, though, and the pots are now bent away from the board. I'll check again for out-of-place solder.
 
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falco_femoralis said:
Verify whether you are getting something closer to VCC or VREF at pin 1 of the IC

Reading thru your volt findings, if you compare pins 1-3 of the IC to pins 5-7, you can see there's a voltage diff between pins 3 and 2 and this is why the LED is constant on. Check to see if VREF is present at R11, and go back and check your IC socket. Try pressing on it to see if it changes the continuity/resistance thru various pins
Click to expand...

Thanks for responding. I'm getting a VREF of 4.28V measured at the C101 positive leg. I'm getting 4.28V at R11 too.

I've pushed around on the IC looking for changes in the brightness of the LED and saw none, but I'll get the DMM on it and see what some pressure does to it.
 
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comradehoser said:
I concur with @falco_femoralis. I had actually typed "diode situation suggests a ground fault."

What about C3, though? Isn't that the cap you kludged up? Could be an incomplete solder/solder crack or second component leg going awry. Kludgery usually involves a greater danger of enshittification.

Technical terms.
Click to expand...

Kludgery can definitely enshittify. The scratches were just enough to get through the factory solder mask, though. I was pretty thorough in checking the traces. It probably would have been fine without the added UV-cure mask, but I had some and the process is entertaining. You're correct that the goop is opposite of C3.
 
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andrewsrea said:
Your issue is definitely at pin 2, most likely a mechanical short if the effect works out of the box and not mounted. Given the readings of pin 2, it is likely a very weak short, since the readings are indicative of some of the current and voltage still wanting to flow through your meter, giving you a value > 0. This could be a fragment of wire, a solder drip or the back of a pot.
Click to expand...

Okay, I feel like we might be getting somewhere. There's only 20 Ohms' resistance between pin 2 and pin 4 of the empty IC socket. I'm no Nikola Tesla, but to my pitiful brain it seems like it could be exactly what you're describing.

I still can't see anything on the board that would cause that. All I can think to do now is try to flush out from underneath the IC socket with contact cleaner, flux cleaner, or both. And maybe throw it in the ultrasonic with pure isopropanol if that doesn't clear out the partial short.
 
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