SOLVED Mahayana drive losing signal at op amp

[AceHyzer]

Hey all,

I have been working on a Mahayana Drive build. I’ve tested continuity everywhere it should be expected and as far as I understand the voltages are correct as well.

I’ve tried two different AD712s and an NE5532P and get the issue on all of them. Using an audio probe, I get signal up to pin 3 of the op amp and nothing beyond

Op amp voltages
Pins:
1,2,6,7, and 8 - 4.42V
3 - 4.25V
4 - 0V
5 - 8.86V

All legs of both mosfets are between 4.3 and 4.5V

I’ve checked and rechecked solder joints, continuity, measured pots, tested the Mosfets with my DCA75, I just can’t figure out why the signal isn’t making it past the opamp input.

Any ideas? Thanks!

 

[Nostradoomus]

did you sub BS170 for the 2n7000 by any chance? Also it’s probably lighting but that 470k resistor looks off.

Test continuity based on the schematic of the whole section going from pin 2 to pin 1 of the opamp, sounds like something is shorting out or off there

 

[AceHyzer]

did you sub BS170 for the 2n7000 by any chance? Also it’s probably lighting but that 470k resistor looks off.

Test continuity based on the schematic of the whole section going from pin 2 to pin 1 of the opamp, sounds like something is shorting out or off there

Thanks for the quick reply. I did not sub BS170s, I used 2 2n7000s. Also double-checked the 470k resistor and it was the lighting, it checks out. Measured it with the multimeter to be sure and it came out at 471k.

I've just checked continuity for every component between pin 2 and pin 1, everything is checking out as expected.

 

[AceHyzer]

Adding a photo with better lighting

 

[Nostradoomus]

Very odd. Maybe consult this thread and see if you have the same kinda issue https://forum.pedalpcb.com/threads/possible-mahayana-board-problem.19487/

 

[DailyDovetails]

The voltages seem odd. I would expect 8 to be 9V. Out of curiosity does it work if you pull the Mosfets? The resistors all seem right. The caps I can’t really check. Otherwise i would wonder if maybe there is a component leg or something running to positive somewhere it shouldn’t.

 

[Nostradoomus]

hold on, pin 8 should be supply voltage yeah, beat me to it @DailyDovetails

 

[Nostradoomus]

Check the mosfet voltages too

 

[AceHyzer]

Rookie mistake, I measured them correctly and wrote them down backwards on 5-8:
updated Op amp voltages
Pins:
1,2,5,6, and 7 - 4.42V
3 - 4.25V
4 - 0V
8 - 8.86V

Mosfet voltages:
Q1
D - 4.42V
G - 4.42V
S - 4.30V

Q2
D - 4.42V
G - 4.42V
S - 4.32V

Tried pulling the Mosfets, no signal past pin 3 still.

 

[Nostradoomus]

How are you testing it? Measure your pots and check continuity there too, this is getting weird haha

 

[AceHyzer]

Haha for real though. I'm pretty baffled by this one.
I have two different testing rigs, one from fuzzdog I believe it's called "the simple tester", and an auditorium from pedalpcb. I've tested several circuits on both, so I know it's not the testing rigs. I also hooked this board up to both to make sure something hadn't gone funky there. I'm stumped!
I measured all the pots, they're measuring correctly and I got continuity on them as well. I just checked to make sure the traces to the 2n7000s were as expected and everything looks right to me and I'm getting continuity as I would expect there as well.

 

[Nostradoomus]

Give your board a good scrub with iso and a toothbrush, I wanna see it cleaned up. You never know!

 

[Nostradoomus]

Can you confirm continuity between pins 1-2 of each mosfet? just for fun

 

[AceHyzer]

Yeah, when I read that other thread I checked continuity between pins and all checks out. I'll give it a good scrub tomorrow and give it another shot. Thank you so much for all your help!

 

[Nostradoomus]

All good, we'll figure it out!

 

[AceHyzer]

Update: I put a couple new 2n7000s in and it worked! The other two are still testing fine on my DCA75, so that's strange, but I'm gonna roll with it. Thanks again for all your help!

 

[Nostradoomus]

Weird! Glad it's going tho