Q2 son of ben

[Jighead81]

Drain of Q2 is reading 9.16v, and then sometimes will read a lower voltage as it’s dropping lower and lower til it’s near zero. Q1 and Q3 biased up fine and are staying put, not dropping or reading 9.16v. I’m stumped, checked everything 10x, used working fc j201’s from a good plexi drive build. Sorry, this is my first post, but super perplexed here, lol. I can post pics if needed tomorrow. Any ideas?

trimmer and drain resistor measure good and aren’t shorted

 

[fig]

What kind/value of gain pot did you use?

 

[Jighead81]

What kind/value of gain pot did you use?

Alpha A1M. Think I should swap it out?

 

[benny_profane]

Can you provide the voltage readings on both sides of R9?

 

[Jighead81]

Can you provide the voltage readings on both sides of R9?

Both ends of R9 were the same as the drain on q2. Voltage dropping fast

 

[fig]

Alpha A1M. Think I should swap it out?

A1M is the correct pot.

Posting pictures might get you playing quicker.
Are you getting any sound, bypass or engaged?

 

[Jighead81]

Can you provide the voltage readings on both sides of R9?

A1M is the correct pot.

Posting pictures might get you playing quicker.
Are you getting any sound, bypass or engaged?

Getting sound in bypass, but silence when engaged. If I crank the amp up and turn the volume up on the pedal, I can faintly hear the guitar.

 

[fig]

It could be that trimpot I suppose. You might want to wait a bit though and see if someone else has another suggestion.

 

[benny_profane]

I would suspect the trimmer as well. What is the substance all over the board and the trimmers? If you cleaned the board with IPA, you have to be careful about contaminating pots, trimmers, and sockets.

 

[Jighead81]

I would suspect the trimmer as well. What is the substance all over the board and the trimmers? If you cleaned the board with IPA, you have to be careful about contaminating pots, trimmers, and sockets.

Yes, used some IPA this morning to clean it to see if that might help, thinking maybe some splash was arcing somewhere but no luck. This issue was happening before any cleaning though.
Going to swap out the q2 trimmer on my lunch break and report back. Luckily it’s in the middle so I won’t have to remove any pots.

 

[Jighead81]

Swapped in a fresh trimmer for q2 and still have voltage dropping, but no more random readings of 9.16v on the drain.
Weird thing about this also, was that it was working fine before and after I boxed it up but I hadn’t biased it correctly yet, so it was a little farty and didn’t sound good. I biased it per the instructions in here, closed it up, plugged into it and now nothing once engaged.
Not sure where to go from here. Just made an audio probe and plan to go through and see where I’m losing signal, but I can’t wrap my head around why the voltage is dropping. It would make more sense if q1 and q3 were doing the same thing

 

[Jighead81]

Swapped out the A1M drive pot, now q2 is holding steady at 4.02v!!! Gonna run home and try it out. Fingers crossed

 

[Jighead81]

Sounds great, thanks for the help guys! I guess the trimmer and the pot was bad.