SOLVED Weirdness with Mach 1 Build

[fable_instruments]

Hi all, I've been working on a pair of Mach 1's and have an issue with one of them that I'm having a hard time diagnosing. The pedal works normally throughout the full sweep of the volume and tone pots, but with the gain pot the sound starts to gate and fart out as soon as you go past noon-ish. I subbed the OPA2134 for a TL072 in these build and thought that might have been the issue, but the other pedal functions perfectly through the full sweep of all the pots. So I'm at a bit of a loss. What do you folks think it could be? Bad pot? Bad clipping diodes? Any help would be appreciated.

 

[Mcknib]

Sounds like something's mis biased but could be a few things

You're better posting some nice clear pics of your build, both sides of the board and offboard wiring so it can be traced

Post your IC voltages they may indicate something

 

[fable_instruments]

Here are the photos. The pedal passes sound in bypass and the LED works fine. Not sure how to measure the IC voltages though (total noob here)

 

[Mcknib]

Set your meter to 20 VDC

DC voltage symbol is the line and dashed line


Red probe in V for volts, black probe in COM (common or ground)

Put your black probe on a ground point like the gnd pad, IC pin 4, etc and red probe to each IC pin

You could for ease ground your black probe by simply putting it in one of the enclosure screw holes like this



Pins are numbered like this pin 1 to the left of the notch down to 4 and up in a U shape from pins 5 to 8



Then just post voltages by pin number
e.g

1. 4.89
2. 4.89
3. 4.89
4. 0
5. 4.89
6. 4.89
7. 4.89
8. 9.12

 

[fable_instruments]

Ok so the meter I got only gives VDC read outs in whole numbers but here they are (all pots on the pedal maxed):
1. 5
2. 4
3. 2
4. 0
5. 5
6. 5
7. 5
8. 8

While I was at it I took the voltages from the pedal that is working right for reference:
1. 4
2. 4
3. It was switching between 1 and 2 (so 1.5?)
4. 0
5. 4
6. 4
7. 4
8. 8

Hopefully these are still useful being whole numbers and all.

 

[Chas Grant]

Have you tried swapping the TL072? I've had a couple of them that did the same thing, threw a NJM4558 in there and the pedals worked perfectly.

 

[fable_instruments]

Yeah I’ve tried switching the TL072s out (don’t have any other dual opamps on hand unfortunately). I even tried the TL072 from the functioning pedal in the non-functioning one just to rule out a bad device. Could a melted gain pot cause this issue I’ve been having?

 

[Mcknib]

As far as I can work out your VREF should be 4.89v coming from your voltage divider R101 and 102

I wouldn't think 0.11v would mis bias it too much

I'd do what @Chas Grant says as a quick check swap your op amp from your working circuit to this one

All your components I can see look to be the correct value and orientation, can't see the drive pot value or volume pot and a few caps

I'd guess it's something in the feedback loop throwing it out with you saying the drive pot affects it

You could check your drive pot changes resistance as you turn it - all the way down between 1 and 3 should be in ohms and all the way up K ohms, you won't get totally accurate readings it being in circuit but you'll be able to check resistance varies as you turn it

It's A taper so in the mid position it's only around 40 to 60K a tenth of the resistance give or take it's tolerance + or - 20% so shouldn't really be full gain farting

I'm guessing here not having it in front of me, but this is what I'd check, it's quite often a process of elimination once you've done your visual checks

It may well be a faulty drive pot but I think you're better trying to find out before you start removing components especially board mounted stuff

Check your drive pot first then

I'd continuity check and or reflow components in and around the feedback loop C7, D2 to 6, R9, the drive pot, R11, 12, C5 and 6 all you need to do is hold your iron on the solder joints for around 3 or 4 seconds until it flows nicely, you need to be careful with the diodes they can be damaged by heat so 4 seconds on one side, let it cool then reflow the other side

 

[Chas Grant]

Looking at the circuit close up, it looks like you have a lot of cold solder joints. On the attached image, the green circles are good solder joints, the red ones are very suspect. On the green ones you can see where the solder flowed through the hole from the back side, this is what you want on all the joints. A cold solder joint is normally caused by one of a few things, solder iron not hot enough, iron not on joint long enough, or not enough solder applied can be considered cold also. Try reflowing all the joints on the board and see if that helps.

 

[fable_instruments]

I finally got it solved. I had strong suspicions about the gain pot so I elected to work on that first. Pulled it out, scrubbed the pads with IPA, popped a new 500K in, fired it up and it worked like a charm. Sustain for days and no fartiness or gating even with the drive pegged. Thanks @Mcknib and @Chas Grant for all the help