DOD FX92 Bass Grunge malfunctioning - Anyone able to help?

[zentropa]

I bought one of these a couple of years ago for a good deal that was kind of beat up. I had to resolder a few wires internally but it worked fine until a few months ago, it stopped outputting signal when bypassed when on my pedal board (it would output signal when on). When I isolated the pedal and had it sitting on a small bench, I found that it would output signal when bypassed if there was gravity pulling on the cable attached to the output jack. I replaced the jack and then it wouldn't output bypassed signal at all. I will save sharing all of the crap I have tried in the meantime, but it is back to refusing to output signal when bypassed except occasionally, if I jiggle some stuff around, I can get it to briefly output signal when bypassed but as soon as I set it down, it doesn't.

My pedal shop got a new repair person, so I dropped it off for repair, only to get it back unfixed and charged for the labor of the non-repair, citing they couldn't find a schematic. I emailed them the schematic asking for assistance, but it turns out this person is an amp tech that has no formal electrical engineering education and mostly fixes things by messing around with them.

After doing some digging online, I'm guessing I have a bad transistor somewhere that controls the switching of the signal routing when bypassed? Or possibly one of the input buffer JFETs failing?

I was wondering if anyone would be able to take a peak at the service notes/schematic and take a guess as to what I should try replacing.

As of now, the pedal seems to function normally when activated, but does not output signal when the pedal is bypassed. (If it isn't working 100% when on, I haven't owned one of these before so I don't have any point of reference). Mine is revision A1

http://bee.mif.pg.gda.pl/ciasteczkowypo ... Manual.pdf

Thanks.

[laurie]

The symptoms you describe are a mechanical fault " if I jiggle some stuff around, I can get it to briefly output signal when bypassed but as soon as I set it down, it doesn't".

Likely a dry solder joint, or a hairline crack in a trace.

It will be in the signal path: R9 - Q8 - C9 - Q6 (or the surrounding components).

Do you have, or can you make a signal probe?
https://diy-fever.com/misc/audio-probe/

[zentropa]

Thank you. I'm picking the pedal back up from the shop today and will take a look at it. I don't currently have the materials to make a probe, but the shop I'm stopping by might have those. I'm currently a bit limited in my transportation situation, so it would probably take me a few weeks to get to a shop that has the materials to make one unless Amazon or something has an assembled version I can buy.

[Pepe]

You don't really need an alligator clip for the ground. My audio probe consists of a standard jack (that I put a patch cable into), the capacitor and the wires, pretty much like in the lower picture of the link that Laurie posted.

[zentropa]

DOD FX92-02-resized.jpg (198.6 KiB) Viewed 115435 times

DOD FX92-01-resized.jpg (186.19 KiB) Viewed 115435 times

DOD FX92-03-resized.jpg (178.54 KiB) Viewed 115435 times

I can see some damage to the traces. Oddly, that damage appears to be caused by the resistor being added, which was actually a factory addition as part of the A1 revision. Q8 is directly to the left of the resistor tacked down.

[chromandre]

is there a jumper lead not connected to this pad?

[zentropa]

is there a jumper lead not connected to this pad?

That was just the LED, which I reconnected after taking the photo. The repair person ripped the wire off and didn’t reattach it.

[laurie]

Did you make any headway with this repair?

One of the things I've seen a bunch of times with these pedals is the solder joints where the pots are connected to the board crack and become intermittent.

Maybe first thing is reflow the solder on all the potentiometer legs.

[zentropa]

It is fixed now. Thank you, it was a brittle joint on one of the switching transistors (either Q8 or Q6) that needed a reflow. I also had to replace almost all of the wires connecting to the board from the jacks/switch/LED/etc. because the tech that had looked at it tore the wires off the board and there wasn't enough slack left to reassemble the pedal correctly (he had jammed everything back in, causing the card to flex).

I was never a fan of the normal Grunge, but the bass Grunge ends up being a lot more usable on guitar due to the wet/dry mix.

[zentropa]

It will be in the signal path: R9 - Q8 - C9 - Q6 (or the surrounding components).

I actually had a couple of quick questions. Are you able to tell from the schematic whether there should be unity gain if the wet/dry mix is panned all the way dry vs. all the way wet? Mine is significantly quieter all the way dry and it gets louder the wetter I blend. I'm not sure if it is supposed to be this way or not.

Also, are you able to tell me which transistor is the input buffer? After searching for info on fixing these, I have found that this is a common failure on this model. I have both J201 and J113, I'm just wondering which one to replace if mine ends up failing (or if the quieter dry blend means it isn't performing optimally).

Thanks.