DOD FX90 question from a newbie

[tbelcher98]

Trying to troubleshoot (and fix) my childhood FX90....When in bypass, it passes audio normally, but once engaged, the delay doesn't seem to be working. If I go 100% wet, I can hear faint audio, but it's not delayed. I've done some basic troubleshooting with a multimeter, but I don't trust myself. Seems like 9V is reaching the PCB when the pedal is engaged (and the input is plugged in). When I measure at the first diode after power reaches the board, it doesn't seem to be passing anything voltage. If I put the probes on both sides the diode and multimeter in diode mode, I get nothing. So do I replace the diode? Is this common? Anything else I need to check before I start surgery?

Thanks in advance.

 

[Feral Feline]

Use your DMM in beep-mode to test between the non-striped end of the diode and ground, it should beep;
then check the striped side with ground, it should not beep. If it does, then the diode is kaput and needs replacing, 'cause it'd be sending all your power to ground.

I found this schematic online, I believe the red dots are meant to be trouble-shooting points.



If you get power sorted out and still have a weak dry signal and no wet, I'd audio-probe it.

Follow the path of the dry signal, then follow the path of the wet signal.


I'm not the best trouble-shooter, so you should get a second opinion from another more experienced forumite.

 

[jwin615]

Note, most DMMs will only read a diode in one of the two possible positions. You should see 9v across the diode AND be able to test it in diode mode.
I'd check the 51ohm resistor and parallel cap as well. If the diode blew, it was probably an incorrect polarity adapter and could have overcurrentrd that resistor and or damaged that cap.