MXR Phase 100 passes signal,but no effect

[stan]

I have an MXR Phase 100 that passes signal,but has no effect. No voulume dip when unit is turned on or off.
I checked for broken wires and gave it a general once over.
Can anyone point me in the right direction as to how to troubleshoot this little beast? Test point voltages ?

[Mark Hammer]

The fact that you get a clean signal without any volume loss suggests that there is a discontinuity in the phase-shift path.

If the tone was different between effect and bypass, that might suggest a problem with the LFO, such that it was "stuck" at one setting and did not sweep. But you have not noted anything like that.

The only audio signal that passes through the 4-position switch is the feedback signal, but since the unit can sweep without any feedback at all, that would suggest the problem is not in the switch, or any wires running to it.

Given that, it would appear - at least from the information you've provided - that either one of the op-amp chips is blown, or else there is a cracked resistor or other discontinuity between two op-amps.

If you have a meter that will let you measure AC voltages in the under-2-volts range, then I would suggest that you feed a music/signal source to the pedal, whether guitar, radio, or signal generator, and measure the AC voltage at pins 1 and 7 (the output pins) of each of the op-amp chips carrying the audio signal (the ones with 10k resistors and a .01uf cap around them). You should see an AC voltage that corresponds to peaks in the audio signal. If you see no signal at one of those points in one of those chips, that would point to either a blown chip, or a discontinuity leading up to the input of that chip.

[stan]

it looks like 3 out of 4 transistors (replaced all 2SC1849) were cooked and a diode 1N914. Replaced and tested. This thing sounds great.

Thank you Mark for the Op-amp troubleshooting tip, which is now in my note book.

[Mark Hammer]

My pleasure. Happy phasing!