MXR M-101 Phase 90 not phasing

[thedefog]

I've repaired a couple of these before, but this one is a bit more difficult.

I replaced the first obvious problem, which was a burnt out diode. First it was replaced with a 1n914, but I got awful distortion from it. Thinking it was an opamp, I replaced the first TL082 chip. That did nothing, so I swapped the 2n4125 with a 2n3906. Still no change.

Then I realized that it should have been a zener diode, and that fixed the distortion issue. However, I am not getting any phasing of the pedal at all. The signal passes through, but no oscillation occurs.

The 2n5952s are most likely good, since when I touch them they make a nice little phase sound for me. All of the voltages read fine on the TL084 & 82.

Did I mess up by swapping the 2n4125 with a 3906? I have had good luck with replacing pretty much any low voltage pnp or npn with 3906s or 3904s.

[mdh]

When you say that no oscillation occurs, do you mean that you've probed the circuit and found no sign of a working LFO, or just that you don't hear phasing at the output?  The first thing I would do is make sure that the LFO is going.  You should see a voltage fluctuating in the range 2V to 7V or so at the positive end of a 15uF (or so) electrolytic cap near the inverting input of one of the op-amps (I would guess that this would be the dual, and that the quad would be devoted to the phase shift stages).  Next thing I would do is verify that the zener is oriented properly, and is rated for somewhere around 4.7-5.1V.  Finally, I would set the rate to maximum, and twiddle the trimmer, listening for phasing at the output.

[e178453]

That burned out zener seems to be a common problem with these.  I ended up replacing both op-amps on a surface mount version of this pedal along with the diode.  That worked.  As I recall, the voltages on the bad ones looked good.  NTE cross reference says the 2n3906 will replace the 4125.  MDH is right, check the LFO first.
 

[thedefog]

When I probed around, I see no sign of a working LFO. So I'll check that 15uF first. I installed sockets on the ICs, so I'll try swapping them out with different ones as well. The pads are so small on these dunlop boards that they were obviously not meant to be hand-soldered... Or repaired for that matter. Thanks for the tips. I'll give it a shot.