MXR Flanger 117 not working

[lowell]

I've read a few other posts about this pedal but neither seem to be this issue.  I just replaced the PT in this pedal, the original was bad.  I have +15v and the Vs voltage a well now.  When the effect is engaged the pedal hums and there is flanger effect on the output and all controls do change the effect... however no signal is getting through.  I've signal traced the signal.  It stops after R35 on the input of U2 pin6.  I've checked voltages around this and resistors too and they all check out.  There is SLIGHT signal on the output of U2 pin7.  Not sure what to expect here so I need some help.  Should there be audible signal at U2 pin6?

Schematic
http://media.photobucket.com/image/recent/bainzy007/flangerm117_12.jpg

[PRR]

> stops after R35 on the input of U2 pin6.

Correct. That is a "virtual ground summing point". There's signal current, but the signal voltage is essentially zero.

Signal at U2 pin 7 should be similar to U4 pin 7. Perhaps 2/3rd the volume.

Signal at A1 pin 2 should be same as U2 pin 7.

All these signal-pokes need a DC blocking capacitor so the signal-tracer does not short-out the DC bias voltage at these points (and so your sig-trace does not freak-out about DC coming into it).

> the PT in this pedal, the original was bad.

What else happened around the same time? A simple PT failure should not cause other problems. A lightning-spike or beer-spill could take out the PT and other parts. Solder-splash or wrench-slip while changing the PT could cause other problems.

[Fender3D]

Sometimes I find usefull reverse probing, I mean hook effect output to amp and feed signal from the last active stage back to input.
Humming usually means dead or highly defective parts... maybe half dual op-amp or DC capacitors

[cjlectronics]

Check the DC filter caps after the secondary of the transformer. Leaky caps will cause hum in the output.

[lowell]

I'll check those points out and report back.  Yeah I replaced the only electrolytic in there with new.

[lowell]

So after poking around, leaving it, then coming back it was working perfectly.  Then I turned the feedback knob and it is now humming/oscillating again.  No matter what knobs I turn it's stuck again.  unplugged it for a bit, plugged back in and still oscillating.  Gonna resolder everything.

[lowell]

Okay so resoldered most everything and still no worky.  One thing of note is this.  I'm inserting 1k into the input.  The signal is fine @ U4 output.  But the following stage U2 pin7 has a pitch 2 octaves down from 1k along with the 1k.  This low oscillation is also on the output.  I think it's being amplified throughout the circuit.  Not sure why this is happening or why it's causing there to be no signal on the output.  Any ideas?

[lowell]

Oh and of note is that the lower oscillation is also on U2 pins 1 and 2.

[Govmnt_Lacky]

Check you Regen pot and its connections. Also, check to make sure that when you affix the board back into the enclosure (assuming this is an MXR flanger and not a clone) that nothing is shorting to the Regen pot lugs.
If you notice, according to your original post, the signal is disappearing at the point where the Regen is entering the chain.

Worth a shot 

[oldschoolanalog]

Got access to a 'scope?

[lowell]

I do.  But wouldn't help because I have no experience with this circuit and wouldn't know how to interpret the results.  I was gonna study the IC datasheets ... But now the dang thing is working.  I took all the pots and board out, was moving stuff around and it started working.  I tried, really tried to get the thing to act up again and it wouldn't.  Put back together and it seems fine.  I'm thinking it'll be back at somepoint but well see.

[Paul Marossy]

now the dang thing is working.  I took all the pots and board out, was moving stuff around and it started working.  I tried, really tried to get the thing to act up again and it wouldn't.  Put back together and it seems fine.  I'm thinking it'll be back at somepoint but well see.

Sounds like something must have been shorted out.

Did you replace the 470uF filter cap while you were at it? (preventative maintanence)

I fixed one for someone a while a back, that filter cap was completely dried up. Put a new one in and it worked great, and without the HUMMMMMM.