maestro mps2 ..... where to start?

[iainpunk]

071's might work as well. preforming quite the generic function here.
probably the LM308 would as well, given accommodation for the comp cap is jerry-rigged on. would give the phaser that ''slew rate smoothness'' haha

cheers

[Strategy]

Hi all, back with the Oberheim P-100, "almost" the same circuit as Jimi's MPS. I've had progress, following RG's "how to trouble shoot pedals fried by overvoltage" thread, and also thanks to wiring photos from Jimi, I have this gem passing dry signal and phased. BUT, no sweep, have tried all manner of trimmer tweaks and it's like playing the phaser manually, can't find the sweep. I'm bumping up agai st the edge of my troubleshooting experience, can anyone share some suggestions?  Measure voltages on those first two 741's perhaps?
...can a potentiometer get damaged in a "catastrophic" overvoltage event? The owner of this pedal said it was on a pedal board that had a "power surge" in which every pedal died. I had to replace all ICs, electro's, and transistors to get it this far.
Thanks all!
Strategy

[Rob Strand]

BUT, no sweep, have tried all manner of trimmer tweaks and it's like playing the phaser manually, can't find the sweep. I'm bumping up agai st the edge of my troubleshooting experience, can anyone share some suggestions?  Measure voltages on those first two 741's perhaps?

It would be a good idea to make sure the LFO IC5+IC6 is working.
If you set the speed to low you might be able to see IC6 pin 6 going on and off.
Increase the speed and see if it increases.

If that's too hard to see, wire a 10k resistor and a RED LED and connect it from IC6 pin 6 to ground.
The LED should go off and on with the speed.



IC5 pin 2 about 4V
IC5 pin 3 about 4V
IC5 pin 6 will have triangle wave (centered around 4V) you might be able to measure that.

IC6 pin 2 about 4V
IC6 pin 3 will bobble up and down (maybe 3V to 5V, something around 4V anyway)

If the LFO isn't working then there's no way you will get any sweep, regardless of the trimpots.

If your LFO is working progress to the trimpots and perhaps check your JFET orientations.

[Strategy]

Ok, a covid reinfection had me off track for a few weeks but I'm back and tested ICs 5 and 6. The LFO is not O'ing, and i get way higher voltages across the IC pins, up around supply voltage:
IC5
1 - 8.35
2 - 8.13
3 - 8.34
4 - 8.36
5 - 8.36
6 - 8.29
7 - 8.29
8-  0

IC6 -
1 - 8.36
2 - 8.35
3 - 8.29
4 - 8.36
5 - 8.36
6 -8.26
7 - 7.96
8 - 0
There's not a lot upstream of these ICs, so I feel there's a potential to hone in here but I'm puzzled. Advice sincerely appreciated!

[Rob Strand]

Those voltages look totally whacked.

Upfront: Are you sure the IC's are inserted correctly?  Are you sure you are reporting the IC pins correctly?
This is the pinout when viewing from the top of the IC (not the solder side of the PCB).


You should be able to use your DMM continuity beeper to confirm pin 4 is connected to ground and pin 7 is going to +7V.  In fact pin 4 of all ICs should connect together and to ground and Pin 7 of all ICs should connect together and go to +7V.

If that's not the case check the IC orientations.    If you want to check further follow some of the IC pins and make sure each pin connects a part that matches the schematic.

It might also help to check the voltages on the other IC's. That might highlight an error, or a problem on a larger scale eg. a wiring error, broken track, bad solder joint.

[Strategy]

Thanks Rob!
I counted the IC pins on the correct order.
They are correctly oriented.
I will try the other troubleshooting you recommend.
As I mentioned before as well, effected signal passes and I can modulate phase by turning the trimpot - it sounds really nice. So I believe the issue is confined to the LFO.
So, the 741s I used for IC5 and IC6 are from a different source and batch from the ones in the ohasor section. Could bad ICs turn up a result like i'm seeing? Sadly Im all out of 741s, so I'd need to order more to test
Thanks again for all the help!

[Rob Strand]

Thanks Rob!
I counted the IC pins on the correct order.
They are correctly oriented.
I will try the other troubleshooting you recommend.
As I mentioned before as well, effected signal passes and I can modulate phase by turning the trimpot - it sounds really nice. So I believe the issue is confined to the LFO.
So, the 741s I used for IC5 and IC6 are from a different source and batch from the ones in the ohasor section. Could bad ICs turn up a result like i'm seeing? Sadly Im all out of 741s, so I'd need to order more to test
Thanks again for all the help!

The fact pin 4 doesn't have 0V is a sure sign of a large scale problem.    If the ground/0V isn't getting to the ICs then the circuit has no hope of working.   Check the continuity of 0V everywhere, jacks, DC inlet, pots.  Something isn't right there!

[pinkjimiphoton]

071's might work as well. preforming quite the generic function here.
probably the LM308 would as well, given accommodation for the comp cap is jerry-rigged on. would give the phaser that ''slew rate smoothness'' haha

cheers

308 would work fine. i don't use tl0xx for anything, i don't care for the sound of jfets personally

[pinkjimiphoton]

i know your prob, strat... replace the trimmers. they're hosed.

[Strategy]

woah, really? one of 'em gives me phasing when I turn it. did you have to replace the trimmers on yours?

[pinkjimiphoton]

not on this one, i was lucky to get them back to life with a bit of effort, but the last one i'd had a few years ago, yep, it was the damn trimmers, doing the same kinda thing... i could kinda hear it phasing, but i couldn't get it to fire right. one is the bias for the jfets, the other is the amp for the oscillator if memory serves... effectively, one you set for the "cleanest" signal, the other you set for the least distorted sweep.

[Rob Strand]

Don't overlook the fact if the opamp voltages are way off there's no hope of the thing working.   The 0V issue you have is a very significant problem that has to be fixed.

[Strategy]

Yeah as Rob suggests, before I go at those trimmers I need to trace through the entire LFO section of the circuit and double check ground throughout that section. I haven't had time the last week or so, getting ready for a gig.

Depending on what happens with that, I'm going to test the speed potentiometer as well as the trimmers, I've had new potentiometers fail in new projects, who knows with a cheap pot in a 50 year old pedal that got 'fried' in a catastrophic overvoltage event...

But again I think the issue is confined to the LFO Section of the circuit, ground for the audio section seems to have continuity. Passing effected audio great. Just not LFO-modulated effected audio.

[pinkjimiphoton]

Yeah as Rob suggests, before I go at those trimmers I need to trace through the entire LFO section of the circuit and double check ground throughout that section. I haven't had time the last week or so, getting ready for a gig.

Depending on what happens with that, I'm going to test the speed potentiometer as well as the trimmers, I've had new potentiometers fail in new projects, who knows with a cheap pot in a 50 year old pedal that got 'fried' in a catastrophic overvoltage event...

But again I think the issue is confined to the LFO Section of the circuit, ground for the audio section seems to have continuity. Passing effected audio great. Just not LFO-modulated effected audio.

ended up selling mine, so i can't quite remember which trimmer is which, but one sets the bias to the jfets, and the other is the for the gain of the lfo. on the one i'm talking about, corrosion had built up enough on the trimmer, and people had f'd with it so much in the past, it just couldn't make good connection, it was maddening, cuz i was def getting lfo sweep i could read on the chip for it.
i ended up kinda bending the trimmer so the wiper made better contact and caiging the bejesus out of it.
if that trimmer goes open, no lfo at all, at least not useable.
i socketed and replaced all the 741's in the circuit, and replaced all the tropical fish with green meemies instead, as it seemed fish weren't too fresh.
fired right up after that.  i DID buy a set of jfets for it from smallbear, so i may build another one on perf or something.
hope ya got it sorted out bro!

[Strategy]

Thread wrap up: Oberheim Phasor FIXED, finally. Took a break from it this winter due to work and gigs. Anyways, I wasn't getting supply voltage on Pin 7 of the 741s in the LFO section because there was a break in continuity around one of the electrolytic caps I replaced. I tested continuity after soldering in new parts, but must have been in a hurry/skipped over this one. Lesson learned.
If there's a PCB pattern for this phase shifter out there I'll likely build one. It does only one, simple thing but does it really musically.
thanks to all who helped with troubleshooting pointers.
-Strategy