ATTN: MFOS Soundlab MKII owners...voltages

[kdmr]

I'm troubleshooting the filter section in my build. Wonky voltages firstly led me to U1 (TL072) which was improperly seated in the socket. None of my filter IC outputs were showing voltage (U1 pin7, U2 pins 5 and 12) referenced in Ray's documentation. Now beginning to question whether the unseated pins would have actually caused burnout of the chips since replacement of the TL072 yields the same erroneous voltage reading at pin 7 (showing 5v if i remember correctly) all of my +/- 12v pins and U1 bias input are showing proper voltage, as are the coarse and fine cutoff pots.

i've ordered a new batch of LM13700s to double check the error with new hardware, but i'm beginning to suspect that the problem is elsewhere on the board or in the wiring. ive reflowed all the PCB pads and checked everything for continuity based on the schematic.

Hoping someone with a working unit might do me a solid and shoot me the voltages i should be looking for mpf102s at the pin 5/pin 12 junctions so when my 13700s arrive i can hopefully zip this project up. Such a drag because i've audio probed and calibrated every other "module" on the board and its sounding so rad.

i can post the full voltage readouts for all the pins if someone wants to help tackle this, but some working voltages are all im after!

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[kdmr]

alright heres a loop for ya...

U1
1. -165mv
2. 4mv
3. 0
4. -11.73v
5. 0
6. 5.44v
7. -10.15v
8. 11.83v

pins 1, 2, 3, 5 test positive for continuity to ground


this is where it gets weird. every time i disconnect and reconnect power, the LM13700 gives me different readings. ive used 4 different units, these are the readings from the last 2 trials

U2
1. -10.69v both times
2. 0 both times
3. -140mv gradually becoming less negative/ 67mv
4. 18mv gradually increasing
5. 0 both times
6. -11.73 both times
7. -11.23 both times
8. -7.12v/ 1.52v
9. 5.65v gradually decreasing/ 800mv gradually increasing
10. -6.73v gradually decreasing/ 2.25v gradually increasing
11. 11.83v both times
12. 0 both times
13. 0 both times
14. -5mv both times
15. 0 both times
16. -10.59v both times

pins 1, 8, 13, 16 show continuity to ground, as do pins 10 and 11 intermittently.

i'm getting new batteries for my DMM tonight because this cant possibly be right, RIGHT?

i guess i'm looking at both 330p at the gate of each MPF102 next...ugh

still getting audio at the mixer out, when the vca is modulated by VCO1 i get audio at the master out and can engage LFO mod, S&H mod and hard sync as well as all functions of both envelopes...thats great and all, but i may have to put the project down for a week and come back fresh.

[gdavis]

[kdmr]

Yeah it's all screwy...like I'm looking at the schematic trying to figure out how these readings are even possible. Thanks a mil for the input though...it's priceless to have another mind throwing out ideas. I'm going to double-check everything with fresh batteries...it's been a month or two of pretty heavy use.

[kdmr]

Oh and the continuity test is the audible test on my DMM. From the panel to each pin.

[gdavis]

Aw, it's not that bad. We're getting it narrowed down, just keep going and we'll find the culprit.

Like I said, not enough current getting through the feedback loop. Could be R7 or R8 is not the correct value, bad Q1 or Q2, short or bad connection in that area.

I usually don't rely on the DMM continuity test on populated boards. Maybe just to look for shorts between points I know shouldn't have any sort of connection. Components can make it a bit unpredictable.

[kdmr]

I cleaned between all the pads in the area last night in case of any bridges. My last resort is usually to go through and make sure all the components are correct because I'm usually pretty careful...maybe I'll push that chore up the agenda...

[kdmr]

R7 is correct and Q1 base voltage basically equals U1.1

couldnt get out to grab batteries last night but i'll be back at it tonight......

[kdmr]

Update: officially in "needle in the haystack" mode.

Both trimmers check out
Q1 and Q2 check out on hfe test, thinking they are not faulty.
All components are properly placed
No pin to pin shorts on my sockets

Posting transistor voltages tonight

I'm flooding this thread with all readings for my own records and hopefully once it gets ironed out it'll help someone else down the road.

[kdmr]

Oh my god it was Q1.

Thanks a million...guess I'm off to match transistors