pedalpcb.com duo-phase troubleshoot

[ymjon]

Hey all,

Looking for some help troubleshooting my build.

Have it all put together but LFO A is not fully working. It is ramping up but not ramping back down, can someone point me in the right direction to get this functional?

I've put the link to the build docs which has the schematic.

https://www.pedalpcb.com/docs/DuoPhase.pdf

Edit:
Just got home and took some measurements with all knobs fully CCW:

DC 9.04v


IC1a

1 5.9mv
2 4.9mv
3 82mv
4 -7.8v
5 2mv
6 2mv
7 2mv
8 9.03v

2a

1 -8.2mv
2 3.1mv
3 1.8mv
4 -7.8v
5 1.7mv
6 4.4mv
7 16.9mv
8 9.04v

3a

1 14.6mv
2 5.4mv
3 1.8mv
4 -7.8v
5 1.6mv
6 2.6mv
7 0v
8 9.04v

4a

1 -3.7mv
2 1.4mv
3 1.6mv
4 -7.8v
5 1.8mv
6 2.8mv
7 6.6mv
8 9.03v

5a

1 2.8v
2 83mv
3 82mv
4 -7.8v
5 2mv
6 1.9mv
7 2.1mv
8 9.03v

6a

1 -6.4v
2 1.6mv
3 -6.3v
4 -7.8v
5 1.6mv
6 -29mv
7 -6.3v
8 8.4v

7a

1 9.03v
2 5.42v
3 2.1mv
4 -3.04v
5 -7.8v
6 4.3v
7 5.6v
8 9.03v

Q1a
E 2.2v
B 2.8v
C 9.03 v

D1a
C 8.36v
A 9.03v

D2a
C 2mv
A 83mv

Thanks in advance,
Jon

[bluebunny]

Have it all put together but LFO A is not fully working. It is ramping up but not ramping back down

Just to clarify...

do you mean it's doing this:	instead of this?

[ymjon]

Have it all put together but LFO A is not fully working. It is ramping up but not ramping back down

Just to clarify...

do you mean it's doing this:	instead of this?

Sorry about that. For clarification, i do not own a scope so I am not sure what the LFO is doing in that sense but what I am visually seeing is the led for LFOA fade in to full brightness then cut to black instead of fading back out.

[bluebunny]

OK, sounds like the sawtooth (on the left).  Need someone with LFO smarts to stop by now.   

[ymjon]

So looking thru the schematic it looks like the LFOs are based around IC6? Does that look right?

Based on that idea I measured IC6 A and B to compare and got very almost opposite readings on pins 1 3 and 7 as seen below.
6a          6b
-6.35v   7.56v
1.6mv   3.0mv
-6.28v   7.74v
-7.76v   -7.74v
1.6mv   3mv
-28.8mv   3mv
-6.27v   7.4v
8.35v   8.32v

Visually it looks like LFO B is functioning properly so would it make sense to replace IC6A or is it possible that something feeding into it might be bunk? I'll have to order in another tl072 so I can't confirm for myself at the moment.

I compared the rest of the ICs and they had some variances as well, here are those measurements

1a           1b
5.9mv   3.9mv
4.9mv   3.2mv
82mv   3mv
-7.8v   -7.7v
2mv   3mv
2mv   2.5mv
2mv   2.5mv
9v           9v

2a           2b
-8.2mv   0v
3.1mv   6.2mv
1.8mv   3mv
-7.8v   -7.7v
1.7mv   3mv
4.4mv   3.6mv
16.9mv   8mv
9v       9v

3a           3b
14.6mv   13mv
5.4mv   7.8mv
1.8mv   3mv
-7.8v   -7.7v
1.6mv   3mv
2.6mv   2.8mv
0v           1.6mv
9v           9v

4a           4b
-3.7mv   2.5mv
1.4mv   6.5mv
1.6mv   3.3mv
-7.8v   -7.7v
1.8mv   3mv
2.8mv   6mv
6.6mv   10.5mv
9v           9v

5a           5b
2.8v   2.9v
83mv   112mv
82mv   114mv
-7.8v   -7.7v
2mv   3mv
1.9mv   2.7mv
2.1mv   2.6mv
9v           9v

7a           7b
9.03v   9.03v
5.42v   5.22v
2.1mv   3.1mv
-3.04v   -3.2v
-7.8v   -7.74v
4.3v   3.9v
5.6v   5.5v
9v           9v

The diodes and transistors measured the same on both sides.

Thanks

[Rob Strand]

So looking thru the schematic it looks like the LFOs are based around IC6? Does that look right?

Yes.

IC6A.pin 1 should toggle +/- 8V.  If you set the Rate to low you should see it.

Check the power rails to the chip.  Should be + and - V on them.

If IC6A.pin 1 is just going in one direction then getting stuck on the rails then there's an issue with the feedback loop around the two opamps being broken.  The most likely cause would be a wiring error on the Rate pot.

EDIT1: Are your 470nF caps non-polar or films?
EDIT2: Those 120R resistors look pretty small.  Maybe bump them up to 470R.

[ymjon]

So looking thru the schematic it looks like the LFOs are based around IC6? Does that look right?

Yes.

IC6A.pin 1 should toggle +/- 8V.  If you set the Rate to low you should see it.

Pin 1 is not cycling between + and - just holding at -6v

Check the power rails to the chip.  Should be + and - V on them.

For the power rails do i measure voltage between 9V+ and pin 4(vcc-)?

If IC6A.pin 1 is just going in one direction then getting stuck on the rails then there's an issue with the feedback loop around the two opamps being broken.  The most likely cause would be a wiring error on the Rate pot.

The rate pot is a pcb mount and I've checked continuity with the schematic and everything checks out

EDIT1: Are your 470nF caps non-polar or films?

470nFs are all non-polar

EDIT2: Those 120R resistors look pretty small.  Maybe bump them up to 470R.

I can switch out the 120R for the 470R but that is what is in place on the functioning LFO B side

Thanks

[Rob Strand]

Pin 1 is not cycling between + and - just holding at -6v

OK thanks.

6a          6b
-6.35v   7.56v
1.6mv   3.0mv
-6.28v   7.74v
-7.76v   -7.74v
1.6mv   3mv
-28.8mv   3mv
-6.27v   7.4v
8.35v   8.32v

For the power rails do i measure voltage between 9V+ and pin 4(vcc-)?

I can actually see the -7.76V from your measurements.  The good thing is the +/- power rails look fine.

The rate pot is a pcb mount and I've checked continuity with the schematic and everything checks out

OK it must be something around the IC6 opamp.
I can see your measurements show it's stuck but I can't quite work it out yet.

Maybe this test will help:   Measure the voltage on the wiper of Rate Pot A  and the voltage on IC6 pin 6 when Rate Pot A is on Minimum and on Maximum.   The aim here is to see of the voltage on IC6 pin 6 is stuck at the small -28mV or gets into the volts region.   It's weird to be stuck on that small voltage without the output going high.

The voltages on IC6 pin 6 and IC6 pin 7 are the key at the moment.   My best guess is maybe pin 7 isn't going high or pin 6 isn't working.   If you have a different dual opamp you might be able to replace IC6 just to get it debugged and going.

I can switch out the 120R for the 470R but that is what is in place on the functioning LFO B side

It sets the maximum LFO rate.  The small 120R value causes two problems.  The first is the pot will have a rapid jump in speed near the end; not very usable.  The second is opamp offsets might cause the LFO to produce assymetrical waveforms only at the max setting.

[ymjon]

The rate pot is a pcb mount and I've checked continuity with the schematic and everything checks out

OK it must be something around the IC6 opamp.
I can see your measurements show it's stuck but I can't quite work it out yet.

Maybe this test will help:   Measure the voltage on the wiper of Rate Pot A  and the voltage on IC6 pin 6 when Rate Pot A is on Minimum and on Maximum.   The aim here is to see of the voltage on IC6 pin 6 is stuck at the small -28mV or gets into the volts region.   It's weird to be stuck on that small voltage without the output going high.

The voltages on IC6 pin 6 and IC6 pin 7 are the key at the moment.   My best guess is maybe pin 7 isn't going high or pin 6 isn't working.   If you have a different dual opamp you might be able to replace IC6 just to get it debugged and going.

Okay so voltage at the rate wiper at min is -29mv
Wiper rate max seems to be cycling between 0v and -3v

Pin 6 rate min is 29mv
Pin 6 rate max is -3.3v

Unfortunately I dont have anything else to swap it out with at the moment so I'll have to order some parts

[Rob Strand]

Wiper rate max seems to be cycling between 0v and -3v

The good thing is it is varying.   

However, now there's another thing that doesn't make sense:  Pin 1 previously was at -6.35V.   When the Rate pot is on full you would expect the wiper to swing to the same voltage as pin 1 ie. -6.35V but you are only seeing -3.3V.     If I saw that I'd start thinking there's something wrong on the wiper of the Rate pot, like it's shorting to something.   It's possible to get a fault like that if the opamp is blown but I wouldn't expect it.

Pin 6 rate max is -3.3v

OK, that at least follows the wiper voltage.  One last thing you could check:  When the Rate wiper is set to max.  Check the voltage on IC6 pin 7.  If it is still at -6.3V  then it is extremely likely the opamp is dead.  If it has changed to some other voltage then there maybe some other issue.

[ymjon]

Wiper rate max seems to be cycling between 0v and -3v

The good thing is it is varying.   

However, now there's another thing that doesn't make sense:  Pin 1 previously was at -6.35V.   When the Rate pot is on full you would expect the wiper to swing to the same voltage as pin 1 ie. -6.35V but you are only seeing -3.3V.     If I saw that I'd start thinking there's something wrong on the wiper of the Rate pot, like it's shorting to something.   It's possible to get a fault like that if the opamp is blown but I wouldn't expect it.

Pin 6 rate max is -3.3v

OK, that at least follows the wiper voltage.  One last thing you could check:  When the Rate wiper is set to max.  Check the voltage on IC6 pin 7.  If it is still at -6.3V  then it is extremely likely the opamp is dead.  If it has changed to some other voltage then there maybe some other issue.

Okay I switched the two rate pots but the problem still persists.

Pin 7 is going between around -0.5v and +1.5v with rate at max

[Rob Strand]

Pin 7 is going between around -0.5v and +1.5v with rate at max

That's weird.  It's like the opamp is trying to do something.  Maybe even OK.

You could try:
- Disconnect pin3 of the LFO A Depth pot.  To remove the effect of a problem at or after the Depth pot A
- Replace the 470nF cap.   Assuming the cap has an issue.    If you don't have 470n use anything close
  but it must be a non-polar electrolytic or film.

[ymjon]

Pin 7 is going between around -0.5v and +1.5v with rate at max

That's weird.  It's like the opamp is trying to do something.  Maybe even OK.

You could try:
- Disconnect pin3 of the LFO A Depth pot.  To remove the effect of a problem at or after the Depth pot A
- Replace the 470nF cap.   Assuming the cap has an issue.    If you don't have 470n use anything close
  but it must be a non-polar electrolytic or film.

Sorry just recovering from the holidays, looking at what I have in stock and the closest to the 470nF would be 330nF if that is close enough I can switch it out. Otherwise I can order some stuff from tayda. Gonna heat up the iron and try to unsolder the depth lug to see if that helps.

Thanks again for all your help