Merman signal issues

[Mr.H]

I just got my Merman build completed on Saturday, and have run into problems.  There is no bypass signal, and switching signal is only present when a high gain pedal is in front of it.  I did an audio probe today, and got mixed results.  From the first image, I have marked the parts that are giving a clean signal, and other parts that are giving a screeching signal.  At first, I thought it was an op-amp issue, so I swapped out the op-amp that had a screeching signal (OPA2134) with a TL072CP, but still got the same results.  I put the original op-amp back in, then stepped away as I thought about it for a while.  I went back to do another audio probe, and now I am getting a whistling signal from the pedal, without having the probe touching any components.  I tried to probe, but no signal came from it because the whistling is still happening (whistling occurs when both Drive and Level are dimed out).  I thought that maybe it was because of the location of the op-amp, so I moved the op-amp on the left to the right, and the right one to the left, but the original one on the left kept whistling.  Maybe it's an issue with the op-amp.  Not so sure. 

I have only been doing pedal DIY since April 2020, so I don't have a lot of experience with troubleshooting these matters, and any help is much appreciated.

[Mr.H]

Oh, the diodes have the strip on the Anode side; they are D9E's I got from Pedal Hacker Electronics.

[jimilee]

Have any picks of the switching board?


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[Mr.H]

here they are

[jimilee]

Wait, so how are you testing it?


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[madbean]

AFAIK, using OPA2134 in place of TL072 should be fine here. They can certainly tolerate the voltages. Your resistor values seem correct from what I can see in the pics.

If you have no bypass signal then that's the first place to start. Your switching board looks good from the pic but first verify you've put the correct transistors in their appropriate places (if you haven't already). The writing on the relay is hard to make out but it should say TQ2-L (for latching version).

You can actually audio probe the switching board and that might be a good idea. Audio probe the J2 pad with the circuit in effect mode. If you have output at the J2 pad then something might be amiss with your output jack. If you have no audio at the J2 pad then something is wrong with the switching circuit itself. At that point you'll want to check some voltages like the three leads on the LP2950 and pins 1 and 8 of IC1. And, of course that you have ~9v going from the switching board to the Merman PCB.

[Mr.H]

The correct transistors are in their appropriate places, and the relay is a TQ2-L-5V.  Here are some voltages:

9v Jack = 8.81
Jack to SB = 8.79
V+ @ SB = 7.65
V+ on main PCB = 7.65

LP2950
1) 5.02
2) -0.4
3) 7.65

IC1
Pin 1 = 5.02
Pin 8 = -0.4

When I have the Level knob turned up, and the Drive knob turned all the way down, and doing an audio probe, I get a clear signal at various points (haven't traced it all the way thru the circuit yet).  When I turn up the Drive, with the Level dimed out, a pitch comes out of my amp, and the more I raise the drive, the higher the pitch becomes.  With the drive dimed out, and turning up the Level, the pitch rises in volume and pitch.  If I have both Drive and Level dimed out, I can affect the pitch by turning the Treble knob.  I can get similar results with the Comp and Warm knobs; the Bottom knob doesn't affect the output.  I can "play" with the knobs, varying the degree of pitch with each turn.  When the pedal is acting like a pitch pipe, I cannot do an audio probe.  Mystery to me.

[madbean]

When you say IC1 are you referring to the one on the Merman board (which is a TL072) or the one on the switching board (which is the PIC that does the relay switching).

You don't need to do a full comparison, but it might be a good idea to get voltages on pins 8 and 4 of IC1 and IC2 on the Merman board. That would tell us if you've got proper power to them.
What you are describing could happen if the diodes were not conducting for some reason (IOW, they are not clipping the signal through the Drive path which would cause the whole thing to become unstable at mid to full gain settings). But, I'm not convinced that's the issue here yet.

[Mr.H]

Yeah, I had measured the IC on the switching board.

Here is the measurements on the Merman Board:

IC 1
Pin 4 = 0.8
Pin 8 = 7.63

IC2
Pin 4 = -7.12
Pin 8 = 3.75

One thing I forgot to mention is that when I engage the pedal, I can hear a ping.  When I disengage the pedal, it sounds like a pop.

[madbean]

IC1 looks good. IC2 has a problem. You are under by about 4v. Pull that IC out and measure the voltage again to see if it jumps up to  what you have on IC1. If so, the IC could be the problem, or possibly there is an issue with soldering.

[Mr.H]

I pulled IC2 and got these readings:

Pin 4 = -8.29
Pin8 = 16.44

I reflowed the solder joints and got these:

Pin 4 = -8.30
Pin 8 = 16.45

I put the IC back in and got these:

Pin 4 = -6.59
Pin 8 = 13.22 (it varies though, always increasing)

[madbean]

Yeah I forgot about the split rail thing so your voltages with no IC look much much better. So it might be that IC. Maybe next try putting your IC1 in it's place a do another voltage reading as a test. Or, try a different one. We need to determine if the actual device you were using is faulty or not.

[Mr.H]

I changed out IC2 for a TL072 I had in another project.  Here are the voltages:

Pin 4 = 7.93
Pin 8 = 8.43


I would say the voltages are looking better, but I am not very familiar with this, yet. 
After that, I tried adjusting all the knobs, and the "pitch pipe" noise is gone =:-)
Now I have no bypass signal still, and I am not getting any signal when the pedal is engaged.  My LED also stopped working.  I did remove the board from the box for troubleshooting.  Not sure if that is causing this new mess of problems or not (but I didn't have bypass signal to start with).

[Mr.H]

Forgot to mention that I can still audio probe the circuit when the pedal is engaged.

[Mr.H]

I spoke too soon, the "pitch pipe" problem is still present.  I soldered up a broken ground wire to the switching board, and the LED came back on when engaged.  I plugged my stereo headphone output into the pedal, and the volume is extremely low, even with the Level on the pedal dimed out, and when disengaging the pedal, there is no bypass at all.  There is still a "ping/pop" sound when engaging/disengaging the effect.