Mini Mu Help

[rhaas]

Hi!

I just built a Mini Mu and I'm having some difficulty figuring out where I went wrong. The pedal powers up and switches on/off correctly, but when it's on, no sound comes through (I do notice some scratching noise when I turn the Level pot, but no actual guitar signal). Here are the voltages, and I've attached some pictures.

IC1 OPA134
1   8.2 (should be -8.4)
2   -0.3 (should be 0)
3   0 (should be 0)
4   -8.2 (should be -8.4)
5   0 (should be -8.4)
6   7.5 (should be 0)
7   8.5 (should be 8.8)
8   8.2 (ignore)

IC2 LM358
1   -0.97 (should be 7.57)
2   4.2 (should be 2.24)
3   4.2 (should be 4.35)
4   -8.2 (should be -8.44)
5   0 (should be 0.027)
6   - 0.95 (should be 0.03)
7   7.2 (should be 0.36)
8   8.5 (should be 8.8)

IC3 TLE2074
1   -6.5 (should be 0)
2   -6.5 (should be 0)
3   0 (should be 0)
4   -7 (should be 8.77)
5   0 (should be 0)
6   -6.3 (should be 0)
7   -6.5 (should be 0)
8   -6.3 (should be 0)
9   -6.5 (should be 0)
10  0 (should be 0)
11  -8.2 (should be -8.4)
12  -6.5 (should be 0)
13  -6.5 (should be 0)
14  -6.5 (should be 0)


Any help is greatly appreciated!

[madbean]

I'd start by pulling the three ICs off the main board then re-compare some voltages (IOW, check the pins of the bare sockets) to see if they start to align with the ones in the doc. Particularly check where you are getting -6.5v and it should be ~0.

[rhaas]

Ah, thanks! I didn't realize these were supposed to be the measurements without the ICs installed.  :) The new measurements:

IC1 OPA134
1   0 (should be -8.4)
2   -0.3 (should be 0)
3   0 (should be 0)
4   -8.9 (should be -8.4)
5   0 (should be -8.4)
6   -2.7 (should be 0)
7   8.9 (should be 8.8)
8   0 (ignore)

IC2 LM358
1   0 (should be 7.57)
2   -4.2 (should be 2.24)
3   4.4 (should be 4.35)
4   -8.9 (should be -8.44)
5   0 (should be 0.027)
6   - 5.8 (should be 0.03)
7   -5.7 (should be 0.36)
8   8.9 (should be 8.8)

IC3 TLE2074
1   -2.9 (should be 0)
2   -2.9 (should be 0)
3   0 (should be 0)
4   0 (should be 8.77)
5   0 (should be 0)
6   -2.9 (should be 0)
7   -0 (should be 0)
8   -2.8 (should be 0)
9   0 (should be 0)
10  0 (should be 0)
11  -8.9 (should be -8.4)
12  -2.8 (should be 0)
13  0 (should be 0)
14  0 (should be 0)

[jimilee]

On the backside of the board, check for solder bridges. Also, a lot of your solder joints should be reflowed so the solder flows through the board.


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[madbean]

It's not that you should measure empty sockets for voltages. It's that when voltages are off by so much, it's helpful to remove the ICs sometimes to see if some problem areas are resolved. That way you may be able to identify if a specific IC is causing the issue.

[rhaas]

Thanks, all! I inspected the board and couldn't find any solder bridges. However, I did see that I missed one of the socket pins for the TLE2074, so I soldered that (and reflowed anything that looked like it could use it). I'm now getting sound when the pedal is engaged, though the envelope effect is barely noticeable and the up/down switch doesn't seem to make a difference. Here are my new voltages with ICs installed. In summary the TLE2074 seems fine, the LM358 seems fine except for pin 2, and the OPA134 has a few anomalies (pins 1 and 5, in particular). Any ideas?

IC1 OPA134
1   8 (should be -8.4)
2   0 (should be 0)
3   0 (should be 0)
4   -8 (should be -8.4)
5   0 (should be -8.4)
6   0 (should be 0)
7   8.3 (should be 8.8)
8   8 (ignore)

IC2 LM358
1   7.1 (should be 7.57)
2   0.03 (should be 2.24)
3   4.1 (should be 4.35)
4   -7.9 (should be -8.44)
5   0.02 (should be 0.027)
6   0.03 (should be 0.03)
7   0.5 (should be 0.36)
8   8.3 (should be 8.8)

IC3 TLE2074
1   0 (should be 0)
2   0 (should be 0)
3   0 (should be 0)
4   8.3 (should be 8.77)
5   0 (should be 0)
6   0 (should be 0)
7   0 (should be 0)
8   0 (should be 0)
9   0 (should be 0)
10  0 (should be 0)
11  -7.9 (should be -8.4)
12  0 (should be 0)
13  0 (should be 0)
14  0 (should be 0)

[rhaas]

Update: I double-checked all resistor & capacitor values, removed the pots so I could inspect and reflow all solder joints, reinstalled the pots, double-checked the optocoupler orientation, and even tried a second pair of optocouplers... and I'm still seeing largely the same voltages as the ones in my last post.

The two discrepancies that seem the most suspicious to me are the reversed polarity on pin 1 of IC1, and the 0V reading on pin 5 (which should be ~-8.4V). Do either of those point to a specific area where I could focus my troubleshooting?

[jimilee]

Do you have another 134 or single opamp you can try?


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[rhaas]

I have one spare OPA134 and I'm seeing the same values with that chip. They're both from the same Mouser order, so maybe they're both defective (seems unlikely) or I ordered a subtly incorrect part (somewhat more likely ).

https://www.mouser.com/ProductDetail/Texas-Instruments/OPA134PAG4?qs=7nS3%252BbEUL6vHBXizjGOrwg%3D%3D

[Bio77]

Is the white wire from your power jack connected to the + terminal of the switching board?  I can't see from the picture.

[rhaas]

Is the white wire from your power jack connected to the + terminal of the switching board?  I can't see from the picture.

Yes, it is (specifically, the square pad all the way to the left).

[jwin615]

Have the correct type dpdt on-on-on switches?

[rhaas]

Have the correct type dpdt on-on-on switches?

Just checked and I can confirm that Mode and Range are on/on/on, and Drive is on/on.

EDIT: These are the on/on/on switches I used: https://stompboxparts.com/switches/dpdt-toggle-switch-on-on-on-pcb-pin-short-bat/

[jwin615]

Have the correct type dpdt on-on-on switches?

Just checked and I can confirm that Mode and Range are on/on/on, and Drive is on/on.

EDIT: These are the on/on/on switches I used: https://stompboxparts.com/switches/dpdt-toggle-switch-on-on-on-pcb-pin-short-bat/

I'm driving at the moment so can't confirm anything but there are two types of dpdt on on on switches. Type 1 and type 2. The center position is inverse between the two types.
I'm not super familiar with this circuit or the circuitry around those switches to know if it could be causing any issue. Just stating an obvious but maybe not so obvious possibility.
I believe generally type one is the more common unless otherwise notated

[rhaas]

Type 1 and type 2. The center position is inverse between the two types.

I'd love to get to the root of this, as painful as it would be to remove and replace those switches. The switches I got from SBP are advertised as Type 2. I don't see a particular type specified in the build docs, but this thread suggests that Type 2 would work (unless I'm misinterpreting the diagram posted there, or the picture on the Small Bear product page that @jjjimi84 linked).

https://www.madbeanpedals.com/forum/index.php?topic=30743.0

@madbean Do you know if Type 1 or Type 2 is correct?