Electro Harmonix Metal Muff weak signal

[almavague]

I'm relatively new to electronics and I'm trying to troubleshoot my Electro Harmonix Metal Muff. When the unit is on bypass, there's no sound. When it's on distortion, there's a very weak distorted sound. The leds both function as normal. The power supply is good, and I've double checked with a battery and it produces the same problem. I've removed the pedal from the enclosure with no improvement. The switches appear to be working well. I've tried looking at the schematic and doing continuity tests where I'm able and found nothing abnormal. I've tried to test voltages, and haven't found anything abnormal, with the one exception noted below.

There are two points which seem strange, but I don't know enough to say if they're normal or not:
1. The power supply gives 9.5v. However the positive terminal on the power supply to the ground spring that contacts the enclosure, there is only 8.75v. And then there is 0.75v between the negative power supply terminal and the ground spring/enclosure.
2. I see some sort of clear substance on all of the larger solder joints (I assume these are the hand soldered joints?). It could be corrosion, but it seems pretty consistent on all of these joints. Do soldered joints typically have some sort of protective coating on them? These joints are on all of the pots and on the connections to the switches. Do I just need to clean the PCB?

I've provided the voltages below. All the voltages are in bypass and were taken with the ground spring. Can anyone help me troubleshoot this pedal? I don't know how to go beyond the steps I've already done.

https://tuxdoc.com/download/metal-mu...hematicpdf_pdf

U1
1 4.3v
2 4.6v
3 3.85v
4 0
5 3.9v
6 4.3v
7 4.3v
8 8.5v

U2
1 4.3v
2 4.3v
3 3.9v
4 0
5 4.2v
6 4.3v
7 4.3v
8 8.5v

U3
1 4.4v
2 4.4v
3 4v
4 0
5 3.9v
6 4.3v
7 4.3v
8 8.5v

U4
1 4.3v
2 4.3v
3 4.3v
4 0
5 2.2v
6 4.4v
7 4.4v
8 8.5v

Q1
B 4.1v
C 8.5v
E 3.7v

Q2
B 4.1v
C 8.5v
E 3.6v

Q3
B 42.9v
C 8.5v
E 3.6v

Q4
B 4.8v
C 8.5v
E 4.6v

Q5
B 0.6v
C 4.8v
E 0v

Q6
B 4.3v
C 8.5v
E 4v

Q7 (this one is on the PCB but not on the schematic)
B 8.4v
C 8.5v
E 8.5v

[Jazz P Bass]

The 'clear stuff' is flux from when it was soldered.
Not a real issue.
D2A is the reason for the voltage drop from the battery to the circuit.
Normal,
Q7 is right above & to the left of Q1.

[almavague]

Thanks for the response! So if everything so far looks normal, what should I look for? Would other voltages help to troubleshoot? What would be the next step?

[Helmholtz]

Q7 is a JFet, used as a source follower, so no base, emitter, collector. Gate should be at 4.5V (measurement requires a high input impedance meter). Source should be 5V to 6V.

Is the 4.5V supply fine?

[almavague]

4.5V supply voltage (taken at C40 +side) measures 5V.

As for the rest, I'd be very grateful if you could help me understand, I'm a noobie

What is the gate? What is the source? Is a "high input impedance meter" like the 2Mohm setting on my multimeter?

[Helmholtz]

A JFet (Junction Field Effect Transistor) is another kind of transistor (note different symbol in schematic). Its legs are called gate, source and drain, corresponding to base, emitter and collector of a "normal" bipolar transistor.
https://www.onsemi.com/pdf/datasheet/pn4393-d.pdf

Measuring gate voltage requires a (professional grade) voltmeter having an input impedance of 10M or higher. Most meters are only 1M, which would load down the gate voltage to 50%.
Meter input impedance is typically found in the manual/specs.

But you don't need to know the gate voltage. Just make sure the source voltage is around 6V.

[g1]

But you don't need to know the gate voltage. Just make sure the source voltage is around 6V.

He gave voltages for Q7, just mis-labelled as E,B,C. All of them were around 8.5V so it seems jfet is not running right.

[Helmholtz]

He gave voltages for Q7, just mis-labelled as E,B,C. All of them were around 8.5V so it seems jfet is not running right.

Yes, I meant after replacement. No type of transistor would work with all terminals at same voltage.

Also 5V at C40 looks fishy with a supply voltage of 8.5V.

[almavague]

Thanks! Here's the update:
I double checked the voltages on Q7, and they're all 8.5v. I looked around with a multimeter in this area of the circuit. Everything seems normal (connectivity between components, normal resistances, normal voltages on capacitors). Can this transistor be replaced? It's very small. Here's a link to a photo of the PBC:

https://drive.google.com/file/d/1371...ew?usp=sharing

As for the 5V at C40 - I think it's my lack of experience that's confusing you. Here are the true voltages
+/- at the power jack 9.5V
+ power jack and ground spring 8.5V
+/- C40 5V
+ C40 and ground spring 4.3V

Does that seem normal? Should I be noting voltages taken from the negative on the power jack rather than the ground spring?

[Helmholtz]

Forget the ground spring. For circuit operation the ground reference is at the negative terminal of C40 (or C39).

[almavague]

Would it be helpful if I retook all the voltages from C40?

And is it actually possible to replace the transistor? The reference number on the schematic is MMBF4393, so is this transistor the right part for the replacement?
https://fr.farnell.com/on-semiconduc...3/dp/2453367RL

Or do they make the same transistor in a larger form with long leads, and would that be easier to soder to the board?

Do I need special tools? I have a multimeter, sodering iron, soder and general purpose tools (pliers, screwdrivers, etc.)

[Helmholtz]

And is it actually possible to replace the transistor?

It's tricky but can be done, e.g. look here: https://www.youtube.com/watch?v=LADgxZgyJ04

The Farnell part looks right.

As the Fet seems to be shorted, this would explain why your distorted signal is weak and why you don't have a dry signal.
The Fet is used as dry signal buffer ("source follower") and if its input is shorted, it loads down the input signal to the distortion circuit.

[Jazz P Bass]

"general purpose tools (pliers..."
A sharp point tweezer should be in your kit.

[Helmholtz]

Test if the distortion is working with the Fet removed.

[almavague]

Thanks a lot for all the great advice! I'm going to try that this week... I'll keep you posted!