Moodring - Q1 Voltages

[Jlkenne08]

Hi all,

I've put together the Moodring 2017 version recently and have signal coming through which is great.
I have a loud noise coming through the circuit when I dial up the Ring pot that is making it unusable.
So far I've done my best to diagnose the issue, but cannot come up with a solution.

Q1 voltages are reading way off from expected:

D 8.80
S 8.77
G 0

All other voltages seem close to the guide.

I've swapped Q1 over to a MPF102 as I couldn't get a hold of a J201 at the time.
Can you think of any reason for these off readings? Could this explain the noise coming through the circuit?

I've audio probed the circuit and the noise seems to start at R13 before Q1 which makes me think the two are related.

Any input would be greatly appreciated!

[madbean]

I'm confused here. Q1 one is the transistor in the feedback path but you wrote REG1, which is the LM78L05 regulator powering the brick and PT2399.

[Jlkenne08]

Sorry about that, I did mean Q1 and not REG1.
I've updated the original post.

[madbean]

MPF102 should be fine as a substitute. Here are some possibilities:
- MPF102 is a bad part.
- There is a solder bridge between the Drain and Source pins or some kind of continuity that should not be there.

First thing to do:
If you have a continuity checker on your multimeter see if there is continuity between D as S which are the square and middle pads (there shouldn't be any).

Next step:
Reflow solder under Q1. Inspect for solder bridges or anything that looks unusual on the pads.

Further Step:
Remove Q1 and measure for DC on the middle pad where Q1 was. There shouldn't be any DC there.

Last steps:
Socket and replace Q1 with a different device (another MPF102, J201, 2n5457, etc).