can someone take a look at these BSIAB II voltages, please?

[vince76]

Hi, I have some problems on this build, the effect is working but I'm not happy with the sound and have feedback problems.
I take new voltage readings and it seems something wrong on Q3 ?. here are the readings (GGG board etched by me) battery powered 9.0v.

Q1
G   0
S   0.4
D   4.46

Q2
G   2.65
S   4.46
D   7.99

Q3
G   0
S   0.07
D   3.85

Q4
G   2.65
S   3.86
D   7.99

Q5
G   0
S   0.87
D   3.91

[MikeH]

What's the reading on your battery?  Looks like it's only about 8 volts.  I'd hook up a new battery before any further debugging.

Also:  Check the bias on Q5.  It might not be getting enough juice.

[vince76]

hi mike, and thank you for prompt reply
the bat is 8.7.... there's the dropping due to diode
anyway with fresh battery on Q3 I have 0; 0.08; 4.15
is that wrong?
V

[Ice-9]

The voltage on the source of Q3 looks very low about 0.75 is nearer correct you have got 0.07 unless thats a typo

[vince76]

yes I think the voltage on Q3 Source is too low (0.07, not a typo) maybe the transistor bad?

thanks, V.

[MohiZ]

The Q3 source seems perfectly OK to me. It has only a 120 ohm source resistor, after all. There's about the same current flowing through both Q1 and Q3.

What seems odd to me is Q2 and Q4 gate voltages.... they should be half the power supply, yours look well below that. Maybe check you have the correct resistor values there. What does the pedal sound like now?

+1 for the Q5 bias, too...

[vince76]

ok guys I found the problem here...It came out to be a broken trace (incredible was invisible). Q5 is ok if you compare to ggg specs. The feedback has stopped and the sound is full.
V.

[tjburton]

Vince,
I have a similar problem, with similar voltages... can you remember where your broken trace was?

I'm hoping you have the board from GGG? Thanks for any assistance... this is driving me nuts. I've replaced all the trannies and a few caps.

Thanks for any info you can provide,
Tim