Big muff q4 collector high? Pulling my hair out :(

[composition4]

Hi,

I'm built a big muff using GGG PCB (the GGG tuned version). When bypassed, sounds fine, when engaged I'm getting a very very quiet sound, almost not there.

Audio probe at the "fx input" on board is silent... I think this may be a problem? I checked for shorts to ground, but nothing found.

Also, transistor voltages (2n5088) for q1, q2 and q3 seem fine, but q4 collector voltage is 9.54v (battery is 9.66v). This seems wrong to me, as does 0.00v for both base and emitter.  The only thing between +9v and the collector of q4 is a 10k resistor R24, as per the layout on GGG.

When I take out q1, q2 or q3 the already very very faint sound gets even quieter - but when I take out q4 there is no difference at all.  I hope I've given enough information, I tried swapping transistors but no luck.

Thanks, help would be much appreciated - I really don't know what I'm missing here...

Jonathan

[theehman]

Did you check your switch wiring?  What's on the emitter of Q4?

[AL]

I agree with EH Man. Check your switch. Or just take the switch out of the circuit completely and see if the circuit works.

AL

[composition4]

Thanks for suggestions,

It's not the switch, tried it without but still no luck

q4's collector is only 100mV below the power supply (battery), and the emitter is at 0.00v and base is also at 0.00v... I'm not sure if this is a separate issue from the fact that there's no audio at the input of the board though..

[R.G.]

Divide and conquer.

>> If there is no audio coming to the board, then it can't put audio out.

>> If the collector of Q4 is at the power supply and its base is at 0V, then it could not pass audio even if there was audio at the input.

You have at least two problems.

To find the first one, use your audio probe. Follow the audio from the input jack lug to the switch, then to the PCB. Don't quit until you get audio to the board. If you suspect your switch wiring, then use an ohmmeter to figure out which pin of the switch connects to which other. If you're using a 3PDT, it is especially easy to get the square 3x3 terminal matrix turned 90 degrees wrong.

To fix the second one, you need to explore why the base and emitter of Q4 are at ground. The base is held up above ground by two resistors. Does the junction of those resistors show the right voltage, as predicted by ohm's law? Does that voltage match the one at the base, which is presumably connected by a solid copper trace? If the divider is wrong, the base MUST be wrong. One possibility is that the lower resistor is shorted to ground. Or that that upper resistor is open. Or the base could be shorted to ground. Or to the emitter.

The right amount of information is what is asked for in "What to do when it doesn't work."

[composition4]

Solved!

Thanks for your help guys - RG, you got me on the right track. It was actually only one problem. The switching and the rest were all fine as I thought...

I put a 100R resistor instead of a 100k resistor which went from ground to the base of q4.  So there was obviously very little resistance to ground through the circuit, which I guess would explain why there was no audio at the input of the board either when it was connected up.  As you said, RG, "Or the base could be shorted to ground" - it virtually was!

Thanks again for helping out... and making me think logically about WHY it wasn't working.

Also as a side note, I used solid-core wire for this build and I think I'll be using it more often. A lot easier to keep neat, although quite a bit of excess wire on this build - I may clean it up later.



Jonathan