Big Muff, little sound :(

[carboncomp]

Hi guys,

Iv built one of the tonepad triangle Big Muffs but have run into a little trouble.

the circuit was working fine till I went to pop it in a box, then all the voltages dropped and now there is very little effect on the clean tone going into the circuit.

Here transistor values im getting at the moment.

BATTERY = 8.74v

Q1
C 4.33v
B 3.49v
E 4.18v

Q2
C 0.69v
B 0.08v
E 0.67v

Q3
C 4.24v
B 0.69v
E 0.12

Q4
C 3.81v
B 1.65v
E 1.06v

and some voltage reading off the power rail.

[DiscoVlad]

If it stops working when the circuit is boxed up, check for shorts to the casing, or pinched wires.

Two things are immediately obvious:

• Q2 definitely looks wrong, it should have similar voltages to Q1.
  
• Q1's Emitter voltage is too high. It should be below the voltage at the base.

[Jdansti]

The red trace should have the same voltage at any point along the trace, but you're showing a 1.3V drop at the 12K resistor next to Q2. Could you please post photos of both sides of the PCB?

[carboncomp]

Double post

[carboncomp]

Sorry you're  right the voltage is the same all along the red line....measured the wrong spot!

May be hard to post photos as my cam is playing up.

I have fluxed resoldered and checked each connection but still low voltage across the transistors?

Any other mesuments I can take to help debug?

[Pyr0]

Make yourself an audio probe and start probing from the input on, when you loose the signal it's a good area to start looking for shorts, opens etc.

[carboncomp]

I have a audio probe, but the trouble is I don't lose audio at any point....it just sound flat and lacking fuzz or amplification all the way round.

[LucifersTrip]

I have fluxed resoldered and checked each connection but still low voltage across the transistors?

Any other mesuments I can take to help debug?

The voltages you already took should narrow it down. You should be checking connections and making sure you have the right values around Q1 and especially Q2.   You'll need to look at the schematic.

[carboncomp]

That's what I have been doing but all the valves are right :-(

Is the voltage supposed to drop down so low at the 12k resistor just before Q2 (drops to 1.25v)?

[DiscoVlad]

Is the voltage supposed to drop down so low at the 12k resistor just before Q2 (drops to 1.25v)?

It's the 9V rail, so no. The other side of the 12k resistor is the Q2 collector which should be around 4V. FWIW, I measured my Russian Big Muff, and the collectors are all around 4V.

Using Isopropyl alcohol (rubbing alcohol/tape head cleaner) and an old toothbrush, scrub all the flux residue away. CHECK AGAIN with a magnifying glass for solder bridges.

Check that Q2 is the right way around!

[carboncomp]

Yep Q2 is the right way round, even just took it out and tested it in case it was broken.

really is a clean soldered board and i can't see any shorts when using a jewellers loop.

Here are the voltage readings from that area if anyone want to help me out as im clearly at a impasse

[Jdansti]

What are the values of the following resistors supposed to be according to the BOM:
R4
R10
R20

Look at the color codes or measure their resistances to see if they match the BOM.

[Pyr0]

There's too much current flowing thru R10, which is causing the large voltage drop across it. There must either be a short between tracks or an incorrect resistor near that area. Recheck the value of R10 , R11 and R9.

[petey twofinger]

great thread title

good luck man .

[carboncomp]

Hey guys, checked the resistor values and they seem to be right

R4 22k
R10 12k
R20 390k
R11 150R
R9 390k

[Pyr0]

You mention in your first post that the circuit was working fine until you boxed it up, I know this is kind of stating the obvious, but maybe that is a hint.
What changed during the boxing up process ? Did you rewire the pots ? Are they all wired up as before  ?
Are there any bad solder joints to the pots and on the various ground points and ground connections to jacks etc? Buzz them out with a meter.
Do you have your input and output jacks ground connected together or are you relying on a ground connection through the enclosure ?
Did any slivers of solder fall on to the the board........etc.

This is a weird one. Good luck trying to fix it.

-Alan

[Jdansti]

Looks like its time for photos of both sides of the board and the off board components. Could you please post some photos?

[carboncomp]

Here are some photos, guys (thanks for your help with this).

http://postimage.org/image/v6tjju5bb/

these are really large one, so thought id pot a link rather then in the tread!

[Jdansti]

Thanks.  I'm still reviewing, but the first thing that jumps out at me is that R8 is missing from your board. Another thing is I would use some isopropyl alcohol and a tooth brush or nylon bristle brush and clean all of the flux from the board. Use a hair dryer or heat gun on low to dry the board before firing it up.

[Mike Burgundy]

There may be more too - without R8 Q2 won't bias properly, so that's a big hint -but:  Are you sure C1,3,4,7 etc are 1uF or .1uF? They look an awful lot like ceramics to me, suggesting much lower values.... This would kill anything but the upper register, so if you get Q2 to bias and are left with very thin sound, the cap values are up for review.