Big Muff troubleshooting

[croquet hoop]

I'm having some trouble with a Big Muff I am building (Spasm Chasm specs, Spasm Chasm pcb from CJ) . The most obvious problems are that the pedal sounds weak and muffled, and the sustain pot doubles as a volume knob — when you turn it down, the gain decreases, but so does volumes too. At half the rotation, there is almost no sound passing.

I double checked the values when populating the board, I have checked them again, they seem alright.
I tested with the transistors reversed, and with different transistor types (2N5088, 2n5099, BC550C, MPSA18), no difference whatsoever.
I let it rest for two days.
I re-did the wiring on the vol and sus pots. No change either.

Here are the voltages:

Battery : 8.70v

Q1:

E= 0.05
B= 0.64
C= 3.90

Q2:

E= 0.06
B= 0.63
C= 6.05

Q3:

E= 0.72
B= 1.09
C= 6.70

Q4:

E= 0.89
B= 1.37
C= 4.23

I compared them to those taken on a working P19 build, they are not terribly far apart, so I don't think the problem lies here. I have checked the solder points and that seems OK too, although it's utterly impractical to check due to the board mounted pots.

I think that the most telling symptom is the behaviour of the sustain pot, but I have no idea what to infer from there. Does anybody have an idea?

[pryde]

Do you have a signal probe?

Isolate to see at what stage you are loosing your signal.

Your sustain pot is after the first gain stage (Q1). Is the resistor from lug 1 of the sustain pot correct? It should be ~1k and tied to ground.

[croquet hoop]

I do not have a probe, but that would be a good occasion to make one. Am I really losing my signal though? (since I still hear some sound as long as the sustain pot is past half the rotation)

I just checked the resistor in question (R7 in the mudbunny schematic), the value is right (1k), and it is properly connected to ground (checked with my dmm in continuity mode). I'll try to reflow things around the sustain pot tomorrow, just to be sure.

[pryde]

I assumed you were losing signal when you stated the output was weak. If you feel like the circuit is putting out good volume (just sounds muffled) then it could be a lot of possibilities.

A signal probe will still tell you where you are getting strong, powerful sound vs. weak, muffled sound in the signal path. As you said, good opportunity to make a simple probe.

[croquet hoop]

I reflowed the parts around the sustain pot/Q1, and it seems to work properly now (only affects gain, not overall volume). The sounds remain weak though, but I found out that it sounds the same whether there is a transistor in the socket at Q3 or not. In other words, as of now Q3 is not in the circuit, so I'll try to reflow things in that area as well.

edit: but is it normal that sounds passes when Q3 is removed? There is no more sound when I remove Q2, and according to the schematic their implementation is identical. Is that a sign that there's a short somewhere?

[pryde]

You would still have signal passing if the transistor is removed because the base and collector of Q3 are tied together with the clipping stage. You should notice a drop in volume or some noticeable difference with the transistor removed though.

Are you saying there is no audible difference in or out? Maybe get some pics up

[croquet hoop]

That's it, there is no difference whatsoever in sound whether Q3 is in the socket or removed (tested with different transistors, in both orentations). Tried to reflow things around it, no no change. I'll take pics asap.

[Govmnt_Lacky]

Swap Q3 with another transistor? Duh... re-read the previous post! 

A schematic would do wonders!! 

[croquet hoop]

It's a big muff variant, so except some extra parts near the Tone pot, it's pretty much the same as any other BMP. The parts are not labeled exactly the same as a mudbunny, so I'm joining the schematic of this particular version, as well as the layout (the whole build doc can be found here).

Here are pics of the board, the light makes some solder joints look far worse than they are, but don't hesitate to tell me if you spot something fishy.

[Govmnt_Lacky]

Couldn't help but notice that there are NO successful build posts in the thread you linked to.

Could you have possibly gotten a bad board? Unless you can confirm a successful build on that version of the PCB, you HAVE TO consider that and figure it into your debugging equation. 

[croquet hoop]

Well, the board is from culturejam, and I think it's safe to assume that my building skills are a bigger liability then his designing skills 

I remember a bigger thread about that board, I'll try to find it back. I have a few more of these , so I will put another together and triple check everything as I go. If that one fails too, well, we'll see.

[pryde]

R16 appears to be incorrect (Q3 emitter resistor). Should be 390R and it looks like you have 390K

Let us know

[Govmnt_Lacky]

R16 appears to be incorrect (Q3 emitter resistor). Should be 390R and it looks like you have 390K

Let us know

I concur! Good eyes sir 

Change that puppy out for a 390R  Also, it would probably be a good idea to check ALL of your values again. In for a dime..... in for a dollar!

[croquet hoop]

R16 appears to be incorrect (Q3 emitter resistor). Should be 390R and it looks like you have 390K

Let us know

Hats off to you, sir. I actually managed to misread the BOM and use a 390k instead of a 390R. I'll check the rest too, but it looks like you spotted the culprit.

[croquet hoop]

Okay, replaced it, no change, fiddled a bit with the transistors, and got it to work! So there was something fishy with at least one transistor, but R16 was definitely wrong to start with. Now it roars as expected, thanks a lot for your help guys!