Yet more Big Muff Pi trouble...

[Chawk]

Here's my step by step troubleshooting list

1) I built the Big Muff Pi "Green" version from Tonepad.
2) http://www.tonepad.com/getFile.asp?id=94
3) I followed the layout pretty closely. I did make a few substitutions in the resistors but they were very close subs.
4) I did swap the 2n5088s for 2n4401s.
5) NA
6) It works fine in bypass mode but when the switch is engaged there is no sound at all.


Battery voltage is at 8.15

Q1)
C = 4.68
B = 0.70
E = 0.11

Q2)
C = 8.14
B = 0.93
E = 0.38

Q3)
C = 8.14
B = 1.06
E = 0.55

Q4)
C = 8.14
B = 0.84
E = 0.29

D1
A = 91
K = 91

D2
A = 91
K = 91

D3
A = 84
K = 84

D4
A = 84
K = 84

Volume pot
Lug 1 = 0.00
Lug 2 = 0.00
Lug 3 = 0.00

Tone pot
Lug 1 = 6.84
Lug 2 = 2.78
Lug 3 = 1.24

Sustain pot
Lug 1 = 0.00
Lug 2 = 0.00
Lug 3 = 0.00

[killerkev]

Here's the little bit that I can offer. I'm no electronic master but I have built and played with many BMP's. !st, you should really start with a better battery but moving on:
Q1- C looks kinda high but should work
Q2-Q4 - C voltages are way high

If you have a trusty audio probe I'd probe it from the begining and see if you have sound up and past Q1. Likely, your problem lies somewhere beginning with Q2. Check all solder joints, maybe reflow them. Check your resistors values to make sure you mixed up a value.

[Chawk]

Thanks for the tip on the audio probe. Is there a value for the transistor voltages that I should be shooting for? You said that the voltages I had were high.

[petemoore]

  First Q bias looks good.
  the last three the collector is almost V+, Something common to all three latter stages maybe is putting them out of operational bias, what is needed for that is shown on the schematic, and the DMM can be used to verify or test all these values.
  Collector resistors measure'__'?
  Orientation of transistors is absolutely correct? [check it again again].
  Emitters all have small resistors to ground?

[Meanderthal]

 Agreed- the last 3 gain stages seem to have the collectors pretty much shorted to positive.

[Chawk]

Would that be a cause for now sound? It's completely silent when engaged. I'm pulling my hair out trying to find the faulty component. Do resistors go bad? All of mine were new but they are a combination of carbon and metal types. The caps and trannies are new. Must be a bad trace somewhere. But I don't see it. The weird thing is when I touch the volume pot to the output jack there is sound it's just not distorted or anything.

[TELEFUNKON]

bump 

[Chawk]

BTW I made the circuit following the GGG layout and it fired up on the first shot. So either I royally screwed something up with the tonepad layout or there is a problem with the layout itself.

Side note: I socketed nearly every cap and all of the trannys. I highly recommend doing this as the range of tonal characteristics can change dramatically with a few cap replacements. So if down the road you decide you want a little more low end to come through it's easy to swap what you need in and out.

Matt

[johngreene]

Let's do a little circuit analysis.

Q1: emitter current = .11/390 = 282 uA
      Collect voltage = 8.15 - (282uA*12k) = 4.76V 

So far so good.

Q2: emitter current = .38/390 = 974uA
      Collector voltage = 8.15 - (974uA*12k) = 8.15 - 11.7

This is saying that with the voltage you are measuring on the emitter, the collector resistor should be dropping all the voltage and the transistor is basically shorted. So it should not be possible to measure 8.14V on the collector.

But this voltage and current in the emitter has to be coming from somewhere.....

If I had to guess, I would bet that the collectors are actually tied to the supply voltage and the 12Ks are not in the circuit. Either that or they are not 12Ks but instead are 120 ohms.

--john