Big Muff Pi not muffing

[armdnrdy]

Hi all,

I am very embarrassed to be writing this, asking for help with something so trivial. 

I built a Big Muff Pi, based on the GGG schematic and layout. I built this to blast through the Mutron Flanger build that I completed to showcase the slow sweep and the pedal sweep. I routed my own board and made a few minor changes that are irrelevant to it's non working status.



The problem: The tone control does not change anything tonaly....it acts like a volume control.
There are no solder splashes, no parts in the wrong place, and no bad solder joints. I have checked continuity though out and have lifted relevant components to check for bad parts. I pulled and checked Q3.

My voltages are:

Power supply
8.9v

Q1
Emitter -      .03
Base -          .63
Collector -   4.1


Q2
Emitter -      .03
Base -          .63
Collector -   4.1


Q3
Emitter -      .05
Base -          .66
Collector -   1.1


Q4
Emitter -      1.03
Base -          1.63
Collector -    4.2

Thanks in advance for your help!

[duck_arse]

seeing as the q3 stage is the same as the q2 stage, the voltage at the collector of q3 should, logically, be the same as at q2, no? check your q3 stage resistor values are as marked.

[R.G.]

The problem: The tone control does not change anything tonaly....it acts like a volume control.

Check the continuity on the path that includes R19. If R19 or any point along the path through it is open, the tone control would act like a volume control.

Q2
Emitter -      .03
Base -          .63
Collector -   4.1


Q3
Emitter -      .05
Base -          .66
Collector -   1.1

D.A.'s comment applies. I would expect these two stages to be more similar in voltages.

[armdnrdy]

Thanks for the replies.
I'll lift a few more components and check that marked values are correct. It is very uncommon, but I have sourced caps that are nowhere near the marked value.

Something sooooo simple....a Big Muff! 

[armdnrdy]

A bit more info:
I lifted one side of R19. It measured correctly. (39K)

Here is Q3's voltages with R19 lifted:

Q3
Emitter -      .02
Base -          .61
Collector -   5.8

I also lifted one side of C10. It measured correctly. (.024µf)
With R19 and C10 lifted the first three stages are isolated from the output stage.

[Kesh]

I would check the potential at the other end of R19 from Q3. Maybe C11 is blown/shorted, or maybe the pot is touching ground somewhere.

[armdnrdy]

Before I checked for new posts I changed C11 to the original .01µf, lifted and checked many of the surrounding components, changed Q4, (the voltages looked a bit off) disconnected the tone pot, checked for correct value, (the pot isn't touching ground) changed R14, went through the GGG board routing and my board routing with a marker.....no anomalies.

No change! The tone control still acts like a volume.

I'm running out of things to check and change! This is unbelievable! I'm being beaten down by a Big Muff!  

Even though when pulled and tested Q3 showed good...I'm going to pull it and change it.

Edit:
Just changed Q3 and C9.....no change in Q3's voltages.

Changed out R17 (15K) even though I had lifted and checked it. Still the same! 

[armdnrdy]

Tried a few more things...l removed C7 to isolate the third stage. Took voltage readings...no change.

Here are some images of the layout and the build:

[armdnrdy]

THE BIG MUFF PI IS NOW MUFFING WITH A WORKING TONE CONTROL!!!!!!!!!

If the cap letters look like I'm screaming.....I am!!  

I replaced R15 and got the correct voltage for Q3.

I measured R15 previously when I removed Q3...it measured okay.
When I just removed R15, it measured okay. earlier in the day I reflowed all of the solder joints...so it's a mystery.

There was a hex on this build!  

But now it's gone!