Big Muff Pi Conversion Troubleshooting!

[Jakub421]

Hey everybody.

I bought a new big muff pi just a couple days ago, didn't really like the sound, so I decided to mod it to a Violet Ram's Head.
(This schematic http://www.bigmuffpage.com/images/schematics/KR_1973_V2_Violet_Schematic_1st_version.jpg )

Now the issue is that i get no sound out of the pedal. All of the transistors are getting the correct voltages except for Q3 on the schematic. The base and collector are getting almost 0 volts, and the emitter is getting only about .7 volts, while it should get around 4 - 4.5.

One side of R18 is showing me 8.7 volts from the battery, while the other end is showing me the .7 volts. Any ideas as to which components may be causing this issue?

Thanks!

[armdnrdy]

Just so us individuals who do not know every Big Muff circuit inside and out can try to help you.....

Which components did you switch out?

[LucifersTrip]

All of the transistors are getting the correct voltages except for Q3 on the schematic. The base and collector are getting almost 0 volts, and the emitter is getting only about .7 volts, while it should get around 4 - 4.5.

The collector should be around 4.5V. Did you mean "The base and emitter are getting almost 0 volts  and the collector is getting only about .7 volts"

One side of R18 is showing me 8.7 volts from the battery, while the other end is showing me the .7 volts. Any ideas as to which components may be causing this issue?

Sorry for the obvious answer, but you're looking for components surrounding Q3...but more than that, you're looking for solder bridges around Q3.
...and also Q3 itself, did you possibly heat up a leg and blow it out.

If you have nearly 0V on B & E, they're somehow nearly shorted to ground (E is close to ground anyway). What's the resistance between B & ground?

Regarding R18, one side should be your supply (near 9V) and the other side Q3C, so yeah ~ 4.5V. What's the resistance across R18?

...and the answer to armdnrdy would prob help. what subs did you make?

[Jakub421]

Yeah, I did mean the collector.

The components I switched out were R2, C1, R13, C4, R23, C5, R19, C6, R21, R18, C13, R12, C7, R11, R10, R8, R5, C3, and C2. Pretty much everything that was different, and I have transistors coming in in a few days.

R18 reads at 11.8k ohms.

The schematic to the reissue that I have is here: http://www.bigmuffpage.com/images/schematics/V9%20NYC%20REISSUE%20BIG%20MUFF%20SCHEMATIC_1st%20version.jpg

I just don't understand why I'm getting .7 volts after R18, but the normal 4.5 across R13. I redid all solder connections and checked value, but I cannot for the life of me find out what I did wrong.

[LucifersTrip]

What's the resistance between Q3B & ground, Q3E and ground (re-check R21), and between Q3B & Q3E?
Did you re-check if Q3 was blown?

[Jakub421]

Q3E: 103 ohms
Q3B: 82k
Q3B and Q3E: 84k

How can I check if it is blown?
Could I replace it with a known working BC108 just to test?

[LucifersTrip]

looks reasonable. yes, you can try any si npn with reasonable hfe...just match the pinouts.

[Jakub421]

Replaced with BC108 - did not work.

Resoldered all components near Q3 again - worked great for about 10 seconds, then started sputtering and volume dropped significantly 

No idea whats going on.