Russian Muff Clone - False Transistor Voltage at Q3

[pud3000]

Hi,

i'm building the Russian Big Muff with this Layout:
http://www.fredric.co.uk/misc/green%20russian%20big%20muff%20stripboard%20veroboard%20layout.gif

I build it twice.

And on both circuits i noticed that the volume is extremely LOW. The Clean Signal from the Amp is much louder than the muff... I compared it with my stock BMP and i realized that this isn't normal....

I measured this:

Q1
C: 4,57 V
B: 0,74 V
E: 0,15 V

Q2
C:4,62 V
B: 0,74 V
C: 0,15 V

Q3
C: 0,88 V
B: 0,88 V
E: 0,3 V

Q4
C: 4,56 V
B: 1,6 V
E: 1,01 V

Battery: 9,71 V

Q1-Q4: 2N5088 - brand new...

Volumepot: 100k log
Others: 100k lin

So i came up with the idea that it must be something around Q3, but i cannot find any mistake.  I built it twice, and i get the same Voltages on both circuits, so i double checked with the layout and searched for bad soldering points, wrong parts etc... nothing.

Now my question is if anyone has built the muff with this layout too, or even faced same problems with it?

Thankfull for any advice....

Thx, Tino.

[slashandburn]

Hey man, sorry I cant be of more help, but while you wait on a better answer, have you built one of those audio probes yet? I cant tell you how many headaches its saved me. Its only 3 or 4 parts.

You should pretty much be able to hear where the problem is in the circuit. Its so simple and effective it blew me away.

http://www.diystompboxes.com/pedals/debug.html

[LucifersTrip]

you're lucky that it's q3, because you have a seemingly good working stage (q2) to compare it to, which is identical.

find the schematic that your layout is based on...maybe something like this:
http://oi10.tinypic.com/8bqlhjc.jpg

and start by measuring the in-circuit resistances of the resistors surrounding q2.  are they the same as the in-circuit  resistances of the resistors surrounding q3?

[R.G.]

It is possible, but very rare, for two transistor pins to have exactly the same measured voltage. So rare that when you find it, you probably ought to immediately suspect that they're either shorted together, or that they're connected by a resistor and no current is flowing through the resistor.

Based on the voltage evidence, I'd guess that it is possible that the base and emitter pins are shorted, perhaps in some subtle way. It is also possible that the collector resistor is open. That would connect the base to the collector by the "feedback" resistor, which might be carrying so little current that the two pins would look like the same voltage.

LT is right - find the schemo for what you think you have, and start measuring resistances to power, ground and transistor pins. The differences between the Q2 and Q3 stages are a huge help to you. The trick is to figure out why they are different.

[pud3000]

Thanks guys!!

I looked on the circuit again, for hours. No wiring mistake, no shortcuts around there.

The only difference to the Q2 stage was R15: the layout says it has to be 470k. I built those two boards in parallel and after assembling the circuitry around Q1 and Q2 (twice) i went out of 470k resistors. So i decided to use 500k's. According to the bigmuffpage.com, R15 Reduces gain if the value would be decreased. So, i thought 30k more should not matter. WRONG!!

I soldered the 500k out, took one 470 out of the other circuitboard: WORKS. Measuring Q3: Right Voltages. On the Amplifier: LOUD!

I never would have thought that this circuit is so sensibel about little changes. Lesson learned.

Thank you guys!

[Pyr0]

The big muff circuit isn't that sensitive to resistor changes, it's a great circuit to experiment with.
I would hazard a guess there was a hairline short, and by replacing R15 you just happened to remove it.
Did you try checking with you meter for continuity between collector and base of Q3 before you replaced the resistor ?

Glad you got it working.

[pud3000]

Yes I checked continuity on the Q's. None.

Yeah i thought so, too. But the same hairline short twice? I had the same voltages on the second muff i built so i'm pretty sure its all about the 30 k more at R15.

Maybe 470k is the upper limit for that feedbackresistor to put voltage back into the transistor correctly. I dont't know.

As soon as i get some 470's i will put one into the second unit and check if it solves the problem on that board too.

best regards,

T.

[LucifersTrip]

The big muff circuit isn't that sensitive to resistor changes, it's a great circuit to experiment with.
I would hazard a guess there was a hairline short, and by replacing R15 you just happened to remove it.

+1
almost definitely there was some sort of error corrected after the replacement...ie, maybe a 500 instead of 500K.
you can easily do a test with a 1M pot subbed there and slowly increase, watching the voltage change. there will be little
change from 470K to 500K...

note that if a 470K 10% resistor was used anytime during production, it could be ~ 425K - 515K, anyway

[bluesdevil]

Yeah, Lucifer may be right on the money.... it happened to me when I was sent mislabeled resistors years ago!
Measure that "500k" and see what value you get.