Big Muff Pi Transistor Voltage problems

[killerkev]

Nothing worse than working for hrs. trying to debug, then coming to this forum and writing a bunch of requsted help to only be kicked out and lose eveything you've wrote 'cause it wasn't posted quick enough. Anyway, I'm signed in now forever!  Problem is with a Ram's head big muff Pi. Hrs. of probing with the audio probe and taking voltage readings. The engaged sound is quiter that the the clean bypass signal. Below are voltages:


Battery: 9.36V

Q1
E - 0.01
B - 0.59
C - 7.37

Collector seems pretty high but the audio probe sounds correct...

Q2
E - 0.02
B - 0.61
C - 3.86

This looks good I would think

Q3 (the problem child)
E - 0.11
B - 0.79
C -0.12 

Collector seems to be the problem as indicated by the audio probe almost going silent here.... more info to come

Q4
E - 1.18
B - 1.76
c - 4.37

Looking good her but quiet due to Q3.

I've spent a lot of time desoldering components out of the path to see if there was a negative influence from them but no luck.  I did have continuity between the collector & emittere of Q3 which was blowing my mind but after removing the 150 ohm resistor out of the emitter path & replacing with new removed the continuity but still didn't effect the voltage on Q3 collector. I did remove the original 2n5088 tranny  and replaced with new but same story....

I probably typed more in the first post that was lost but I'm still hoping someone can shed some light on what to look at now.

[bumblebee]

when that happens,press the back button copy it,log in again then paste , no re-type.
cant help ya with the muff as ive never made one and dont know a lot about them.

[R.G.]

It is very likely that the collector of Q3 has too high a resistance to +9V. Measure the voltage on each end of the collector resistor. If they're the same, no current is flowing through the resistor and there is an open circuit from 9V to the collector.

[killerkev]

You're refering to the 15K resistor that feeds the power to Q3 right? I've done that, one side is the battery voltage and the other side of the 15K resisitor is 0.12V, the same problem voltage. I took that resistor out of the path and like you said, it has the same high battery voltage on either end. What about that continuity between the collector and the emitter? I found another post where the guy seems to have the problem I'm having, no with the voltages but with how the pedal is sounding.

http://www.diystompboxes.com/smfforum/index.php?topic=59122.0

[R.G.]

You're refering to the 15K resistor that feeds the power to Q3 right? I've done that, one side is the battery voltage and the other side of the 15K resisitor is 0.12V, the same problem voltage.

OK, one problem eliminated.

There's only a few ways to get the collector down that low. One is a short between emitter and collector. You posted that you had that, but fixed it. Is it possible that it's not quite fixed?

Maybe test that with your DMM's resistance setting while the power is off to the pedal.

[killerkev]

With the original carbon comp 150 ohm resistor I had continuity between the collector and emitter of Q3. I replaced it twith a new metal film resistor which DID stop the continuity but DID NOT fix or even change the voltage readings. I guess since it's a new day, I look at it a again this afternoon. You suggest that I test the resistance between the emitter and collector while the pedal is off? What am I looking for? It should read infinitey if there was no short right?

[R.G.]

Actually, it may have some reading less than infinity because of the other resistances in the circuit. But it should definitely be more than, say 10K.

It would help to use a magnifier to examine the copper traces and soldering very carefully under that transistor.

[killerkev]

The reading is 68K with the power off. An interesting tidbit... with the power off there is no short between the collector and emitter.  Once I turn on the power, the continuity alarm sounds.  What do you make of that? I still have the new metal film resistor in place.  As far as solder bridges, there is plenty of room between all three legs of the tranny.  I triple checked the tansistor on the other side to ensure that the legs are not touching eachother.  I have about two hours to dedicate to it today.  Anything else?

[R.G.]

Once I turn on the power, the continuity alarm sounds.  What do you make of that?

Only that something is turning the transistor on hard. But we already knew that. What's going on is that something is providing more current to the base than it needs to operate properly.

(1) Are you sure that the transistor that is in there is the correct type number?
(2) Are you sure that the transistor that is in there has the leads stuck in the correct holes? (i.e. pinout)
(3) what is the voltage on both ends of the emitter resistor?
(4) what is the voltage on both ends of the resistor between collector and base?
(5) what is the voltage on both ends of the capcitor between collector and clipping diodes?
(6) What is the value of the collector-base resistor, both reading the color code and by measurement?
(7) Ditto for base-to-ground resistor.
( what is the DC voltage on both ends of the 8.2K input resistor? Maybe the preceeding stage DC blocking cap is shorted and feeding in too much current from the previous stage's collector.

[killerkev]

(1) The transistors are the original 2n5088 Fairchilds
(2) Absolutely the correct pinout
(3) 0V ground side / 0.11V emitter side
(4) 0.11 V colletor side / 0.76 base side
(5) 0.1 um capacitor diode side is 0.04V / 0.77V on the other
(6) This was interesting....color code: Yellow/Purple/Yellow/Silver 470K right? Measured value: 32.7K!! Should I be taking these out of the board to measure? I measured the other 470K resistors by Q1 &Q2 and had readings of 98.4K & 127K. What's the deal here?
(7) color code: Brown/Black/yellow/ Silver 100K right? Measured value: 58.6K!

This one has 10K between Q1 & Q2 and again between Q2 & Q3. 0.6V on each side between Q1 & Q2 and 3.6V on Q2 side / 0.77V on Q3 side.


I took voltages on all components. I will post a picture with all the voltages so you see everything later to night once the little one goes night night.

[R.G.]

This one has 10K between Q1 & Q2 and again between Q2 & Q3. 0.6V on each side between Q1 & Q2 and 3.6V on Q2 side / 0.77V on Q3 side.

OK, we found the problem. At least we found A problem. The correct value for that resistor is to have the same DC voltage on both ends, because one end should be DC blocked from the preceeding stage by a capacitor. We find instead that the end toward the previous stage is at a higher voltage.

That can only be because there is DC coming from Q2's collector through the "DC blocking" capacitor. Check to see if the capacitor between Q2 collector to the resistor to Q3's base is shorted, solder shorted, or inserted backwards if it's an electrolytic.

[killerkev]

They all happen to be ceramic in this one. I will just put a nice new film cap there to see if anything changes. Stay tuned!

[petemoore]

  Simply pulling one end of the cap will effectively block DC.

[PerroGrande]

In general, it is necessary to measure resistances out of circuit.  There could be various resistances in parallel that will change the reading.  Also, that said, resistors *rarely* fail unless some large currents or temperatures are involved.

I'm a little bit curious -- are you using a schematic that is the same as the one posted on GGG?   http://www.generalguitargadgets.com/pdf/ggg_bmp_ram_sc.pdf

If so, I'm a little curious about the orientation of the 1uF capacitor marked as C9 on the GGG schematic.  It is shown as an electrolytic, but the polarity seems opposite of what it should be.  If that cap is shorted, or just leaking like crazy, that could sufficiently pull up the base. 

So -- petemoore is absolutely correct -- simply pull that cap (C9) and see what happens to the voltages...  The stage won't have the clipped feedback at AC, but at least you'll see if that is the culprit. 

[killerkev]

Yes, I'm am using the schematic from GGG. There are no electrolytics on this board and the capacitor is a 0.1 instead of 1uf as stated in the schematic. I think that 1uf is a typo. Refering to the GGG schematic, I believe R.G. is refering to C7 not C9 right R. G.? By taking out C7, wouldn't I be breaking the circuit?

[PerroGrande]

C7 is a coupling capacitor that removes any DC offset introduced by stage 2. 

I think you're right -- I think RG is talking about C7.    I was theorizing about problems with C9, which is part of the clipping/feedback in stage 3, throwing the biasing of this stage out of whack. 

However, if DC is leaking through from the previous stage, which it seems to be based on your readings, then the same end result is possible.  If you remove the cap, it will effectively break the signal path.  However, it might be interesting to see the Q3 voltages with C7 out.  If stage 3 is otherwise working properly, I'd expect to see voltages that are less like Q3 slammed "on".

[killerkev]

  BINGO!!!

I pulled one leg of that C7 capacitor and the voltages were:

B - 0.61
E - 0.03
C - 3.58

Also, no more continuity between the emitter and collector!!! Now I need to put it all back together and plug it in!

Ok, did that and did a quick test at real low volumes since family is sleeping and sounds like it's back to normal. Will have to test it out tomorrow.
Thanks R.G. for hanging in there with me. I believe I learned somethings here, maybe not a complete understanding of what a DC blocking capacitor really means but I'm sure this tread will help others in the future.

[PerroGrande]

Good! 

A DC blocking capacitor (also known as a coupling capacitor) does exactly what the name implies (when it is working properly, of course) -- it blocks DC.  That is -- if the output from the previous stage contains a DC component, the capacitor removes it.   Capacitors, in simple terms, "pass AC and block DC".  So, what is left after the signal passes through a capacitor in series (like the one you are working on) is the "AC" component -- or, the waveform that we are interested in (audio) without a DC component. 

If one were to look at it on an oscilloscope, an audio signal without a DC component (i.e. passed through a coupling/DC blocking capacitor) would be a waveform centered on the zero (center) reference line on the scope.  The same signal with a DC component would show the same waveform, but no longer centered on the scope, but lifted (or dropped, if the DC voltage is negative) to a different center point.

What happened in your case --

the capacitor in question wasn't doing its job...  so the signal was getting through, but so was some DC offset.  The next stage in line (Q3) was designed to expect NO DC offset in the signal. The biasing was set based on the assumption of a "zero-centered" audio signal.  What it got, thanks to your cap, was one that was offset.  This threw the biasing of Q3 off -- enough so that it slammed "on" and pulled down the collector to near ground -- regardless of how small the input signal was.  Result -- no sound past that stage.

This is a fairly classic case of multi-stage troubleshooting.  Follow the signal with an audio probe, isolate the first point at which things fail, and lock in on the culprit.  I've fixed countless amplifiers that went "dead" due to a bad coupling capacitor (typically an electrolytic) somewhere in one of the early stages. Not all that different from a pedal circuit.