Triangle PNP Big Muff Pi - Oscillation problems

[morales72]

Hi to all!

I've built this clone last night using carbon composite resistors, mixed ceramic/film capacitors, 1N914 diodes and 2N5087 transistors. I've tried to emulate the one in http://www.singlecoil.com/triangle/triangle.html
Sounds great, smooth and creamy but I cannot get rid off an ugly oscillation back ground sound. It changes its frequency by rotating the sustain pot. I've checked the PCB and grounding a dozen times and tried several transistors, even NPN with reversed polarity.
I usually tend to think that's about my mistake, since I always find a shortcut or a bad connection but I'm starting to think that maybe some component is working wrong.
I also get lower collector tensions, between 3 and 4V, but never over that. I've changed collector to negative Q1 resistor to 10K and I got 4.11V at Q1 collector to ground, but didn't solved the oscillation problem. when I unplug Q2 or Q3 from their sockets, I still get fuzz, less, but without the oscillation so I guess the problem is around those two Qs. I will post the exact voltages tonight when I get home.
Please, this is the first time I can't solve the problem by myself. I would really appreciate some tips, if it's not much to ask.

Thank you,

Fernando.

[geertjacobs]

Oscillation is often a matter of unwanted feedback between output and input.
Shortening the wiring can help.

Any chance of posting a picture of your wiring?

[morales72]

Thank you geertjacobs for your kind answer. Yes, definitely this evening I will.

[morales72]

This is the populated PCB and pot wiring:



These are transistor voltages:

Q1
C = 3.42
B = 0.55
E = 0.03

Q2
C= 2.08
B= 0.64
E= 0.10

Q3
C= 3.41
B= 0.55
E= 0.03

Q4
C= 3.25
B= 1.56
E= 1.13

You'll notice that it's mirrored  sorry, my dislexia plays tricks on me when drawing PCBs...

Any help will be appreciated.

Fernando.

[morales72]

Recent update:

I removed one by one the components following the schematic and I've found that replacing the 50nf capacitor from Q2 stage made it sound better. I tried with another identical one and it worked. I still got motorboating so I removed the 390K resistor from the same stage and the sound (and fuzz too) was gone. It gave Q2 collector a value of 6.82V so I tried different resistors from the same and classic values, but they just changed the noise frequency. I did the same fro Q3 stage, but nothing changed. So I guess the problem is around Q2. I'll keep you updated.

[springtank]

M72,

The original schematic doesn't seem to have ANY filtering caps on the power supply.  You might try adding one or more 100uF caps across the power supply to see if the problem is lessened.  Feedback betweeen stages through the power supply could be causing your trouble, and it is always a good idea to have some kind of power supply filtering.

I would try adding 47-100uF caps to the power supply as close to the transistors as possible.  Remember to get the polarity correct, and good luck.  If that doesn't do the trick, the Goddess of Electrons is trying to tell you something.

Brian

[morales72]

Hi, Brian, thank you for your answer. I tested with a 9v battery and with a regulated power supply that's quite silent regarding hum.
Anyway, I did put a 100uf cap between the poles and some of the noise went away, but the main oscillation/motorboating didn't.
Could the low collector voltages generate that sound? I'll keep replacing components to see what happens.
That lady you mention, probably hates me...

Fernando.

[col]

I had the same problem with mine when I built it. I grounded everything to the backs of the pots and the case and shortened all the wires as much as possible. It's now fine apart from if fuzz and vol are both on full when it becomes uncontrolable. I used BC184c transistors but I have also built several with BC549cs and they were fine.
Try changing the value of the caps across the BC junctions of the transistors as this sometimes helps but it will alter the tone of the pedal. Try small increments (pF) by piggybacking on the existing ones to keep this to a minimum, (I can't remember what the existing values are.)
Good luck with it, this has been my gigging fuzz for years now.

[morales72]

Thank you for your answer, col. I pull off the 390K resistor from Q2, and the noise was gone. I replaced with a 100k film cap   and it fuzzed again, different tone of course. I will try your suggestions this afternoon. Since I mirrored the circuit, I got the tone and volume pots on the opposite side, so I used longer wiring for them. This could be one of the problems. I'll keep you updated.
I also got a reissue BMP that's always on my rig, but I usually combine it with a TS808 clone before it, so I get more midtones.

Cheers,

Fernando.

[johnyboy_pt]

Cheers,

I have an electrically unmodded (except for true bypass) store-bought 2000-ish Black Russian BMP Reissue, with this exact same problem (I believe). I have not altered the effect circuit in itself in any way (I just mounted the PCB "as is" on a different case and wired it up for TBP), but I still got the same problem. I experimented with the suggestions provided here so far, but I got the same results as Morales72 (changing resistor values changes noise frequency, nothing really just "cures" it).
One thing I did notice is that the "noise" is more like a tuned frequency (like a dial tone), and according to my tuner its about 415Hz, with the Sustain knob acting like a band-pass filter for it (besides its regular function). Running the Muff at 7.5V seems to change the tone to more of a motorboat-like frequency ramping as you describe.
I remember reading about some mod for the BMP that makes it self-oscillate. Maybe that's what mine's doing, although it didn't do it before and I did not change anything in the circuit...

Morales72:
Did you manage to solve the problem in the end?
If so, could you kindly share your solution?

Much appreciated if anyone can help, I've googled myself hopeless trying to solve this...


John

P.S. (In case it helps): At 9V flat, I'm getting around 4.60V on all collectors but Q4, which puts out 5.50V.
This is the circuit: http://i1016.photobucket.com/albums/af286/fenderamerica/Big%20Muff/SOVTEKBMP.png

[LucifersTrip]

tried to emulate the one in http://www.singlecoil.com/triangle/triangle.html

then why use such high gain transistors?

similar to the Triangle I built, those transistors use 60's button style. I have a bunch of those types in my collection and the hfe's are rarely over 150, most below. It is possible that the gains could be higher, but I'm just playin' the odds...I have some of the transistors (2N5133) used in the original Triangle and the hfe's are < 150. The datasheet shows they can be as low as hfe 50

I also get lower collector tensions, between 3 and 4V, but never over that.

...which is exactly what will happen if the transistors have higher gains

here's an approximate chart for Q1 (and probably close for Q2/Q3. Q4 is biased differently)

hFE ___ Vc
1 _____ 8.2
3 _____ 7.77
10 ____ 6.7
30 ____ 5.54
100 ___ 4.65
300 ___ 4.28
1000 __ 4.14
3000 __ 4.1

In the end, I used Q1 - Q3 ~ 80 hfe and Q4 ~ 125, which gave me closer to 5V on all collectors.
http://www.luciferstrip.com/fuzz/bigmufftriangle-finale.jpg

Also, my 80's Green Russian, which I kept because it sounded so good has 5.45V on Q4C

Thread:
http://www.diystompboxes.com/smfforum/index.php?topic=98421.0

[morales72]

Wow! A lot of water under the bridge in this one...

then why use such high gain transistors?

I guess at that moment were the only ones I had around, plus they were the recommended from different websites. After that, I got a bunch of the button style transistors you describe, though not the famous 2N5133. I didn't try them since the clone sounded so good. Nowadays I'm using the second clone I made of the BMP based on one of the earliest versions of the Ram's Head which I prefer better. I always carry the Triangle as backup though.

Anyways, you awaken some curiousness inside me and I will try those cool looking transistors when I have the time. I might even build an 80's russian clone too! Thank you!

Best regards,

Fernando.

[morales72]

Morales72:
Did you manage to solve the problem in the end?
If so, could you kindly share your solution?

Hi, John! It's true I read your replay after Lucifer's and I was going to answer you but I got winded up with my baby daughter.

It's embarassing but true story: there was a little, almost unseen shorcut between two tracks. Don't remember which ones exactly but when I cut it the pedal worked like a charm. This probably won't do it on your BMP since it's a commercial PCB, instead of the "old school" psychedelic traced I build. Anyways, please, check if there's no remaining solder in between tracks.
I know it's not a great solution but this build taught me never to give up on checking PCB tracks and wiring before blaming components, a common rookie mistake. Most of the time, they are OK, specially in these simple builds.

Hope this helps and sorry for the delayed answer.

Best regards,

Fernando.

[johnyboy_pt]

No worries, thank you very much. I apologize if I came across hasty but I really need to get this pedal back up and running ASAP...
I read somewhere on the EHX forums that sometimes older Sovtek BMP's get hairline cracks in the PCB traces, which can cause unwanted capacitance and consequently all sorts of "additional" sounds to the fuzz.

So between your case (unwanted short) and the one over there (unwanted PCB trace cut), looks like I'm in for some serious time troubleshooting this baby 

[morales72]

No worries, thank you very much. I apologize if I came across hasty but I really need to get this pedal back up and running ASAP...

No problem, I've been there 

I read somewhere on the EHX forums that sometimes older Sovtek BMP's get hairline cracks in the PCB traces, which can cause unwanted capacitance and consequently all sorts of "additional" sounds to the fuzz.

That was the other option I forgot to mention.

So between your case (unwanted short) and the one over there (unwanted PCB trace cut), looks like I'm in for some serious time troubleshooting this baby 

Debbuging, debbuging and then some more debbuging...but come on, we do this for fun!

Best of luck with your project.

Regards,

Fernando.

[johnyboy_pt]

OK I'm almost ashamed to admit this: it was the power plug.
For some reason, the plug I was using to plug batteries and PSU's into the pedal was acting up. After going through the circuit once again checking for shorts and cracks, I started fiddling with the plug (I remembered it made some squeaky noise whenever I tilted the barrel), and after another round of absolute silent despair (the pedal, in "ON" mode, just hummed like a lightsaber - cool but not what I was after...) and an "anything goes" plug-swapping, everything is back to normal.

Lesson learned: always make sure your plugs and jacks are working stable and fit snugly.

THANK YOU VERY MUCH! This site restored my confidence in messing around with analog electronics =)