Debug help - Big Muff Triangle (tonepad) - reverse voltage applied to circuit

[scondon]

Barely an hour after proudly finishing and boxing this build, I applied a few seconds of reverse voltage to the circuit.    It now sounds decidedly underpowered for a BMP.  I did some searching and found R.G.'s very helpful "Repairing Power Fried Pedals" post (http://www.diystompboxes.com/smfforum/index.php?topic=55003.msg421856#msg421856 ), but this circuit doesn't have IC's and I didn't use electrolytics (was this a mistake?).  Should I replace the diodes?  Please lend me your debugging expertise.

1. Name of Project: Big Muff / Gran Mango, from Tonepad
2. Schematic: http://www.tonepad.com/getFile.asp?id=94  (triangle big muff variant)
3. I followed the project fairly closely, though I added a mid boost/cut toggle switch at c11 which works beautifully.
4. I substituted regular poly caps for the electrolytics. transistors are all 2N5088, diodes are 1N914.
5. negative ground
6. sounds pretty  good with sustain at max, but not like a bmp. very little distortion before then.

Measurements:
battery = 8.2 V

Q1
C= 4.33
B= .725
E= .141

Q2
C= 1.23
B= .7
E= .09

Q3
C= 4.14
B= .7
E= .13

Q4
C= 3.7
B= 1.6
E= 1.1

Thanks for any help or suggestions you might have!

[jlullo]

was it in working order before you applied the reverse voltage?

Q2 looks a little gnarly to me.... i'd swap out that tranny with a new one, and check the diodes.  If you didn't try it before you *possibly* toasted the transistor, it could always be a cold solder with one of the resistors around Q2

that's no problem that you didn't use electros either, as long as you used the recommended values!

[scondon]

was it in working order before you applied the reverse voltage?

Q2 looks a little gnarly to me.... i'd swap out that tranny with a new one, and check the diodes.  If you didn't try it before you *possibly* toasted the transistor, it could always be a cold solder with one of the resistors around Q2

that's no problem that you didn't use electros either, as long as you used the recommended values!

Thanks for the reply jlullo.  no, I didn't get a chance to test it before possibly toasting it.  I don't have a replacement tranny on hand but I've got one on order and will try swapping Q2 and all the diodes out. 

[jlullo]

You might not need to swap out the diodes.  What voltage readings are you getting on them?

[scondon]

The diode voltages:

D1 & D2:
.572
.701 (d1 cathode & d2 anode)

D3 & D4:
.574
.707 (d3 cathode & d4 anode)

You said Q2 looked off - is this because the collector is only measuring 1.2 V?  Should it be closer to 4.5 like the others?  Also, would reverse voltage toast any of the caps?

thanks again for taking the time to help, I really appreciate it and am learning a lot.

[petemoore]

You said Q2 looked off - is this because the collector is only measuring 1.2 V?
   Should it be closer to 4.5 like the others?
  Yes
   Also, would reverse voltage toast any of the caps?
  If it got to them, Iow if they're damaged, yes, I'd replace them if I think they've been RV'd.
  Otherwise, did you use a power supply reverse polarity protection diode? If you did..I've RV'd circuits and the RP diode did it's job protecting the circuit from RP.

[jlullo]

yeah, like pete said, Q2 is off.  Diodes look ok...

first, i'd check the resistors around Q2 to make sure they are the right values, and then make sure there are no cold solders...

[scondon]

Hmm, removed and socketed Q2 and Q4, then swapped them.  the swapped transistors read the same - Q2 is still low at the collector.  This means that the problem is not with the tranny, right? 

R10 looks to be the voltage divider supplying power to Q2, I checked it and re soldered it, it's fine.  Where to look next, the caps? 

Also, R8, which goes to ground has nothing in it per the tonepad parts list.  Should it still be wired or should it be left empty?

No, I didn't install a RV protection diode.  I will definitely do so in the future!

[jlullo]

I will check out the schematic when I get home, but possibly.  I can't remember without looking at the schematic.  Hold tight for a bit!

[petemoore]

Hmm, removed and socketed Q2 and Q4, then swapped them.  the swapped transistors read the same - Q2 is still low at the collector.  This means that the problem is not with the tranny, right? 
  If they're biasin'...just about for sure they're good, sockets are nice to have.
  R10 looks to be the voltage divider supplying power to Q2, I checked it and re soldered it, it's fine.  Where to look next, the caps? 
  If actually RV'd...no tellin' they might be and are what they are, I'd change 'em though.
  Also, R8, which goes to ground has nothing in it per the tonepad parts list.  Should it still be wired or should it be left empty?
  I'm having trouble bringing the schematic up or figureing out what R8 would be doing...
  No, I didn't install a RV protection diode.  I will definitely do so in the future!
  Cheap insurance.

[scondon]

sorry, tonepad doesn't allow direct links to files.  The project page is here http://www.tonepad.com/project.asp?id=3, then click on tonepad_bigmuff.pdf , the following page has the download link. 

I don't know enough to understand the purpose of R8, but it's a high value in the other Big Muff versions - 100k.  It's between the output of the sustain pot and the base of Q2.

[scondon]

Success! I got it working by replacing all of the caps. I learned something along the way:

Q2 apparently biases at 1.22 volts on the triangle version.  R8 (R9 on the GGG version here: http://www.generalguitargadgets.com/pdf/ggg_bmp_tri_sc.pdf ) helps to bias it.  I put a 100k resistor in R8 and it biased "correctly" at around 4v, but because of a bad cap somewhere it still didn't sound right.  After I recapped it I plugged it in with the resistor still in R8 (ggg's R9) and it worked. It sounded huge and sustainy and a little brighter than I was expecting it to, so I removed R8 per the schematic.  It sounded less gainy and slightly smoother and darker.

jlullo and petemoore, thanks again for all your help.

[jlullo]

wow, i wouldn't have guessed that it was the cap!  I was sure it would have been a cold solder somewhere

i'm very happy you got it up and running.  enjoy it- my triangle is one of my favorites!

[petemoore]

  "Murphy's Capacitor"...

[scondon]

Q2 apparently biases at 1.22 volts on the triangle version.  R8 (R9 on the GGG version here: http://www.generalguitargadgets.com/pdf/ggg_bmp_tri_sc.pdf ) helps to bias it.  I put a 100k resistor in R8 and it biased "correctly" at around 4v, but because of a bad cap somewhere it still didn't sound right.  After I recapped it I plugged it in with the resistor still in R8 (ggg's R9) and it worked. It sounded huge and sustainy and a little brighter than I was expecting it to, so I removed R8 per the schematic.  It sounded less gainy and slightly smoother and darker.

Just an update on biasing Q2: the resistor which biases the base of Q2, R8 on tonepad's schematic, is empty on a few triangle schematics floating around, including tonepad's.  Other versions have 82k in that position (see http://pisotones.com/BigMuffPi/psst/BMP_versions.htm for both schematics - tonepad's is listed there as "triangle (second edition)".  it sounds much better with the 82k resistor in there. 

The schematic also omits the 560 pf cap parallel to the 390k resistor between collector and base on Q1.  Posts I've seen at other places regard these omissions in this version of the schematic as errors.  Also play around with Q1's emitter resistor - should it be 100 ohms?