Big Muff Burn-Out

[Uma Floresta]

A tale of woe:

So I was working on some modifications to a USA Big Muff, when the signal cut out. I noticed that the positive line from the AC adapter had come off from the board. No big deal. I soldered it back on where I thought it had been (I wasn't 100% sure it  was the right place at the time). Well, it WAS the right place, but I didn't know that ANOTHER wire had come off the board, and I still wasn't getting a signal, so I assumed I had soldered the wire in the wrong place.

So, I soldered it in another place. Stupidly,  I put it in the same place where the positive line goes to the LED. And then the LED exploded.    I quickly cut the power and removed the wire. Only too late I found out about the OTHER wire that had been disconnected (from the footswitch to the in jack), that had caused the signal to cut out. Everything is wired properly now. The LED is replaced but won't light up (and it seems if you breath on these wires wrong, they pop off the board or the jacks -- thanks EHX!   ) Unhappily, the fuzz signal is now very strange, like it's not getting enough power. The sound is splatty, with not much sustain.

I'm sure I burned some component or components out, but I'm not sure which. Does anyone have an educated guess? Is there a way to test for a damaged component? Here is a picture of the front and back of the curcuit board. Please ignore the red & yellow markings -- they're related to some mods:





FYI the red wire toward the middle in between the two blacks is the positive LED wire. The other red wire at the top and a little to the right is where the adapter positive wire goes. I'm assuming burned out components would be close the point where the positive LED  wire connects to the board?

Any help would be greatly appreciated. Needless to say, I'm frustrated!   

[amz-fx]

If I'm understanding this correctly...  you put the power connection on a wire that goes directly to an LED, which has the other end grounded. Yes?

Large current flows through the LED (for a microsecond) until it burns out with a flash...  current then can flow back the other direction to the circuit because the LED is now open and it should power up the fuzz...  but it should not have blown out anything (and obviously it did).

The thing for you to do is to take a voltmeter and start following the voltage from the power connections (for 9v)  and at the collector of each transistor.  The dc voltage at the collectors should be about 4v to 5v and if they are not, you can do some more tests to narrow it down.

Post your voltages.

regards, Jack

[R.G.]

The thing for you to do is to take a voltmeter and start following the voltage from the power connections (for 9v)  and at the collector of each transistor.  The dc voltage at the collectors should be about 4v to 5v and if they are not, you can do some more tests to narrow it down.

That is to say, read the sticky "Debugging: What to do when it doesn't work" and follow those directions as closely as you can.

[Bernardduur]

Cool; my pics are used again

[Uma Floresta]

Thanks for the input guys. I'll check it out tonight and post the voltages. I apprecite the help. 

[Uma Floresta]

Okay, it was painstaking, but I measured all the voltages on every point of the circuit. The results are here:





Please note that the emitter resistors for Q1, Q2, and Q3 have been removed, socketed, and jumpered on my Big Muff, as per the "Creamy Dreamer" mod. C8 is also socketed, but the original .01 uF cap is in place currently.

[amz-fx]

The new LED is either in backwards or burnt out as well... the voltage at each end of R23 is about 9.3v when it should be 9.3v on the power side and about 1.8v on the LED side.

regards, Jack

[Bernardduur]

Yes; R23 it too high (voltage)

[Uma Floresta]

The new LED is either in backwards or burnt out as well... the voltage at each end of R23 is about 9.3v when it should be 9.3v on the power side and about 1.8v on the LED side.

regards, Jack

Ahhh, thank you. I've never replaced an LED before, and I tried to put it in the same way as the original had been, but I must have gotten it backwards -- it shouldn't be burnt out, it was new when I put it in. Thanks for the help, gentlemen.   

[Uma Floresta]

Okay, so I switched the LED around, and now it lights up. No change in sound, though, or not much. The voltages are now 2.97 at the LED side of the resistor, and 8.66 at the other end of the resistor.

I have noticed that when I completely pulled even the jumper from R16 (emitter resistor for Q2) so that there isn't anything there at all, I get MUCH more gain, approaching normal, but not quite there -- maybe 60% of normal.

[343 Salty Beans]

Do you have an audio probe? It's the PERFECT tool for pinpointing a problem. I know there's a diagram SOMEWHERE...I really should have bookmarked it. It's the simplest thing in the world, though. Solder a .1uf cap to the tip of a mono jack. Solder a long wire (1 foot should do it) to the sleeve of the jack, then solder an alligator clip to the other end of the wire. Then plug a guitar cable into the jack, and run the other end to an amp (at low volume...some parts of the circuit can be really loud!). Have a friend strum the guitar (which goes to the Muff's input...the Muff's output goes nowhere). Then take the capacitor's other lead and poke it along the signal path (at the solder joints). Start at the input jack and move along it until the signal starts getting 'splatty'.