Dead Big Muff Pi v9 (NYC)

[Jrm]

I replaced the 220 uF cap and I'm still getting the same readings. Dammit. Now what?

[52 Bill]

I replaced the 220 uF cap and I'm still getting the same readings. Dammit. Now what?

What were the readings without the cap?

The schematic that you linked does not show these parts, does it?

[Jrm]

I was getting 9v on either side of the 100 ohm resistor where power enters the board after removing the cap. The best schematic I could find for this board/version does not include C14 and C15 (the one I replaced) which are the two biggest electrolytics on the board.

[52 Bill]

I was getting 9v on either side of the 100 ohm resistor where power enters the board after removing the cap. The best schematic I could find for this board/version does not include C14 and C15 (the one I replaced) which are the two biggest electrolytics on the board.

And does the fuzz work without the cap? Is the cap in backwards?

[Jrm]

I didn't test to see if it worked with the cap out of the board, and I imagine that it wouldn't since the 9v doesn't have anywhere to go after the 100 ohm resistor where it enters the board. The cap was installed correctly.

[52 Bill]

I didn't test to see if it worked with the cap out of the board, and I imagine that it wouldn't since the 9v doesn't have anywhere to go after the 100 ohm resistor where it enters the board. The cap was installed correctly.

I'm sorry, but I am really confused here.

From your description I assumed that this cap is basically just a decoupling/filter cap that connects across the 9 volt dc line coming from the power jack. So my vision of the circuit is +9 volts coming from the dc jack goes to the pc board and connects to the first side of the 100 ohm resistor. The second side of the resistor connects to the positive side of the 220uF cap as well as to the 9 volt circuit buss. The negative side of the cap is connected to ground.

If I am wrong, please describe the circuit in more detail or post close up photos of both sides of that part of the pc board.

[Jrm]

You're exactly right, Bill. The 9v comes off the power jack and connects to the board at the 100 ohm resistor, and from there goes to the 220 uF cap. From the positive side of the cap it connects to another 100 ohm resistor that goes to another 220 uF cap and on through a few resistors to the transistors, and the negative side is connected to ground.

I really can't believe that there's not a schematic out there that includes the switches and jacks.

[52 Bill]

You're exactly right, Bill. The 9v comes off the power jack and connects to the board at the 100 ohm resistor, and from there goes to the 220 uF cap. From the positive side of the cap it connects to another 100 ohm resistor that goes to another 220 uF cap and on through a few resistors to the transistors, and the negative side is connected to ground.

I really can't believe that there's not a schematic out there that includes the switches and jacks.

If that is in fact correct, then try the thing without the cap and see if it passes signal. I don't see why the voltage should die at the second side of the 100 ohm resistor unless the cap is shorted.

If the voltage is back up to 9 volts, it should work. It may have a little more hum with the ac adapter, but it should work.

[Jrm]

I removed the cap and checked it with the meter again, but now I'm getting 4.6v on the other side of the 100 ohm resistor now. The pedal passes a clean signal, but still no LED and no sound with the pedal engaged.

[Jrm]

Figured it out. The second cap was bad. I was poking around and chasing the signal path and continuity between the legs of the other big 220 uF cap, pulled it and ran some tests and confirmed that it was dead. Installed a fresh cap and it fired right up. LED and everything.

I guess the lesson here is to just pull all the big electrolytics at the get-go and go from there?

The input jack is all banged to hell, so I'll be replacing that too, but it all came down to that one cap. Thanks for all the help, as always. Sorry this one was so annoying.

[g1]

This was one of those times were you just have to have the schematic, or draw one out.

[52 Bill]

I guess the lesson here is to just pull all the big electrolytics at the get-go and go from there?

No, your first clue should have been the low voltage on the second side of the 100 ohm resistor. In order for the voltage to drop that much, something had to be pulling it down, like a shorted cap or protection diode.

I thought that you said that the voltage came back up when you removed the first 220uF cap. Perhaps the problem was a solder bridge across one of the caps.

Glad to hear that it is working again.

[Jrm]

Well it was definitely a learning experience. When electrolytes go bad can they give a reading as shorted? I didn't see any evidence of a solder bridge at all, and it's a clean board with lots of room and very easy to read.

The voltage DID come back up when I initially removed the first cap, so I have no idea how or why it would have read low again after removing it the second time. Maybe some of the wires were bending in weird ways at the board. It's tricky keeping everything orderly once you pull it all out of the pedal.

Thanks again for all the help, Bill. I'm trying to work on writing out schematics. Is there any software you'd recommend to assist with that?

[52 Bill]

Well it was definitely a learning experience. When electrolytes go bad can they give a reading as shorted? I didn't see any evidence of a solder bridge at all, and it's a clean board with lots of room and very easy to read.

The voltage DID come back up when I initially removed the first cap, so I have no idea how or why it would have read low again after removing it the second time. Maybe some of the wires were bending in weird ways at the board. It's tricky keeping everything orderly once you pull it all out of the pedal.

Thanks again for all the help, Bill. I'm trying to work on writing out schematics. Is there any software you'd recommend to assist with that?

Caps can go bad by going open circuit or by going short, you never know. If it goes short it will pull down the voltage line, if it goes open then it won't filter out the hum from the ac adapter. You just have to learn to read the signs.

I don't do any schematic software, so I can't help there. I'm sure somebody here will offer up a few suggestions. I still draw them up by hand.