How to troubleshoot a Big Muff Pi?

A friend of mine dropped off his dad's old Big Muff (ram's head version) that had been sitting around forever and asked me if I could put a true bypass switch in it since the original had broken and was missing. Because of the missing switch and a broken battery connector I had no way to test the pedal first.

I installed the TB switch using this link: http://www.singlecoil.com/docs/vintage_muff.pdf

The board was quite a bit different from the ones shown in the picture but I'm pretty sure I got everything hooked up properly and fixed the battery connector. I've got bypass but the pedal doesn't work when I hit the stomp switch. I can hear the power turn on and off when I flip the battery switch and I think this switch is on it's last legs as it's very loose and will pop if it's touched once it is on. I get a fairly low volume white noise once the effect is turned on and I get a very faint guitar signal when the volume is dimed that doesn't sound like it has any effect added in. I can also hear a slight change when the tone pot is turned through it's range.

Is there anything that would just typically go bad on one of these when left sitting for years or do these symptoms point to anything common? How would I even begin to troubleshoot this assuming I didn't mis-wire the new switch and can it be done without special tools other than a DMM?

Thanks.

Is there anything that would just typically go bad on one of these when left sitting for years or do these symptoms point to anything common?

The electrolytic caps, for starters. They dry out and the pedal will either sound terrible, or nearly inaudible. EH was notorious for "dangling" the power supply filtering cap off the corner of the board. And it's too small, as well - originals were ~10uF, when they should be 470uF or higher, for quiet operation.

All of the mechanical stuff on the old Muffs seems to fail as well - pots, switches, and the power switch you mentioned. The pots get so full of crud that the signal will drop out altogether. The mid 70's Muffs used plastic shaft pots that tend to warp, anyways.

...But perhaps the WORST element of those old Muffs is the use of thin solid core wire that isn't attached to the board with a through-hole method. Some of the connections get loose, or wires come off altogether. To compound matters, all wires will be the same color (like cream or something), with no designation as to where they connect on the board, should one snap off.

I replace all of the old wire at a minimum. Get some stranded 24 gauge, and in different colors to help make life easier. I also replace (or just bypass) the power switch with a stereo input jack to switch power that way. Don't skimp - get a nice Switchcraft or similar.

Thanks for the tips, 11 Gauge.

I did a little bit of investigating and testing last night, but with a pedal I really don't know what I'm looking for. The pedal supposedly worked 7 years ago when the switch broke, but nobody remembers how they determined that it was the switch so I don't really know if that was the only issue at the time.

I'm pretty sure that the Tone and Volume pots are working properly and I would guess that I'm getting about 70% of the volume of the guitar when I have the pedal engaged and the Volume pot dimed. The Sustain control makes absolutely no difference in the sound or output of the signal and the signal sounds more or less like the guitar with maybe some tone suck...in other words I don't hear any fuzz effect with the exception of what might be coloration from the Tone control.

I did wiggle tests on all of the wire leads and those seem to be fine. I didn't get any funny noises or anything and even tapped their solder connections to the traces without hearing any pops. I tried testing the pots in the circuit and the Volume and Tone pots appear to act somewhat normal in that I can get a 90k reading across them and see some change when I test the wiper to an outside lug.

The Sustain pot seems to read open regardless of which lugs I test across. Do you happen to know what type of readings I should get on this pot? Would a bad pot in this location allow signal to get through to the output without adding any of the fuzz effect? If the pot is working correctly should I at least see some kind of a resistance change when testing it while still soldered into the circuit?

I don't know whether I'm chasing the right component or if I'm just drawing the wrong conclusion because I can't hear this pot making any difference. The pots are soldered to the board in the same manner that you described the wires being soldered so I'm a little bit leery of removing it unless I know I'm looking in the right area or know that I should be getting a reading on this pot.

Here is a picture of the pedal right now. I think all of the wires were original, judging by the solder joints except for the battery connector. I had to extend the blue wire that goes to the switch because it was originally connected to the input jack before adding the true bypass.

http://i78.photobucket.com/albums/j109/milocj/EH%20Big%20Muff/DSC00219.jpg

Thanks again for any and all help.

I checked the readings on the Sustain pot again and was getting nothing across any of the terminals so I jumpered in a spare 250k pot and everything worked fine. It makes me wonder if the pot had gone bad and the owner just assumed it was the switch since there was no perceived effect.

Now I just have to try to find a 100k log, D-shafted pot, preferably with those long tabs so I can solder it to the board. Any ideas? I checked the EH-New Sensor website and didn't see any of the long tab pots.

I checked the readings on the Sustain pot again and was getting nothing across any of the terminals so I jumpered in a spare 250k pot and everything worked fine. It makes me wonder if the pot had gone bad and the owner just assumed it was the switch since there was no perceived effect.

Now I just have to try to find a 100k log, D-shafted pot, preferably with those long tabs so I can solder it to the board. Any ideas? I checked the EH-New Sensor website and didn't see any of the long tab pots.

The pots on the old Muffs just go bad - not a matter of if, just when.

Folks don't realize that EH sourced the cheapest components they could find. For this reason, almost all of the mechanical parts will fail.

The Sustain pot is equivalent to a volume control in a tube amp - it simply either passes signal onto the next gain stage, or shunts some to ground. And the 100K value isn't critical - the gain stages seem to saturate well prior to full gain settings, and there's a minimum gain resistor (1K) between the pot and ground. 50K - 500K should work fine. Log taper is preferable to get a usable sweep.

Any pot with lugs will work. Just use some heavier gauged solid core for the "transitions" in between the pot and the board.
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Don't be surprised if/when the other two pots fail. It would make good sense to simply replace them. The Level s/b replaced with a log taper - 10K to 100K is fine, IMO. The Tone s/b replaced with a lin taper - 50K or 100K is fine. If it's currently wired up "backwards," so that the sweep goes from treble to bass, simply reverse the wiring of the outer two lugs.

Any sized pot will work, from Alpha minis to full sized CTS. The minis might require bigger outer diameter washers to work with the enclosure.

Thanks again for the info. The 250k pot seemed to work fine though I haven't played through a Big Muff since the late '70s so I didn't know if it made any big difference compared to the stock pot.

Do you happen to know if anybody makes D-shaft pots anymore or will I just have to flatten one side if the owner wants to keep the original knobs?

Is it best to use desoldering braid to remove the old pot or would I get away with less heat on those traces if I just clipped the pot from the terminals and pulled each terminal away with pliers as soon as the solder melts?

I'll ask the owner if he wants me to replace the other two pots at the same time. I can't tell who made them but they have a funny feel at the extreme ends of their travel.

You can probably find D shaft pots if you dig around enough.

It's typically easier to a) replace the knobs with something similar looking (and save the originals) or b) get some split shaft knobs and snap off one half of split portion.

The best part of the old Muffs is the circuitboard itself - they are nearly indestructible. You won't need braid. A solder sucker will make things easier, though.

If the other two parts are gritchy anywhere in the sweep, you can count on them to fail.

I wouldn't be surprised if one of the other pots may soon be on it's way out. They sweep fine but the broken one feels almost like it has a notch at each end and one of the others feels like it's getting that same little click/notch.

I'm going to recommend to the owner that I replace all three. I don't really know much about pedals but he dropped it off when I fixed a tube socket for him and asked if I could look at it. I wondered if a "snapped off" split shaft would be the same as a D-shaft. Maybe I've got an old one I can break and do a test fit with.

I noticed that you recommended a linear taper for the tone pot. Would that make a big difference as the schematic shows a log taper? I can't see the backs of the pots without removing them so I don't know what is actually in the pedal.

Thanks.

I noticed that you recommended a linear taper for the tone pot. Would that make a big difference as the schematic shows a log taper? I can't see the backs of the pots without removing them so I don't know what is actually in the pedal.

Thanks.

The linear taper for the tone pot is almost essential. Any taper will work, but since it's a "blend" tone circuit with treble on one "side" and bass on the other, only linear will give you 50/50 at noon on the dial.

I find that most of the old pedals tended to use linear for most everything, which again will work, but is really too "abrupt" for the Sustain and Level controls - log works much better.

I was only going by the schematics that I could find and those showed log for all three pots. Maybe I can move the tone pot over (if it actually is a log pot) and replace that one. I can't read the markings on it.

I took the broken pot apart and the fiber wafer and carbon ring were both broken loose. I'm sure that supporting the entire board from the connections on the pot wasn't the best thing to do for long life, and I think that this pedal has been laying around opened up for a couple of years waiting for a new switch. The battery and just handling the pedal may have put enough pressure on the board to break the pot. I'll try to find some way of supporting the board a little better.

Thanks again.