There's a gost in my pedal, I think?

[Wales]

I built a big muff replica called the violet ram's head from ggg http://www.generalguitargadgets.com/pdf/ggg_bmp_vram_sc.pdf And added some modifications on it controlled by switches. The first mod is a DPDT that jumpers the firs pair of diodes D1 & D2. The second mod is a 4PDT that swaps the standard 1N914 diodes for LEDs. The third mod is a 3PDT that jumpers the resistors that connect the emitters on Q1, Q2 & Q3 to ground.

Now the problems. When I first plugged it in and played it everything worked fine. All pots worked fine and the mods also worked fine. After about a half hour the pedal started to break up and clip notes when i was trying to hold them. The pedal had no mods on at that point. I understand the LEDs for didodes create some clipping, but it was turned back to diodes. I had an engagement to go to so I left. Returning hours later I tried the pedal again and everything was back to working order... for about fifteen minutes. The same problems were occurring, as you hold a note or chord after a second the notes stops. So i opened it up and took the voltages readings below

With no mods on                                         With D1&D2 jumpered                                With LEDs replacing diodes                      With emitter resistors jumpered
Q1 C-3.76                                                   Q1 C-3.76                                                  Q1 C-3.76                                             Q1 C-3.57
     B-0.61                                                        B-0.61                                                        B-0.61                                                  B-0.58
     E-0.03                                                        E-0.03                                                        E-0.03                                                  E-0.00

Q2 C-2.93                                                   Q2 C-2.93                                                  Q2 C-3.85                                              Q2 C-3.05
     B-0.66                                                        B-0.66                                                        B-0.63                                                   B-0.59
     E-0.06                                                        E-0.06                                                        E-0.05                                                   E-0.00

Q3 C-1.58                                                   Q3 C-1.58                                                  Q3 C-0.10                                              Q3 C-1.48
     B-0.63                                                        B-0.63                                                        B-0.67                                                   B-0.59
     E-0.05                                                        E-0.05                                                        E-0.06                                                   E-0.00

Q4 C-4.54                                                   Q4 C-4.54                                                   Q4 C-4.61                                              Q4 C-4.54
     B-1.67                                                        B-1.67                                                         B-1.65                                                   B-1.67
     E-1.21                                                        E-1.21                                                         E-1.18                                                   E-1.21

After not playing the pedal for a day i took the numbers again with none of the mods on and got these readings

Q1 C-3.78              Q2 C-3.12        Q3 C-2.24-2.34          Q4 C-4.45
     B-0.62                    B-0.66             B-0.63                       B-1.66
     E-0.03                    E-0.06             E-0.04                       E-1.20
You may notice the weird reading for Q3 C, the meter was fluctuating between 2.24 and 2.34 constantly.

I plugged the pedal in again and of course seems to be working fine but for how long. Anyone have any idea where the ghost is hiding in this unit?

[anchovie]

Intermittent fault? Best check connections and soldering, might be a dry joint. Definitely check around Q3 - in your table of voltages you have a reading of 0.1V on the collector when your switch to swap the diodes for LEDs is on, which is a radically different value to the other switch states. The fluctuating reading you've got now with no mods on is 0.7v higher than your no-mods reading from the day before, so if the measurement is not repeatable then something around that part of the circuit must be variable.

[Wales]

Can't find a cold solder or loose wire, but the board and other components have not moved since they were built. So I doubt this is the variable.

I think I have a bad capacitor somewhere cause when I check the Collect on Q3 i either get a normalish reading of 2.43v or I'll get a 0.10v reading. If i get the 0.10v reading and leave my meter on it, it will slowly climb back up to 2.4v ish. There is a leaking and recharging from what i can see.

When i read voltage on my caps should i be looking for a different voltage on each side?
When I read the voltages of caps that go to pots like (C3,C4 or C12) should the one side meter reading vary as the pot is altered?

[anchovie]

Caps block DC, so there's a very good chance you'll see different voltages at each end. The Sustain and Volume pots control the amplitude of your signal at those points and seeing as they come after caps there should be no DC component, so you shouldn't see any sort of DC bias changing when the pots are turned. The Tone control acts as a blend for the signal split between high-pass and low-pass filters.

When you do get the expected sound out of the pedal, does the tone control function the way it should?

[Wales]

Yes when the pedal is working well all three pots have full range of use. When the pedal starts acting up as you turn down the sustain pot the problem gets worse and the volume pot has no more increase after 12:00.

When the pedal i not working properly the tone pot still works fine

[Wales]

On C4 the voltage readings are: 0.64v on the side connected to R8
                                              on the side connected to the S2 0.01v when sustain pot is full clockwise, and 0.76v when full counterclockwise. (and varying in between the two while turning the pot)

On c12 the voltage reading are: 1.62v on the side connected to the base of Q4
                                               on the side connected to the T2 0.51 when tone pot is full clockwise, and 1.57v when full counterclockwise. (and varying in between the two while turning the pot)

So I do have DC fluxuation on the one side of the two caps does this mean bad pots, bad caps, or bad assembler

[Wales]

Is everyone stumped or does nobody care?

[anchovie]

I am stumped. I do think you need to get a scope or audio probe on there so you can deal with waveforms rather than just DC voltages.

[Wales]

I have a homemade probe, a 0.1uf capacitor on the end of a guitar cable with an alligator clip for ground. All the probing i do seems fine. I'll give it another going over though.

Unfortunately the opportunities in my life where i had a chance to get an oscilloscope, I had no use for it so i said no. Now ten years later I'm cursin. so no oscilloscope.

[slacker]

Don't worry about the DC on the tone pot, that's normal, the DC on the collector of Q3 has a path to ground through R19, the tone pot and R18. If you had an intermittent short somewhere round the tone pot though that could mess with biasing of Q3 and give you your funny voltages.

[Wales]

I probed the circut again from beginning to end. There seemed to be tone where there should be and it would reduce in volume crossing resistors or capacitors. The only thing i noticed that was odd was that when i pulled the probe away and then placed it back on the same solder the tone seemed to lower in pitch and when i removed the probe and then touched it again it would be back to normal. I use the output of my tuner to produce a note and play that through the pedal, the note did not change the whole time.

I think the thing that bothers me the most is that the pedal will work fine after being left alone for a bit, and then after about fifteen minutes of playing back to cutting out and acting up.

Its like an old man with exercises he's good for about fifteen and then he sputters out and need to recharge before he can run more laps.

Is there a way to check my pots? I measured the two outside wipers to get the overall value of the pots and checked to see if that coincided with the middle wiper with the pot turned to the extremes.

[Mike Burgundy]

DC on the tone pot is normal in this circuit, but not on the sustain pot. That points to a leaky C3 or C4 but I have no idea how that could cause the mentioned symptoms. Lift a leg on c4 and measure if there's still DC there, do the same (replace C4) for C3. That'll tell you which one's leaking. Other than that...?
Yourr description does sound like the circuit is misbiasing itself with time, which does suggest leaky caps and/or wiring mistakes.
hih

[Wales]

Thank you Thank you Thank you.  I replaced both C3&C4 and that took care of the problem. I played for about forty five minutes, used all mod and pots and everything worked fine. After I changed the parts I still had a DC voltage on the leg of S2, but when I checked the voltage on Q3 collector the voltage was high two's and low three's depending what mod was on, back to normal. Thanks again for all the help.