Big Muff repair - op amp version . Stumped.

[Electron Tornado]

I've got a '79 Big Muff, op amp version, with tone bypass that someone asked me to fix. The sustain control does not work and it acts like it's set at full. I removed the pot and it checked OK with an ohm meter. Using an audio probe, I get the same signal level on each leg of the sustain pot. I have not found any shorts.

Anyone have any ideas what I'm missing?

Here are a couple of schematics:

https://www.generalguitargadgets.com/pdf/ggg_bmp_ic_sc.pdf

scroll down for this one: http://www.diystompboxes.com/smfforum/index.php?topic=58665.40

Photos of pcb (scroll way down):  http://www.kitrae.net/music/big_muff_op_amp_history.html

[EATyourGuitar]

looking at the schematic, that is normal to get a probe signal on each lug of the schematic. using the GGG one https://www.generalguitargadgets.com/pdf/ggg_bmp_ic_sc.pdf

sustain pot R19 has a path to ground through R8 and C4. you could have a broken solder joint, bad ground, or a bad part anywhere around there. that would explain why you have no audible change and no attenuation. if your C4 was bad or not grounded, you would have a negative feeback loop adjustable from about 560k to 569k. if R8 is disconnected, then you have a 10k VR (R19) between these two opamps. since there is already 4.7uf and 8.2k, another 10k will change its impedance to the next opamp and limit current but it won't change the voltage or the volume.

[Mike Burgundy]

That would be my guess, too. Check C4.

[Electron Tornado]

Thanks guys, C4 was the culprit. It was kind of confusing to see the same signal level at both ends of the pot, though. Interestingly, there doesn't seem to be a definitive schematic out there for this version of the BMP.

Here is a schematic that accurately shows how that section is set up. Note R6 and R7 : http://gaussmarkov.net/layouts/opbmp/opbmp-schem.png


Here's another question:

Does anyone have one of these or have any experience with the tone bypass feature? I've read that when the tone section is bypassed, there should be a noticeable, if not large signal boost. This one, however, has the cap and voltage divider as seen in this schematic, so I didn't expect much boost in bypass. In fact, with the tone control set at its mid point, switching to bypass gives almost no volume boost.  http://www.diystompboxes.com/smfforum/index.php?topic=58665.40 

[EATyourGuitar]

if you did it like this, then yeah no volume boost. the two 47k resistors create a voltage divider that will attenuate your signal to half. you could try it with not resistor to ground and make that 47k to 10k or 1k. you could go smaller than 150nf on the output cap if you like it bright.

[Electron Tornado]

Having read other people talk about the tone bypass, and watching a video demo of a recent BMP model, I expected a volume boost. When I didn't get one, I started tracing the circuit on the PCB and found the voltage divider. It's stock, but I wondered why they did it that way back. Maybe a volume boost also resulted in an increase in noise.

Anyway, the voltage divider is stock and I'm leaving it stock for the owner.

Thanks again for everyone's help.

[EATyourGuitar]

full volume tone bypass does not add noise. attenuating adds noise because it is going through resistors that generate heat. on top of that, any attenuated signal gets amplified somewhere else with a lesser signal to noise ratio. anyway, that is not a lot of noise or anything so I wouldn't worry about it. the reason EH did it that way is that people would complain if there was this crazy volume boost when all they want to do is EQ or tone knob or whatever. I think if someone modded a muff and added a switch to bypass the tone, it would just be flying wires to the switch and no resistors. not all muffs have that tone bypass from the factory so they don't all have an attenuation circuit.