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I just built a “Green Russian” Muff’n on an older version PCB. The only difference is I used 560pF caps and KT3102E transistors (all are around 550 hFE).
The unit is very noisy when it is off, and when I turn it on I get feedback, and the pitch of the feedback changes when I turn the knobs or toggle the switches (I did the high pass mod, the Bender mod, and the Mid scoop mod).
Thank you for your help!
I’ve troubleshooted by checking continuity, swapping transistors, and changing the resistor value at R25, but I get the same results.
Yes, I checked the pinouts on the KT3102E data sheet and I checked the hFE of all of them.
I see you have R25 socketed. Did you use a 33K resistor in R25?
Have you tied removing the mods to see if they might be errant in some manner?
I socketed R25 and have tried several widely-varying resistor values but still get that feedback drone. I’ve unsoldered all of the jack connections and resoldered the pots to have lugs to make it easier to get underneath the PCB. The solder pads for the PCB’s took a beating from getting unsoldered.
The mod switches actually do what they’re supposed to despite the noise, which makes this even more confounding. Someone from the DIY STOMPBOXES group suggested that I solder straight to the 3pdt switch rather than use Barry’s 3pdt board for better grounding.
Do you agree with that thought? It appears that LED indicator (D8) and the CLR (R27?) are already on the Muff’n PCB. I know that S4-S6 have something to do with the LED. Can they be ignored if I’m using a standard 3mm red LED?
Brooks – “Someone from the DIY STOMPBOXES group suggested that I solder straight to the 3pdt switch rather than use Barry’s 3pdt board for better grounding.” – That’s nonsense. I’ve been using Barry’s 3PDT wiring boards for years. There are no grounding (or “better ground”) issues with that daughterboard.
Everything on the main PCB regarding the LED can be ignored if using one of Barry’s 3PDT wiring boards. Just be sure the LED & CLR is mounted on the 3PDT wiring board and it gets proper ground and +V connections.
As for the noise issue, you want to apply an audio probe to see if you can isolate the section creating that problem.
R18 looks like a 33K not normally significant but with the mid scoop mod may cause oscillation idk
Unfortunately as you know feedback can be caused by a number of things if I’ve eliminated all external causes a non technical method I use to try and find it is probing solder joints with my finger tips and the old wiggling wires test to see if it changes quite often leading me to the source
As Cybercow said audio probing to see if you can find where it starts is a good idea
I too have never had problems with the 3PDT board of all of the one’s I’ve used it’s by far my favourite especially with combo builds so if all solder joints are good and it’s wired correctly I’d personally rule that out
Just for info the S4, 5 and 6 pads are used for the main pcb bi colour LED if you use one
S5 goes to 3PDT lug 5 ground and S4 and 6 are grounded by S5 to which ever colour the 3PDT is switched to when S4 connects to S5 effects mode colour, S5 to S6 bypass colour as you say per the build docs if using the 3PDT board you can ignore them and the main pcb CLR and LED
I spent some time bench testing mods for the Muff’n circuit recently. Through my experimentation I discovered a lot of noises that I wish I had recorded to have as examples for questions like these.
For troubleshooting, I think that the fact that your pedal is noisy when it’s off is a good place to start. If the board is properly bypassed, it should not affect the signal. Verify your cables are good and start testing with no input cable connected to eliminate those variables.
If you have a noise when powered off in bypass mode, check for a polarity or connectivity issue between your jacks. The signal goes from input jack to 3PDT board to output jack. That should be the easiest to trace and verify, this is usually a ground issue. Try a jumper wire between the input jack ground (sleeve) and the output jack ground.
If that is not the issue, but you have a noise when powered on in bypass mode, then you have some cross mojification, That would be some stray voltage or signal. A possibility would be Battery + connected to your signal input before or after the effect board or I/O wiring routed too close to a high gain board.
Billy mentioned external causes. One thing I discovered is that bench testing with an “open” or “no” enclosure can get very noisy if you have a shop full of old fluorescent light fixtures. I have to turn off the overhead lights or put the cover on the enclosure if I want to sound test in the shop.
Thank you both for your help. I fixed my muff. I don’t know if there was a singular problem that prevented it from working, but I replaced the pots (went to solder lugs for easier troubleshooting and the “Mids” 25k pot was apparently fried), replaced R18 to 20K, and replaced the SPDT clipping selector switch because it wasn’t working in all 3 positions. It’s dead silent and Barry’s 3pdt board works absolutely fine.
It has a MASSIVE sound. ?
-Brooks
Brooks – happy to discover you’ve sussed it. Your patience and tenacity has paid off. Happy building!
Patience is a virtue. And many of us are probable running a bit short of virtues lately.
So, what will be your next build?
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