I am trying to repair a Big Muff Pi, a Fran Blanche version, EC3003-A. The tone pot is scratchy. So far I have replaced all the aluminium electrolytic caps. The pot had already been replaced but it was still scratchy so I replaced it again. It is still scratchy! I the measured it and get 3.5V on pin 1 of the pot. The schematic shows a capacitor to pin 3 of the pot and a resistor to pin 1, both connected to the collector of the 2N5088 transistor. Is the scratchiness normal or is there something else going on? The schematic is available here:
http://www.bigmuffpage.com/Big_Muff_Pi_versions_schematics_part4.html (http://www.bigmuffpage.com/Big_Muff_Pi_versions_schematics_part4.html)
That's because there's is no DC blocking cap between stage 3 and the tone control, it's a bug in the big muff. There is DC across the pot so it will always be scratchy.
Adding one will change how it sounds because it alters the collector bias.
Thanks, I'll leave it as it is.
Quote from: CheapPedalCollector on June 22, 2023, 01:41:20 PM
Adding one will change how it sounds because it alters the collector bias.
Not really.. :icon_wink:
Without that cap, Q3 Collector equivalent resistance is 9k35 (10k//144k) where with DC blocking cap will be 10k..
The above 6.5% open-loop gain reduction has a negligible involvement in Q3 stage closed-loop gain which is set by G/(1+βG), where G = Collector resistance divided by Emitter resistance plus intrinsic Emitter resistance (10k / 390+55) and β = Series resistance divided by Feedback resistance (10k/470k)..
@Yazoo: As long as DC blocking cap is much bigger than Tonestack caps bigger value (100nF or bigger in your case), you shouldn't experience any issue.. :icon_wink:
I went back and decided to check for bad solder joints. Sure enough there were 2 really bad dry solder joints where wires were connected - FROM FACTORY. Once I redid these, the noise disappeared. No problems with the tone pot now.
I don't like the way the way EHX did the wiring on these pedals. They tack the wires on to the solder points of other components, no separate holes for the wires.