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JaviCAPMember
Hello,
I have not the Bass Big Muff layout / schematics, just a few pics, but this problem (extremely low output on bypass and clean no processed signal when activated) is nearly always related to transistor problems.
I would point my efforts to the 4 bc547c trannies. Luckily enough, in the bass muff, unlike the Metal Muff, are common TO92 trough hole ones, so you can replace all four in a matter of seconds.
But, there’s always a “but”, the problem could be located too in the op amp IC, a tlc2264c smd, that is not.. lets say “easy” to remove / replace.
If the pedal were mine, would replace the BC547C trannies one by one. If the problm is not solved, would replace the opamp, but if you’re not familiar with this kind of components and have a smd solder iron / station, would take it to the EHX service desk.
Best regards and good luck
JaviCAPMemberQuote:Hi Ashman, maybe it´s a little too late for the response but I´ve received from a friend an MM with the same problem and I´ve found that was the Q7 transistor (I think this is the one that routes the signal in and out the effect, not sure because I have no schematic to check at, can´t find it all over the net) failing.
If you open the MM backcover you will find this tiny SMT transistor near the input jack, almost sorrounded by R55, R57 and D3 (MMBD4148SE), all SMT´s. This transistor is a FET N Channel switch and you can repleace it with the same MMBF4393 (SMT), or an PN4393 (TO-92 packaging, bigger, you have to re arrange the leads to fit it).
I was unable to find either the MMBF4393 nor PN4393 over here so I have repleaced it with an MPF102 (TO-92, fitting it and crossing 2 leads) and it works great.
Hope this helps.Best Regards,
Turco.Hi Turco!
Two years after your post, but I’d like to tell you that this answer has been of great help, as I had the same problem on my Metal Muff (extremely low output sound on bypass and not processed signal, just plain weak sound when switched on).
I used a 2N3819 (TO-92) instead of the original MMBF4393 and now is working great.
And you were right, this transistor is the one routing (JFET N channel) signal to the effect or bypassing it.
Thank you very much! Repair cost was about 25 cents + 3 drops of solder!
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