This bass guitar amplifier had a subtle background rumbling noise, and intermitently it started making pops and machinegun-like cracks. This noise was independent of volume setting, so it seemed power stage problem.
Schematics are kindly and openly available at manufacturer's website. That's a load off...
But power amps TDA7294 were OK (it has 2, bridged). I tried swapping sub-PCBs contaning TDA's but changed nothing at all. So at this point I need scope. It shows the rumbling noise in U2, third output (quad OPAMP TL074CN JFET input), but it's present even before, at U7 and U8 (same type quad OPAMPs). I notice that third output of U7 is the most crazy one (linked to Punch control). When pressing Punch switch things get worse, so I decide to swap U7 with a NJM2902N and all fine!
Schematics are kindly and openly available at manufacturer's website. That's a load off...
But power amps TDA7294 were OK (it has 2, bridged). I tried swapping sub-PCBs contaning TDA's but changed nothing at all. So at this point I need scope. It shows the rumbling noise in U2, third output (quad OPAMP TL074CN JFET input), but it's present even before, at U7 and U8 (same type quad OPAMPs). I notice that third output of U7 is the most crazy one (linked to Punch control). When pressing Punch switch things get worse, so I decide to swap U7 with a NJM2902N and all fine!
no audio out of the instrument channel, but some weirdness is gone and sounds now clean on the second channel. So I put my laptop on there and cranked the music full tilt. Sure enough, it goes crackle and pop with the bass in rhythm. Put my signal geni on it and sure...
