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!