Class D amp, one channel ... funky.

[guitarcarl]

Hello all,

I have a class D stereo keyboard amp, the Groove Tubes SFX-100. One-hundred watts per side. FYI, it uses a pair of speakers in a dipole arrangement and electronics phasing to get a stereo field from it's single 11" X 11" X 18" box.

Of course Groove Tubes just sold their ops to Fender Musical Instruments, who has no clue what to do with servicing these amps, and of course, it's starting to malfunction.

The main/forward speaker is clear as a bell, no problems. The crossfire has the problem: speaker has a hum/buzz (60/120 Hz type, not too much high-freq component), and a paper-sounding distortion on almost all but the most quiet of output.

I've just become familiar with Class D amp theory (by necessity), and must try to troubleshoot it myself, with a bit of help hopefully, from you good folk.

Have not located a schematic yet.

My thoughts:

- The hum and the papery-distortion are related.
- Output transistors probably OK, don't think they'd just partially fry.
- 60/120 Hz hum/buzz leads me to believe the power supply is 'mains transformer+rectifier+filter caps,' verify when disassembled.
- if its the rails, problem would be because left/right rails are split with a decoupling arrangement which has gone bad in the one channel

I could go farther, but into diminishing returns.

So, what thoughts does this august group have?

Thank you much,

Carl

[guitarcarl]

I'll add that the "papery distortion" sounds something like crumpling up a sheet of paper over the top of the signal.

And the hum\buzz isn't very loud, but it's definitely noticeable in the cross-fire channel, while not being present in the front-firing channel.

Thanks again,

Carl