Help with C Audio RA1000 PPA

Hello, all. I am having problems with a C Audio RA1000 Professional Power Amplifier. One channel gives out a sharp spike at 100Hz frequency (with no input) and goes haywire when driven, sounding like the louspeaker is breaking-up. Does anyone have a circuit diagram? Can any of you learned gentlemen suggest how to proceed in the absence of a schematic? The output stage is mosfet (SJ50 and SK???), which is about all I know so far. Thanks in anticipation.

John
 
Ref hF out

have you checked the Zobel resistors (10R 3W) these are situated around the build out chokes in the area in front of the fan.

if these are the brown type theyshould be a uniform colour, if the colur changes around the middle or there is a black spiral line then the resistor is dead.

if they are green with banding, then the banding should be clear and visible if it is faded then again the resistor is dead.

if ou have plastic XLR's then you'll probably find these are loose, at best youwill have a 6dB drop in output, at worst you will get a burst of RF which will take out the Zobels resistors.
If the XLR's are looe either replace them or if not possible, remove any input leads and with a pair of pliers gently lever the pins back about 2mm from th back of the XLR (this will tighten the 2 halves of the connector inside the XLR body, repeat for all 3 pins and of course both XLR inputs.

Finally, on the underside of the main PCB at the rear edge below where the input wires are soldered, the ground track necks a bit and if you have inadvertantly stuffed something inappropriate up the input this track can open circuit.

i hope this helps.

if you do get an emal address i can send the circuits and service manual

lee Basham
Poduct manager for C Audio (before harman killed it off)