Behringer ACX1000

[diydidi]

Hi.
Anyone have this schematic?
Problem is the amp is humming like crazy. Audio reaches speakers just fine. Both send and return jacks have this hum present. Any ideas? I haven't worked on one of these before.

[Enzo]

The amp works but it hums. Does it hum even without any input connection? (probably)

Like any amp, start with the power supplies.If the FX send hums, look at the low voltage rails, I assume them to be 15v rails. Either one missing or full of ripple?

No secrets, the thing uses a couple LM3886 power amps, and the low voltage rails come from a 7815/7915 pair

[diydidi]

Thanx Enzo!
Will check +/-15v rails.

[diydidi]

Rails are clean. 800mV p-p on main rails. what now?? darn difficult without schematic. Check all the caps with my esr meter. Looks good...

[diydidi]

I notice that if I unplug the ribbon cable coming from the front panel preamp board, the humm is gone. So its coming from the preamp?? A ground loop?

[Jazz P Bass]

There is a ACX1800 preamp schematic here, post #3.
Link: http://music-electronics-forum.com/t23748/