Bassmaster xm200 constant hum

[LVNTHEDREAM]

Hey everyone. I have a Bassmaster XM200 that has a constant HUM as soon as the amp is powered up. When this issue first happened it came without warning. The amp was working just fine on a Friday. When I went to play on Sunday as soon as it was powered up the HUM was heard. No cables or instrument connected and the HUM is not affected by the volume or any other controls.

I took it in for repair and after 3 weeks was told they couldn't repair. The issue has now been compounded in that as soon as the amp is plugged in the HUM begins and the power switch will not remove power to the amp. Obviously the repair shop screwed something up but going back to the original problem, has anyone had this issue before? I am not an electronics wiz but it sounds like an isolation issue. Any inputs would be appreciated.

[Enzo]

Everything works until it doesn't.

LOUD hum might mean DC on the output - which can destroy the speaker.

[Jon Snell]

Grab a car rear lamp bulb, 21Watt indicator/brake lamp and wire it in series with the loud speaker. That will ensure you don't cook the speaker.
Power up the amplifier with a jack plug in the EFXRTN socket. That will remove all but the final mixing and output stage. Is there still a hum? If there is there may be, as Enzo states; DC on the speaker output and you must firstly wire the power switch properly before continuing then check the output stage.
If the hum stops, the fault has been isolated to the pre amp stage.
Which ever suits is the way I would proceed.
This amplifier is a crude attempt at class H but still very simple in design.
Check the power supply voltages, zero volts on L1 and the Float voltages, before getting too involved.

[g1]

The repair shop may have miswired the power switch when they put it back together. This is not a difficult amp, if they couldn't fix it, they are not much of a service shop.
Where are you located?
Service manual attached.