Alpine 3519 car amp running hot

[Mark25]

All,
Please help with a problem I have with my Alpine 3519 car amp. One of the channels gets really hot, even with no signal or load connected. The output devices are 2SC3181/2SA1264 the driver chip is a uPC1225H. I have swapped the driver chips with no effect. There does not seem to be any oscillation at the o/p on my 15MHz scope. The o/p devices on the healthy channel have 0.46v across the driving junction, whereas on the unhealthy side they have 0.65v. Any advice or recommended checks gratefully received. Thanks in advance. Mark

 

[JOE DIRT®]

what readings are you getting from the p/s output caps when powered on?

 

[li_gangyi]

Is there a bias setting for these amps?? I'm really new to car amps...and did you check the output transistors...instead of just the drivers...

 

[Mark25]

Joe: about +/-38v, I didn’t do any checks on the ps, as the problem is not on both channels, was that a bad assumption? I have no exp’ with SMPS!

Li: not sure, the blue pots near the driver IC’s seem to control (are wired to) the driver IC’s, not the op devices. The voltage readings are for the output transistors.
Thanks
Mark

 

[jam]

Check for leaky bias transistor. Quick check is to short collector and emitter of bias transistor which will kill the bias on the output stage. This is under the assumption that the bias circuit is a traditional Vbe multiplier.

Regards,
Jam