ML-380 preamp problem.

I have owned one ML38S, and three ML380S and two of them have had similar problem. I put a bit of contact cleaner on the connector that connects the volume control ribbon wire to the board. That solved the problem. I do not know about the other manual controls on the front panel.

Have you unplugged and replugged the preamp? It sounds like the microprocessor is hanging up and might need a hard reboot. Worth a try anyway.

I once had a Classe CP50 that did exactly what you describe. I got used to unhooking interconnects while my McIntosh preamp was on, just set to an unused input, and never had a problem but it turns out the Classe can't handle it. The volume would hang and sometimes it wouldn't switch inputs afterwards. A hard reboot got it back on track and I didn't have any issues after that. And I started turning it off when making connections....

Arthur