Carver PM 1.5 That Just Won't Quit--Power Down

I have a bunch of Carver PM 1.5 amps, and have recently encountered one that will not power down with the front panel switches. I checked both switches (Power and Sequencer) with a meter, and both are in working order--open and close the circuit properly. Any help as to what to check next??? I know a lot of amps have relays on the power supplies thay can become sticky or fused, but the PM 1.5 is not among them. Other than not powering down, the amp works fine--just have to turn it off by unplugging it or switching off the power conditioner.

Many thanks... Frank