Troubleshooting problem Audio Research SP-10 preamp

I've had this component for over ten years with no problems whatsoever.  There are two green lights on the front of the power unit, one of which flashes for two minutes on startup (safety feature, I presume) and then becomes a solid green light as the sound comes on.  All of a sudden the flashing feature is not there, the solid green comes on immediately on startup, and no sound comes from the preamp.  Any suggestions before I ship it back to the manufacturer?