Valcom 760 Transceiver Troubleshooting

I'm trying to solve a problem with a Valcom 760 that appears
to be related to voltage regulation. The symptoms are that the
radio seems to power up normally, then the display digits fade
away after several seconds. It will neither transmit nor receive
in this state. The outputs of several internal voltage regulator
chips jump up from about 4.5V to about 5.15V at that time.

Having no service manual, I can't troubleshoot the unit any further.
Can anyone out there help? Thanks

David Johnson