We does some tests and find one stuck bit in communication between IOU and memory. Measured the communication channel with modern Lecroy scope.
All variable transformers had to be cleaned, now the power supply is stable again. We tested the currents through the transformers and diodes with the clamp ammeter.
A short video @youtube about the Cyber 960, you can hear the 400Hz and the fans.
Fan repair at the console CC545, we disassembled the engine and greased the bearings. Now we know that a second fan is installed, because the first one is now quiet.