I have the same issue with my Beseler. Do this, it works.
Open up the top and remove and reseat all the large connectors from the power transformer to the circuit card nearest the left hand side of the unit.
Usually the AC Center tap from the transformer goes loose. You probably need to remove the other circuit cards to get good access to the circuit card. (Use a cell camera to photograph the connections before you start disassembly)
Do not use any sort of cleaners or burnishing tools on the connectors or the pins on the board. This will wreck them.
If you do not have the confidence to do this find a older fifty something electronics tech to do the work. This technology is in their wheel house.
One other thing, the trigger circuit from your timer to the Beseler is Hi-Z so it can trigger falsely or stay on all the time if you use a digital timer. To fix this place a parallel resistive load in parallel with the AC output of the timer circuit that feed the beseler. An AC adaptor or a 1000 ohm resistor work well. I feed my Graymark to a power bar and plug in the beseler timer connection (two prong AC plug) and the AC adaptor for my darkroom radio to the power bar.