It is really strange that two devices have the same fault.
The easiest way is to turn on the device through the variac and to measure the voltages at several points at
the input of the power source and inside. Then turn it on without variac and see what voltages have changed.
Analyze and try to find the cause. It may take several attempts. The task is complicated by the fact that the
type of the power source is SMPSU.