On Tue, May 30, 2023 at 12:22 PM Adrian Godwin via cctalk <cctalk@classiccmp.org> wrote: > > There was an isolation transformer followed by a variac. The procedure was > to slowly bring up the variac until the control circuit was functioning, > check the waveforms around that, and then increase the supply voltage while > watching the chopper, control and output signals. This allowed the chopper > to work at low current limited by the circuit resistance and slowly > increase with supply voltage until the control circuit backed it off (which > would happen quite quickly with no output load).
I don't normally use a Variac when repairing SMPSUs but I've seen diagnostic procedures in official HP (etc) service manuals which tell you to use such a device to increase the input voltage to the supply. In some cases the the control circuit is separately powered and gets the full voltage all the time, you just vary the input voltage to the chopper circuit. I would assume that an HP service manual of that vintage gave a sensible procedure for that particular design of SMPSU. -tony
