Enough of this flaffing around, should have been just a change of fan
job. Found a "high impedance" 12V ,0.16A fan in a parts mule and that
works fine, incidently from a linear ps amp.
I'll assume its something to do with the 0.1sec or so full tilt, fan
start up function of the control circuit. Instead of initial 2 seconds
or so to audibly confirm to the owner that the fan is working, just a
short kick start before dropping into low temp speed . But potentially
18V over 12V fan for 0.1sec and perhaps some added monitoring variant
somewhere in the SMPS , in this 2013, Mini CMD 121 flavour of mark Bass
, different to the only representative Mark Bass schematic that is out
there.
Anyway no nasty ticking of the SMPS going into protect at switch on and
normal running fan noise and increase speed on warming up.
At least anyone else coming across this problem has now some clues as to
what is going on.
Incidently the usual scratched off ident to the SMPS supervisor chip,
dicussed and collectively identified previously on this board.