Just the thermal LED shows , also with all 5 amp modules unplugged from
the rack. Nothing looks bad , driver transistors and rectifiers, large
Rs and caps sloppy-test cold ok , fuses ok , etc.
Looks like the ST VIPer 12A , SMPS driver IC, data out there, powers a
donkey engine SMPS then the main when the cold side control logic says
ok to do so. I assume a schematic is not out there anywhere

traces and ribbons from PTC thermistor on the main SMPS Tx are ok to the
control logic. I don't evenknow whether the ps will run with no working
PAs plugged into the power bus. Next thing to trace , before probing out
reduced-hot

All the chippery, I've found datasheets for, so maybe resolvable without
schematic. What I thought was a uC (with buried FW of course) is a
crystal controlled second SMPS controller for the main ps, and control
logic is hardwired as CMOS 4000 and opamps

First small SMPS is in burst mode, presumably thermal shutdown (falsely
or validly not determined yet), and cycling to restart every 30mS, so
reduced supply voltage with nasty sawtooth superimposed. At least Viper
12A are easily available, hoping not due to shorted turns on the small
inverter Tx

Replaced the primary SMPS IC and same poor DC supply .
Determined it was not due to excessive load, shorted turns in the
inverter Tx unlikely.
Tried the secondary SMPS powered from bench ps and that brought up the
internal Vref , requiring a supply of 10V , not 8.6V which is just below
the undervoltage lock out. The Vref supplies the "thermal" LED opamps etc.
Went back to the first SMPS and placed 4.7uF across the little 2.2uF SMD
aluminium electrolytic that smoothed the sense voltage and the DC supply
came up to 13.2V. So bad ESR cap again it looks like, falsely telling
the IC that the Tx was overloading, rectified sense voltage was too
high. The opto coupler was not for the primary SMPS, coupling is the
other way round, probably to pass mains AC-ON confirmation, to lock out
the 24V DC option, not used , as only mains use .
Will tidy up cut traces and should hopefully be back into operation.
About 2,500 GBP amp and as a not current model and long wire HV ducted
cabling and transformered speakers etc, perhaps that again if a
replacement 5 ch distribution amplifier was not compatible and all the
other kit had to be replaced also