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