A job for tomorrow but just in case anyone has been here before and knows
the reason.
Loss of one channel. Problem somewhere beween the backside of the input
socket and the preamp output of that channel. Other channel traces through
ok. Vol pot tracks ok

Perhaps owners think they will be charged more for stupidity. You'd think it
was like the doctor-patient consultation.
Perhaps just a chance thing or maybe he plugged a speaker lead in one of the
amp 1/4 inch inputs.
Input to a SMD JRC/ NJM4560 stuck at 7V and its output at +ve rail, 10K in
the line not enough to limit whatever went in there, perhaps high dV/dt,
giving him the benefit of the doubt

Replaced with a pin/function compatible SOIC JRC 4580 and so far checks out
ok.

Thermal/aerodynamic management query.
Sensible , I suppose, the unusual taper in the cooling duct between the 2 PA
heatsinks. So 1.25 inch clear gap at fan end and .25 inch clear gap at the
other end so forcing air down the flutes, which otherwise free air would try
and avoid going into. But this skewness makes the mechanical design awkward.
Why did they not keep the regular 1.25 inch gap and fit a baffle with a slot
or holes at the other end so stopping down from 1.25 to .25 inches in effect
..
Does anyone know whether the aerodynamics of a taper would force air along
the whole length of the flutes rather than just at the far end where the air
outlets ?

I think if I was going to taper the air flow, I would leave the normal
parallel gap and then a divided taper from stiff card or whatever starting
at a vertical folded edge 2 inches from the fan end , down the middle of the
duct and 1/8 inch gap either side, to the heatsinks. So, in plan, a V along
the length of the duct.