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 ?