[SiriusHardware]

Ah, it's the 93C46. It's possible that has gone faulty, so it would be worth trying a replacement. The firmware won't be in the 93C46, as the 93C46 contains only 128 bytes or 64 'word' locations, nothing like enough to contain any significant firmware.

The firmware, and the RAM which the firmware uses as scratchpad RAM, will be in the processor chip, with the 93C46 being used only to hold info which you want to be able to change, but not lose when the power is turned off.

Running back, I think you said you had been unable to source a replacement 93C46? I'll have a rummage and see what I have (I know I have a 93... something somewhere, just not sure it's that exact type).

Also, those PLCC sockets (like the one the CPU is sitting in) have a bad reputation for going intermittent. We have equipment we made 10-15 years ago which originally went out with ICs in sockets like that, but when they are returned for service now we remove the sockets and solder the device directly onto the pads originally occupied by the socket. You need not do anything so drastic but what you could do is extract the IC and gently bend each chip pin slightly away from the chip body so that when it is reinserted, each pin exerts firmer pressure on the corresponding socket contact.

Edit: Unfortunately I don't have one. But I had no trouble finding one for sale in the usual place?