WTB (or trade) National Semi MM57126N Shift Register

[markhen]

I am looking for a MM57126N (IC from National in the 70s - 16x64 bit shift register (1,024 bits)) to repair a Sinclair Enterprise Programmable calc. It is an 8 pin DIP through-hole device.

This part was in the National "COPS" family (Controller Oriented Processor System). It appears to be the memory for the user programs - on my calc everything works correctly, including the user memories, but every program step shows "F 7F" and cannot be overwritten (I'm pretty sure the CPU is reading "1" back from the shift register all the time - I am going to get a 'scope on this shortly).

The MM57126N may have been used in other calcs or devices using the National COPS family (MM578x, MM579x, MM571xx etc.).

I have scoured the obsolete IC houses and parts are apparently in stock somewhere - but prices I've been offered range from $150 to $500 !!!! If I can't source one more economically I will try to emulate the part with more modern device(s) - but I'd rather keep this interesting machine "Stock"?!?

Thanks

Mark