Old MOS Mask-Programmed ROM forgetfulness?

Hi, all, I have a question about old Mask-Programmed ROMs The part in question is the National Semiconductor MM5231. This part is a 2K-bit PMOS Mask-Programmed ROM, generally organized as 256x8, but also can be organized (via a MODE pin)as 512x4 bits. In this particular application, the parts are used as 256x8. I'm wondering if anyone knows if these particular ROMs (from the '72 timeframe) have a tendency for bit rot over the years? I know some of the early MOS ROMs had issues with metallization creep that would cause data loss/corruption. I have an old calculator that uses these ROMs as the micro and macrocode stores. The machine is catatonic, though the power supplies, master clock oscillator and divider circuitry, and the other obvious stuff are OK. I suspect it is probably stuck in some kind of microcode loop, just cycling around doing nothing. I have not yet put logic analyzer on the microcode latches yet, but that's probably my next experiment. Sadly, if one or more of these ROMs (there are 18 of them!) has failed, it likely means that the machine can't be restored to operation, as this is quite a rare machine, and there just aren't many of them left around. I have three different EPROM programmers, but sadly, none of them have the capability to read these parts. I was I had a Data I/O programmer, but alas, haven't come across one with all the Unipak modules I'd need at a price I can afford. Thanks, -Rick --- Rick Bensene The Old Calculator Museum http://oldcalculatormuseum.com