[sdiy] EPROM conversion question

[klosmon]

I've come across a bizarre situation, & was wondering if one of the more 
experienced list members (read: old-timers) could point me towards some 
kind of answer.

The Prophet 5 rev 2 & rev 3.0 use 2708 EPROMS, which require -5 and +12 
volt supplies.  For some years now, I've successfully converted those 
instruments to 2716 ROMS, which are pin compatible -- it's a matter of 
changing socket pin 21 to +5 volts and pin 19 to ground, then copying the 
firmware into the first half of the 2716 chip. This has worked on every 
firmware version I've encountered on these machines
EXCEPT
for the rev 2 version 7.0 -- for some reason, this version will NOT work 
when copied into a 2716.  I've tried copying from several different master 
roms (& they all verify having the same code inside), I know my rom burner 
works, and I've tried a number of different chips (which all work with 
other firmware, but not with rev 2 7.0).

I know this particular techonolgy is approaching antique status;  but there 
are people on this list who've been dealing with it since it was new.

Any ideas?

Thanks.

~GMM