Problem erasing & re-using GM Proms

[ECMnut at aol.com]

Have anyone been able to erase a GM prom & reprogram it?
I always have to solder a regular 27xxxx eprom on before I can
re-program..  I can never seem to re-burn the factory original
prom, which doesn't seem to have meaningful identifiers on it.
My latest is a 27256 like thing from a '93 L35 4.3 engine.
Could I be using the wrong programming voltage?
Thanks,
Mike V
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