For this pattern generator I need the dump of the main prom (D313). It's
a
27C256 with PLCC32 housing in a socket, located in the right corner on the
back of the instrument. Can anybody help me?

What's wrong with the one currently in it? Have you tried reading the data
from it with a lowered VCC?

I can read it, but databit 6 is always '1' the pin is shorted to Vcc,

Ouch. Well, reading an EPROM at a lower VCC will sometimes allow you to
get all the data off a flaky device, but in your case, it probably won't
help. Did you actually measure the short right at the pin with a DMM, or
just assume it was shorted to VCC from the data pattern?

It measures under 25 ohms, so no luck there, the prom is fried anyway, it
gets warm and consumes
too much power. It's a wonder I can read it at all, but without bit 6 it is
no good.