destroyed all the rams TMS4080.
This puzzles me a bit. The -12V rail is regulated down to -5V
using a resistor/zener circuit,. and it's the -5V that's used
by the RAMs. I assume you've checked this voltage.
Or did the -12 disappear, removning the bias from the DRAMs
and cooking them?
The -12V was defective (not on this machine but on another witch
I fixed
(uA723 replaced) and put a 8k board in it.)
I thought hte -12V rail came from a 3-terminal regulator (7912 or
similar). There's a 723 in the _+12V_ supply IIRC though.
That wasn't the problem, the errors are a bit
erratic always the same bit 9
but not always the same address.
Today I replaced the address buffer ('LS368) but that wasn't the solution
too.
I think you're going to have to do some measurements before reolacing odd
components at random. As I mentioned. some of these old HP boards don't
like being resoldered too often, the through-hole plating fails and leads
to more problems. Please don't ask how I discovered this!
I've had
vias fail on old HP boards when
soldering/desoldering components. My next task would be to
check that all pins on all RAMs go to the rignt places and
that none are left floating.
I'll think I'm going to do that I got a current tracer, so I'm going to use
that first.
Using an ohmmeter on the board (with it powered down, of course) will
verify that all the address lines are connected between the RAMs, etc.
-tony