To keep you posted :
I made a error by stating the -12V was wrong, it was the +12V that's why I
replaced the uA723. (mea culpa etc....)
OK, that makes a lot more sense.
The TMS4060's don't have a -12V connection
they get +5,+12,0,-5V.
Indeed. And that's why I was pussled. From what I rememebrm the memory
_board_ takes in -12V from the PSU (it comes from a 3-terminal regulator
there) and brings it down to -5V for the DRAMs using a zener and resistor.
The board is working again, the VCC pin of lowerbit 9
had a resistance of
12.5 ohm to the VCC-line.
Bad trace?
The A10 pin of the same chip didn't connect to
anything.
Ditto?
A little bypassing did the job, so thanks for your
advice.
Yes, I've seen it all too often on old HP boards. The through-hole
plating isn't that good. My HP9830 had a couple of bad connections on one
of the RAM boards (which is the main reason why the machine didn't work
when I got it). Adding acouple of jumpers fixed that. And I've seen it in
other machines.
-tony