31 Mar
2008
31 Mar
'08
2:52 p.m.
Not only did Cricklewood have them, but they arrived Saturday morning.
I'm even luckier in that I live near enough to Cricklewood Electronics
that I can pop over there and pick the chips up the same day ;-)
That effected a partial fix.
The timing generator is a string of four bit shift registers clocked
with 20Mhz.
OK.
Various combinations of their outputs are used to set
and reset
bistables made out of cross coupled 7440's.
As the problem I'm fixing is you can't load memory, which is a one shot
operation, its slow old work.
I don;t know what test gear you have, but I find a simple logic probe to
be more use than a 'scope for this sort of problem. The former can easily
detect narrow pulses that are difficult, if not impossible, to see on a
non-storage 'scope. Of course an 'Advanded Logic Probe' as HP called the
LogicDart is even nicer, but...
Do you have the maintenance manual? Not the printset (schematics), but
the book which is a low-level circuit description? It's well worth reading!.
I would have thought it wouldn't be too hard to keep on hitting the DEP
key and seeing what, if any Omnius signals are generated. And then
totrace back the missing one(s).
[...]
I also return all systems to working order. Apart from
a couple of
VT420's with ticking SMPSU's everything I have restored runs. (Three
Any reason you';ve not fixed those? Ticking SMPSUs are often quite simple
to fix, maybe just dried-up capacitors.
-tony