31 Mar
2008
31 Mar
'08
10:05 p.m.
Oh I forgot.. The 420's ..
I have not done anything because
a) It only happened last week
b) I cant find any drawings.
Rod
-----Original Message-----
From: cctech-bounces at classiccmp.org
[mailto:cctech-bounces at classiccmp.org] On Behalf Of Tony Duell
Sent: 31 March 2008 22:52
To: cctalk at classiccmp.org
Subject: Re: Looking for a M8330 or a couple of SN74194's
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