30 Mar
2008
30 Mar
'08
12:13 p.m.
Not only did Cricklewood have them, but they arrived Saturday morning.
That effected a partial fix.
The timing generator is a string of four bit shift registers clocked
with 20Mhz.
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.
A spare 8330 would confirm if that's where the problem is and I expect
would speed up the repair on the current one.
I'll continue with fixing it and keep an eye open for a spare. Getting
one from the US would be an option.
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
VAX's, Two alphas, a Rainbow 100+, DECMateII, Pro350, Pro380, a DS3100,
a VS3100 and assorted LA printers and VT terminals)
Rod
-----Original Message-----
From: cctech-bounces at classiccmp.org
[mailto:cctech-bounces at classiccmp.org] On Behalf Of Tony Duell
Sent: 28 March 2008 22:52
To: cctalk at classiccmp.org
Subject: Re: Looking for a M8330 or a couple of SN74194's
Hi Tony
I agree that the concept of originality is important in what we
do.
The system I have is original in the sense that all
the parts are from
the right era. However the current configuration is
not that as stated
on the factory label. You could also say that any item
with a
If this is the machine I think it is, I know not only the chap you got
it from, but also the owner before that. IIRC, it originally had a TU56
(single drive versiun, and no I don't mean a TU55) which is currently on
my desk, hooked up to my PDP8/e. And the RX8/e is not original.
If you look at one of the CPU logic boards, you'll see one of the chips
(I forget what) has been replaced. I did that over 10 years ago to cure
a problem with the top 4 data lines in some operation or other. The TU56
was 'payment' for that repair...
date of later than the manufacturing date and not the
result of a
repair or upgrade is not original.
True, and some museums, alas, take it that way, and refuse to instal
replacement parts to keep their machines going, even if said parts are
contemporarty with the machine. Personally, I like all my classics to
run.
However, I also like to use as few spares as possible. If your clock
generator was missing, or physically cracked in half, or... then I would
agree you need an M8330 board. But if you can get your old one to run
again, even by using more modern parts (I would have no problems fitting
LS chips if they would work electrically, for example), then I'd rather
keep the spare board for somebody who really did need it.
Thanks for the tip re Cricklewood I will contact them.
They may only have LS or something like that. As I said, I needed some
7495s, a rather oscure shift register chip that I couldn't find
anywhere.
They had 74LS95, which worked fine in my HP9820. OK, not original, but
at least I can use the machine again.
-tony
31 Mar
31 Mar
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
1 Apr
1 Apr
1:41 a.m.
On Mon, 31 Mar 2008, Tony Duell wrote:
I think ticking PSUs in displays are a sign of a shorted HOT and/or
flyback (had this case several times).
Christian
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
Yes, logic probes are very handy. They are small and easy to use, and I
have found faulty devices most of the times just with the logic probe. The
seven-segment display show transitions and illegal voltage levels (or no
voltage at all), very useful e.g. for dead totem pole outputs (in this
case the voltage of the signal will be determined by the input stage(s)
and lies around 1-2V).
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.
2 Apr
2 Apr
3 p.m.
On Mon, 31 Mar 2008, Tony Duell wrote:
I rarely use a 'scope when debugging digital circuitry. For a long time I
used a cheap Radio Shack (!) logic probe -- I got my PDP11/45 and PDP8/e
running with just that (and a VOM to check the power supplies). After
spending a weekend sorting out a Whitechapel MG!, I bought a LogicDart
and never looked back.
I do have a larger (and better) logic analyser, which has proved very
useful on occasions, but the Dart is a useful handheld instrument that'll
help with 99%+ of faults. Pity it's no longer made....
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
Yes, logic probes are very handy. They are small and easy to use, and I have found faulty devices most of the times just with
the logic probe. The
seven-segment display show transitions and illegal voltage levels (or no
voltage at all), very useful e.g. for dead totem pole outputs (in this
I've seen that too. Particularly in 74Hxx devices for some reason.
Many logic analusers only detect 0 or 1 (they have a single threshold on
the input). A logic probe (and indeed the LogicDart) is better for this
sort of fault becuase they also detect illegal levels.
case the voltage of the signal will be determined by
the input stage(s)
and lies around 1-2V).
I think ticking PSUs in displays are a sign of a shorted HOT and/or
flyback (had this case several times).
Yes, you're right. I'd mentally assumed he'd run the PSU on dummy load,
but I guess not. I know the VT300 series are famous for bad flybacks,
dunno about the VT420.
-tony
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.
6108
days inactive
6111
days old
3 comments
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RodSmallwood＠mail.ediconsulting.co.uk