>
It's also advisable to add a crowbar circuit to the psu the 723 regulator
> IC's and power transistors don't life forever. When they go you will be
> grateful for adding the crowbar circuitry.
Seconded!
I plan to do this. Any pointers from those who have done the mod?
I've not actuiaslly done it, but I will do before I run the machines
again. I would have thoght the traditional SCR + resisotr + zener crowbar
is all that's needed.
-tony
Yeah, but when it comes down to the nuts and bolts of it, we have to figure what
threshold we want for each crowbar, and also how to physically cram it in.
True. The SCR is going to take about 0.6V to trigger, so I'd probably use
seners equal to or slightly above the nominal rail voltage.
Some later HP machines (the 9826/9836 being the one I remember, mainly
because I was giving a talk on it a few days ago) have one SCR connected
across the _input_ to the regulator (that is across the main smoothing
capacitor) with zeners from teh gate to each of the output rails. If any
rail goes overvoltage, the SCR fires and hopefully blows the fuse. Thsi
might be worth considering for the 9825 too.
There's not much space in there, and I hate to cut
traces and have fat wires all
over the place. I'm thinking the best options to add the crowbar circuits would
be:
Remember the crowbar goes in parallel with things (you need to pick up
the votage rails to sense them, and have an SCR which shorts a suitable
point (either hte rail or the input to the PSU) to ground. So there
shouldn't be any need to cut traces. I wonder if there's enough space to
build this as a pgyback to the PSU regulator PCB...
Also, it's good to know which outputs really need
the crowbar. For example,
-12V is regulated to -5V with a Zener, but is also separately regulated on each
RAM board, so is it important to protect the -5V? What's using it?
I will have to shcem schematics. It wouldn't suprise me if it wa used by
the procesosr hybrid module or soemthing like that.
Finally, some of the circuits are fused (+12/+7V), so
it may be enough to place
tack a hefty Zener across the output in some cases to blow the fuse.
I think I would add the normal SCR crowbar.
By the way, does anyone have full schematics for this machine? I am going off
Yes.
There's a lot of information available from
http://www.hpmuseum.net/ This
is the Australian HP Museum site, and is somewhat hard to find your way
around at first. I normally follow the 'documentation' link from the
homeapge and look down the list (since not all manuals are listed under
all the machines they apply to -- an obvious example is that since the
HP9831 hardware is almost indentical to that of the HP9825, you would
need ot look at the latter if you want manuals on the former...)
Don;'t try this site on a dial-up connection :-). There's a lot of
interesting stuff there, and if you're like me you'll read all sorts of
related manuals for intereast...
Anyway, I beleive you can get some HP blueprints from there which apply
to the early versions of the 9825. You can also get 'my' schematics for
the machine (all versions that I've seen) if you can stick my handwriting.
the service manual scan from the HP calculator museum
(Great work, David
The service manuals is a boardswapper guide (and is available for free
from the site I mentioned). It dors contain some useful
information
though. One thing that is there (IIRC), is the schemeatic for the
printer test board. I really must make one sometime...
Hicks!). It would be nice to get the interfacing info
for the ROM modules and
the I/O ports.
In general the I/O module manuals contain schematics... The I/O bus is
not hard to figure out from that.
The ROM modules have internal address latches and address decoders. So
far I've seen 3 types. One uses a pair of HP custom ROM chips (16 bits
wide, with intenral latches and decoders). Another uses more conventional
ROM chips and standard glue logic. And the last is bank-switched, with
the bank control logic being standard TTL parts .Schematics for the first
2 are in the HP9825 diagram set I mentioend, I don't think the last one
(I came across it in the later disk drive ROM) is up there yet, but if
you need it, I can tell you (off-list) who to contact.
-tony