> 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.
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:
1) (No mods to the original parts) Build the circuits on a PCB with a connector
on one end, and contact fingers on the other end for the harness.
2) Or (easier to hand-build) remove the connector from the harness, and solder
the wires directly to a PCB with the circuitry and a connector to plug onto the
PSU.
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?
Rik indicated that if the -12V supply fails, the failure mode is towards 0V,
which can damage the RAM boards. Does that mean the best way to protect that
circuit is to build the circuit relative to the unregulated -20V from the
transformer? Are the RAM circuits OK with 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.
By the way, does anyone have full schematics for this machine? I am going off
the service manual scan from the HP calculator museum (Great work, David
Hicks!). It would be nice to get the interfacing info for the ROM modules and
the I/O ports.
-Dave
