HP 2100A Restoration

Rik Bos hp-fix at xs4all.nl
Tue Aug 2 16:46:25 CDT 2016

> -----Oorspronkelijk bericht-----
> Van: cctalk [mailto:cctalk-bounces at classiccmp.org] Namens Jay West
> Verzonden: dinsdag 2 augustus 2016 23:24
> Aan: 'General Discussion: On-Topic and Off-Topic Posts'
> Onderwerp: RE: HP 2100A Restoration
> There is an HP book specifically on I/O interfacing to the 2100 (and 21MX) I/O
> bus. I think there's also an abbreviated chapter on it in a different manual. I'll see
> if I can dig up the name and if it's online anywhere.
> I've restored at least five 2100's, and run a dual cpu 2100A & 2100S system
> regularly. I have never heard of nor seen a power supply failure on any of them.
> The power supply always came up with little or no pot adjustment, and nothing
> else required. I'd be surprised if your PS needed any attention unless they were
> under water.
> Debugging the memory section is a far different matter. It's far *far* easier to
> troubleshoot if you have a known working set. If not, it's very easy for boards to
> test ok and then later fail and then later test ok (bad upper section of a 16K
> driver, etc.).
> J

About power supply failures mine did have one, when under power after a few minutes there was a loud bang and the system resets.
Which would be repeated every few minutes, some investigating and measurement later I discovered one of the high voltage elco's shorted every few minutes.
Which started the power on cycle and resets the cpu.
After replacing those all voltages were stable and in range and the banging was over ;)

After some testing I concluded there were some faults in the cpu, being the lucky owner of an extender board I was able to fix all the errors.
The HP 2100 service manual and hardware course and engineering reference are a big help fixing the 2100 and as J says a set of known good cards can be of help too.
But knowledge of the working of the micro instructions is also a big help, IRC the micro instructions are described in both the engineering reference and the service manual both can be found on bitsavers.

The only weak point of the 2100 is the use of uCTL a Fiarchild DTL derivate used for IO drivers in the 2100 series.
Brent Hilpert has a lot of info on his site about those gates.


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