HP9100 CRT


I have an HP9100A with a defective CRT (damaged by the parcel smashers). Apparently it has lost vacuum at the base end and has an open filament. Does anybody know of a replacement source or of a company that can repair the tube?


Firstly, DO NOT power that machine up. The electrodes in these CRTs are supported on (brittle) glass support rods. They break, and oyu get inter-electrode shorts. If you are unlucky, you blow transistors on the deflection boards, followed by diodes on the main gating board, and if you're _really_ unlucky, some of the flip-flop transistors as well.
To see just how bad it is, pull the CRT and the EHT generator fuse (on the PSU chassis). Power up, and try something that should result in an error (STOP, STOP, 1, CHS, SQRT is as good as anything). If the error lamp comes on, then the processor is basically working, and it's certainly worth looking for a new CRT. If not, then you have a processor fault to find as well.
OK, back to the CRT problem. It's HP custom (the main 'feature' being that it's very short for an electrostatic CRT). AFAIK, new ones are totally unobtainable now. Having the old one rebuilt is possible, although if it's been full of air for some time, the screen phosphor will need replacing as well, most likely. There are still a few companies who rebuild CRTs who _might_ be able to repair it, but I would think that the cathode/heater assembly would be
very difficult to get. I wonder if it's possible to interface an external CRT to the 9100 (without the modified deflection boards, etc that were used with the external display option). Tap off the signals from the deflection boards, and feed them to a simple 'scope or something. I suspect it would be cheaper than a rebuilt CRT.


Yes, I know about the problems with broken internals to the CRT. This one does not have that problem. Only the filament is open and I suspect that the tube has lost vacuum (tiny peices of glass about the size of salt grains can be shaken out of the base which has come unglued from the neck). It was damaged in shipping last week. The rest of the unit works fine now... it just needs a new CRT.


Hi Mark,
Have you thought about adapting an electrostatic CRT
from a portable oscilloscope? As Tony points out
the CRTs from most scopes would be too big, but
there are some older portable models (the Tek 465
comes to mind, plus various Sony/Tektronix small
portables) with small ones. I haven't tried this
myself but if I ever picked up a 9100 with a broken
tube I think I would, just to get it operational even
though it wouldn't look stock and the display
would probably flicker. The HP9100 service
manual has schematics for the horizontal &
vertical display driver boards, plus some
waveforms, it may require modifying those boards
to get the voltages right but still may be
worth the effort. Good luck!

Alex Knight

The Calculator Museum Web Page


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