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I repaired a HP9100A calculator but still need some help...
04-05-2016, 06:33 PM (This post was last modified: 04-05-2016 06:40 PM by lhf.)
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I repaired a HP9100A calculator but still need some help...
I'm new to this forum and I'd like to say hello to everyone. I'm a collector from China and I've collected few calculators in the past few years, including HP35 and HP45.
Last month I was very lucky to know that someone discovered a HP-9100A in a scrap yard in Hong Kong. This has long been my dream machine. Even though it wasn't in good condition and doesn't work, I still bought it.
The calculator was severely damaged, the keyboard frame cracked into pieces and the aluminum shell was even cracked. Luckily the CRT survived. I did a throughout cleaning and tried to fix it.
Unfortunately I made probably the worst mistake in my life --- Maybe I was too tired staying up all night servicing the machine, I mixed up the horizontal deflection board and the vertical deflection board. Strangely they fit into the sockets very well. When I powered the machine up, the 200V deflection voltage immediately damaged some transistors on the two boards plus some transistors and a resistor in the main body. Future inspection shows that the damaged transistors are all related to one of the core memory INHIBIT/SA channels.
I spent almost one month and finally managed to fix the logic. For safety I disconnected the 4KV generator by removing the fuse, and used an oscilloscope in X-Y mode as display. I used 2N3906 to swap the dead transistors marked 'S24851', and used 2N3904 to replace the '2N2218' which drives the inhibit line. The 2N3906 works in some cases (e.g. on the flip flop board), but fails when used on the sense amplifier. I spent a long time to discover that, during which I tried several models of transistors. Finally I got two transistors from one flip-flop board to use on the SA board and the problem was gone.
When I switch to the original display and reconnected the fuse, there was no HV output. Then I found the power transistor on the HV board (RCA 4 322?) was dead. I replaced it with TIP-41C and finally the machine came back to life, with original CRT working...
I took a lot of pictures during the process and posted on a Chinese forum. It may be the most detailed teardown of a HP-9100A calculator so far. You can see the pictures here:
Sorry it is in Chinese...

Finally, I'd like some help here. I want to know what's the best alternative of the original transistors. Here are the broken ones which I can't find any information:
(From the deflection board)
Motorola 3-016
Motorola 3-203

(From the inhibit/SA board and also the flip-flop board)
Fairchild S24851 007

If you have a spare HP9100A or HP9100B laying around, or just the boards, I'd also like to have the components below so I can repair my little machine just like how it was:

-The keyboard frame;
-The original 'HP-9100A' label on the right of the CRT
-An Inhibit/SA board (For the transistors and a blown resistor);
-The Vertical/Horizontal deflection board;
-5x S24851 transistors (they are widely used in this machine);
-1x 2N2218 transistor from SPARGUE (on the inhibit driver);
-13.7Ohm resistor connected to the 2N2218 on the inhibit driver board;

I can pay for the parts and shipping costs if needed.
I'm also willing to share some suggestions if you are repairing a HP-9100 calculator. Of course the first suggestion is --- NEVER PLUG THE DEFLECTION BOARDS IN THE WRONG PLACE!

Best regards,
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I repaired a HP9100A calculator but still need some help... - lhf - 04-05-2016 06:33 PM

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