HP-9100A after repair and testing
To have a reference while repairing the 9100B the owner brought his very nice looking, but also not working 9100A. This is the first version. The processor seems to be working. You can test this keying : 1 CHS Squareroot. This activates the error light. But the tube stayed dark so first thing I did was checking the power supply. Cleaning the connector, reforming the caps and some resoldering gave all the voltages except the HV. The feedback was around +34V instead of -15.6V, It is a voltage divider between -3500 and +34.8 So that means, no HV.
Underside PSU
When I had the PSU frame free I could measure the HV without removing the CRT. And indeed no HV.
HV unit
The transistor Q1 and HV diode both were dead. This transistor is a real problem. It is a hand picked RCA transistor. A special selected 2n1701. On the powersupply is a second one that regulated feedback. I think they must be matched. According to discussions I found on a forum the beta of the transistor must be not to high. Otherwise the EHT becomes unregulated. The transformer is a selfresonant circuit at 25KHz.
I did a very quick and dirty selfresonance test of the collector coil and this was 27KHz, it should be 25KHz so the primary seems to be OK. I measured the selfinduction of the primairy and secundairy windings.
Now I have to test if the secundairy or the replacement transistor fails. I did not trust this board because the 10KV diode failed. The transformer could be the cause for that. So I replaced the HV board and to be sure the psu board with those from the 9100B. And this is the result:
It’s alive !
temporary for more testing
I replaced the psu board as a test, because all voltages are correct and no bad parts. But that gave a distorted and unstable HV. That is why I think the power transistors are matched.
5 keyboard keyes did not respond, for the rest all functions seem to work. The test: 9 sin cos tan arctan arccos arcsin gives the right outcome (9 and a bit) So I’m afraid I have to take this keyboard also apart.
After taking apart the keyboard and cleaning all keys and contacts all keys work. I think at on moment in time someone sprayed or spilled some sticky stuff over the keyboard, because when I washed the keys, some became sticky until I scrubbed them. Looked like a sort of wax.
I am not good in operating this machine but a little test program I entered did work 
So another nice piece of history saved.
