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General Notes

Received 4/2018.
No response when powered on (by donor).

Initial Inspection

I verified continuity through original power cord and operation of line power switch.

On inspection, the 115V AC and 15V fuses were intact; the 3.4KV HV fuse was blown. A replacement HV fuse blew immediately when power was applied.

Repair

Based on info in the HP Service Manual (09100-90034) and an online article by PA4TIM, I checked Q1 and CR1 on the HV Power Supply board (A20A2). Both were bad.

Fig. 2: Failed Q1 Fig. 3: Replacement Q1 vs Failed Original(red)

PA4TIM suggested that Q1 was equivalent to RCA 2N1701. He also cautioned that it may be matched against a second transistor in the video supply circuit, so I matched a pair of NOS replacements but only replaced Q1 at this time. The failed HV diode was a SemTech SCMS10K, which I replaced with an EDI 2CL2FG.

Fig. 4: Replacement CR1 vs Failed Original(red)

Replacing these two components produced good CRT output and a largely functional machine. Final step at this stage was to replace the two register illumination bulbs, which are CML 7382. Note that these are 14V bulbs being driven at 15V, pretty strange for HP, which is more likely to under-drive components. I'm looking for an 18V replacent to lower the heat output and increase longevity.

At this point, I was able to begin functional testing. I was able to work through the validation steps in the User Manual and Repair Manual successfully with only two problems - the “8” key didn't work and the Degree/Radian switch feels gummy and works inconsistently. Contact cleaner, gentle massage and burnishing of the key contacts with a narrow strip of bond paper solved these problems.

Initial testing of the card reader shows problems - the card drive mechanism works (card is ejected) when either the RECORD or ENTER buttons are pressed, the rubber drive wheel is intact and functional, but data is neither read nor recorded. Cleaning the head seems to have solved the problem, but I'm still testing and learning my way around this machine.