HP 8930A Repair and Demonstation

June 2017

Several years ago, my Dad used to take me to an old electronics store called Mike Quinn Electronics in Oakland, CA. It was a junkyard for old, broken, or otherwise out of date electronics. The imsai.net page has a little write up on this place This is the same place where I bought a box of parts that happened to contain some COSMAC 1802 chips, which I forgot about until a couple years ago.

One item my Dad bought there was a HP9830A. I had never actually seen it myself, or even knew what model it was, he just told me once he had an old desk calculator filled with ROM chips somewhere It sat in storage for years, long forgotten. A few months ago, everything was being cleaned out, and he gave it to me. I tried it out, it didn't come on. He said it was fine, that he had just tried it out recently, and it just had a little glitch in the display. Carefully, I pulled each circuit board out, and seated them back in their slots, attempting to power the machine after each board. It came on! And, it did have a glitch in the display. There was a bad column of pixels in each character position. It can easily be seen in this video:

I will post more details, but the problem was with a single 7400 series logic chip in the display driver board: a 4-to-10 line decoder which selects pixel positions. After replacing it, it works perfectly: