HP-80 display problem.

[jlw]

I very recently acquired an HP-80 calculator that works fine except for a display problem/anomaly. It lights segment c (the one that's off only in a 2) for all digits except the space where a dot would not be when indicating low battery with the decimal points.

If I power this externally and lower the voltage to about 3.5V, those extra segments go away and all digits look correct. If I lower it a tiny bit more, the low battery indication comes on. So there's a tiny voltage window for proper operation. Alas, the battery pack is about 3.9V, well outside this tiny window. The AC adapter is even higher.

If I install the HP-80 logic board in an HP-45, it works and displays just fine. I have three HP-45 keyboard/displays, in various stages of fixing their own display problems (single segments wrong), but one is fully restored. For that matter, an HP-45 logic board on the HP-80 display board exhibits the same problem, starting at 4.2V.

I fetched and perused the hand-drawn schematics for both machines (from http://www.hpcc.org/cdroms/schematics5.0/index.html) looking for differences. One thing I noted was that the value of the resistor on the Rset pin (10) of the anode driver chip U1 was 220k on the 80 and 330k on the 45. (It's 300k on the 35, 220k on the 70 and 65, 120k on the 55. The 67 is a different beast).

Per these schematics, part numbers for the keyboard/display board are 00035-60004 for the 70, 80, and the 45, 00035-80004 for the 35. All of mine are 60004.

Examining the boards, two of them have 220k resistors installed, one has a 330k, and the 80 has a 390k. It looks like it had been replaced once. I found a 220k (measured 216k) to replace it (Wow, those resistors are tiny! Not quite SMD, but getting there.). The 390k that came out measured 520k or so. However, this change did not affect the display anomaly.

The anomaly is observable on a scope at the c pin (13) of the anode driver as a whole bunch of extra pulses. They go away when the voltage is reduced.

I was hoping that passive components could fix this problem. I don't particularly want to replace the anode driver (I'm down to one uncommitted good one, and they're not much fun to remove, and this might not be the problem either.). There aren't very many components on the display board beyond these. I also don't want to cobble up a 3.5V regulator, or a voltage divider, either.

Suggestions (besides give up, or only use it on my lab supply)?