Classic display segment intermittent - cause?

[Mark Henderson]

Hi - I recently added a nice HP-70 to my collection. It's in good shape overall, working.

When first powered up the "c" segment in each digit is off (identifying segments from "a" at top, clockwise). I've seen posts about this kind of fault (usually display driver?).

However, after about 5-10 minutes from cold power-on, the segment will behave correctly, for about 10 minutes, then the segment will extinguish again, and stay off (until the calc is switched off and left for a few hours).

So it seems to be some sort of "warm up" behavior. I'm going to go in with a scope etc and see what's going on, but from the collective experience of the forum, what might be wrong, or where should I start looking?

Cheers

[Randy]

A bad anode driver is a likely culprit. Part number is 1820-1024 which is unique to the 70. While other classics use different part numbers in that position, they might function as they all share the same pin outs.

[Mark Henderson]

Probably wishful thinking on my behalf, but I was wondering if this could be a bad solder joint or similar on the inductor?

[Mark Henderson]

Well I finally opened up my HP-70 and did a full restore. Sure enough, the intermittent segment behavior (above) was just a very obvious dry solder joint - fixed in 10 seconds! Now I have a very clean, 100% functional HP-70!! Used Novus plastic cleaners for the first time, and very impressed.