HP-80

[shinji-ben]

I just got a second hand HP-80 in fairly good condition. However, when I power it up with fully charged batteries, it only displayed a few segments faintly (can hardly see). Same thing happened when I used the adaptor. I have dismentled it to clean and have not noticed any loose wire connection. Can anyone advise as to what the problem is and how to bring it to life again ?

[Viktor Toth]

Ben,

I've seen a similar problem with an HP-55 a while back; the problem turned out to be a failed diode. On that machine, you could actually see the faintly lit segments flicker in response to keystrokes, so you could tell that the machine was still alive.

I have not actually discovered the failed diode until after I've desoldered, and replaced, both display driver chips (groan!)

I can provide more details in e-mail if you want.

Viktor

[Matthew Riehl]

I recently purchased an HP-65 with a display problem similar to this. It has a couple of segments out, and the 1st (where the negative sign goes) and 9th digits very dimly show what other digits have display.

For example starting off with "0.00" displayed, there is a ghost "0" in the sign position and the 9th position. If you press a "5" then a ghost "5" is displayed in those two spots. Subsequently pressing a "7" shows the combined segments of the "5 & 7" in these two spots. Other than this display problem works like a champ, card reader and all.

Any ideas of what could cause this problem and how I might fix it?

Matt Riehl

[Viktor Toth]

Matt,

I'd first check for signs of corrosion. This problem is more prevalent in VFD calculators that use higher voltages but I've seen it in LED models as well.

The problem is simple; at the voltages used to drive the display, corrosion that conducts with a resistance of a few 10 kOhms is enough to produce sufficient current for 'ghost' digits to appear. Since the same lines used to drive digits are also used to scan the keyboard, there's plenty of surface area for that much corrosion to accumulate.

Viktor

[Erik Wahlin]

I have seen the same thing with failed anode or cathode driver chips. I know that hooking up the battery polarity wrong will hurt these chips.