HP 20S with very high "off" current

[Neil Hamilton (Ottawa)]

I have a 20S of unknown heritage (S/N 3251SXXXXXX) that burned through a brand new set of 357 batteries in less than one week.

The display of the 20S is very dark -- call it a large dark smudge across its middle. When running, the information is visible and not that compromised.

I measured the current of both the 20S and a functional 10B with the following results:

State            20S      10B

off              1.14mA   <1uA

on (idle)        1.29mA   ~100uA

button pressed   2.7mA    2.03mA

running (!n)     3.2mA    1.94mA

I haven't ripped the 20S apart (yet) but I believe its internals are virtually identical to a 10B. I have previously ripped my 10B apart and my recollection is that it has nothing more than the single IC and the power supply capacitor.

Any ideas of what the issue might be? Could the cap be bad? Is the (dark) display at fault? (It would be worth swapping the keyboard/display assembly between the 2 units to see if it is the display.)

Its not particularly valuable but I wouldn't mind trying to get it working again.

Cheers...

[Katie Wasserman]

I think it's most likely to have a really leaky (i.e., high leakage current) chip. I've got a 32sii, latest color scheme, calculator with the same problem. If so, there's nothing you can do to fix it. However you can try to change the capacitor -- or just remove it to see if that helps the current draw problem. The dark LCD issue might be related only in the sense that the calculator has apparently seen some difficult environmental conditions.

[Neil Hamilton (Ottawa)]

Thanks Katie.

Even though I could not see any cold solder joints on the processor with my hand loupe, several appeared a bit oxidized. I scrubbed with vinegar and then reflowed all the joints. Bingo, the current returned to normal and the calculator is working as expected again (albeit with the dark display).

Maybe you want to give that a go on your 'leaky' 32sii.

Best of luck!