Dead HP97, any ideas? TIA


I picked up a HP97 today, but it has seen better days. It appears that someone certainly plugged in the wrong power supply, possibly even a mains lead (judging by the burnt pin). That however isn't as bad as it seems, as far as I can tell it the diodes went short circuit and that's as far as it got.

I replaced the diodes and it now powers up. A picture of what is displayed is at:

The display is static, and no keypresses or switch positions change it.

Any ideas?

Please email as well as posting to the forum.



Burnt pin? Yikes... poor HP-97.

The round LED above the numeric display is the low battery indicator. If you're using the calculator with an uncharged battery, it may be a while before it goes out. If the battery pack is old, it may have dead cells that are essentially shorts, so you'll never get proper voltage to your machine without fixing the battery pack first. (You cannot use it without a battery pack, not unless you use your own stabilized power supply.)

As for that 1. on the display, it looks bad. Two questions. First, is it always the same display, or does it change after you turn the machine off and on? Second, can you tell whether it is flickering (i.e., if the display is properly multiplexed?) By moving the calculator rapidly back and forth in front of your eyes, you may be able to see the flicker. If the segments that are lit are unusually bright and don't even flicker, that means that the multiplexing circuit doesn't work. (This may be good news; as usual, the more fundamental a problem it is, the easier it might be to repair it.)



Considering that if it was plugged into a mains outlet (and a common domestic lead for a radio/vcr/etc would fit in the 97's charger socket) it's had 240V at 16A I'm suprised the only obvious physical damage was the pin. The 4 diodes were all shorted, but didn't look burnt.

Seeing as I don't have a battery pack it explains the low battery led. Is that battery pack 4.8V?

As for the display, it's not a 1, only the bottom segment is lit. It's the same every time. I can't see any flicker and it does seem brighter than normal (though I don't have another calc for comparison here).



Did you measure the voltage after the powerstabilizer, since you replaced the AC/DC diodes. There might also be some Zenerdiodes, to stabilize the DC power to the calculator and the battery pack, which might have gone down also. Regards, Jan


First of all, make sure that you aren't trying to use the machine with the HP wall plug, but no battery installed. It's the battery that both stabilizes and regulates the rectified voltage, so without it, the calculator will not work (may even end up being damaged, but I don't think so; it's the continuous memory models that get more easily fried this way.)

The battery voltage is indeed 4.8V nominal; 4 NiCd cells in series. You can use a standard 5V power stabilized supply attached to the battery terminals if you have no battery pack.

No flicker means that you probably don't have a clock frequency. Which may mean that only that part is fried, or it may mean that everything is fried. It's been a while since I last took one of my HP-97s apart, so I don't exactly recall how the clock frequency is generated, but I'd just start poking around with a 'scope, looking for signs of life; I'd also check any discrete components, like diodes and transistors, in the hope that the problem is with one of those, because those are relatively easy to diagnose and replace.

Then again, I'm just a programmer, and you know what they say: beware of programmers who carry screwdrivers! :-)



Are you related to Mike Toth from Barnes Engineering around 1980.

I worked with him and we had an HP-97.

Just curios.

Andrew Beiler IBM Global Service


No, to the best of my knowledge we're not related. Toth is one of the most common Hungarian surnames.


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