42S Troubleshooting

[tgwell2]

I've got a 42S that randomly showed a low battery today when I turned it on. It then immediately reset so I tossed the batteries and put new ones in.

I then did a self test and everything passed. I went about using the calculator and in a hurry hit a couple buttons at the same time... the low battery indicator came on again. So, I started pushing buttons and noticed that if I hit all three buttons on the middle or top row of digits (4/5/6 or 7/8/9) it'll flash low battery. If I hold them for any period other than briefly the calculator will reset. It does not seem to do it (at least as severely) if I hold 1/2/3.

I've searched to try and see if this is a common failure but come up empty... I've also cleaned the battery contacts and tried a third set of batteries/confirmed they work in another pioneer.

Was just wondering if anyone had seen something like this before. I may put the calculator on a power supply and look at current, but I'm assuming at this point I'm going to have to open it up.

I've only had the calculator for a couple of years, but had zero problems with it... really hoping it's fixable.

Thanks!

[brouhaha]

The address lines to the RAM chip are the same signals used to scan the keyboard (US patent 5,065,356). There is series resistance (nominally 7K) between the Lewis and RAM signal and the keyboard scan signal, so that pressing multiple keys doesn't cause direct contention of the CMOS outputs of the Lewis chip, and the software doesn't try to access RAM while keys are pressed. However, if you do press multiple keys simultaneously, it will draw more than the normal current through two (or more) of those 7K resistors. Two keys in the same row would draw an extra (roughly) 0.32 mA with fresh batteries, and for four keys about 0.43 mA. That's not a lot of current, but then, the button cells can't supply a lot of current either. (357 silver oxide and A76 alkaline button cells are rated for 0.66 mA continuous, though can provide more.)

Assuming that the problem isn't contact corrosion, the cause is probably a failing RAM chip or a failing Lewis chip. If a Lewis chip is failing, the calculator is toast. The RAM chip is an industry standard chip and can be easily replaced. You can use a 32KiB chip in place of the factory-installed 8KiB chip, and have a 42S with more RAM.

[tgwell2]

(03-08-2024 05:09 AM)brouhaha Wrote:  The address lines to the RAM chip are the same signals used to scan the keyboard (US patent 5,065,356). There is series resistance (nominally 7K) between the Lewis and RAM signal and the keyboard scan signal, so that pressing multiple keys doesn't cause direct contention of the CMOS outputs of the Lewis chip, and the software doesn't try to access RAM while keys are pressed. However, if you do press multiple keys simultaneously, it will draw more than the normal current through two (or more) of those 7K resistors. Two keys in the same row would draw an extra (roughly) 0.32 mA with fresh batteries, and for four keys about 0.43 mA. That's not a lot of current, but then, the button cells can't supply a lot of current either. (357 silver oxide and A76 alkaline button cells are rated for 0.66 mA continuous, though can provide more.)

Assuming that the problem isn't contact corrosion, the cause is probably a failing RAM chip or a failing Lewis chip. If a Lewis chip is failing, the calculator is toast. The RAM chip is an industry standard chip and can be easily replaced. You can use a 32KiB chip in place of the factory-installed 8KiB chip, and have a 42S with more RAM.

This was exactly the kind of information I was looking for! I certainly appreciate the insight.

Today I noticed that I put in LR44s instead of 357 batteries... I bought a new pack of 357s and put them in just to try... and the problem seems to have gone away(for now?). I know there's a slight voltage difference between them, and I'm assuming an impedance difference but I'm a little surprised.

Thanks again!

[BMWgraybeard]

Just a quick thanks to this forum! I have a 42S that I purchased around 1988 and enjoyed using for my entire engineering career (especially after adopting RPN entry). It never failed me, of course. Some of the side texture in the plastic has been worn smooth from handling. The batteries died in it a couple of years ago (I am retired now) and fresh 357's did not revive it at first. I found this forum and in the archive it suggested to jumper the battery terminals for a moment. I would have never expected this to work, but my old friend is back alive again! Unfortunately, my solver programs are gone but I don't have as much use for them anymore. Such an awesome piece of electronics. The manual is here in the house somewhere...
Please forgive this post for lack of technical content, but HP Calculators Rule!