Faulty fullnut 41CV


Hi everyone,
I have a problem with my fullnut 41CV.

It was working perfectly until I put it into storage a couple years ago. I turned it on (new batteries) some days ago and it developed the following:

the display is dim;
the card reader pulls the card slooowly giving read errors;
printer operation causes the display to get dimmer and dimmer until the calc turns off.

I have replaced C1 (470 uF) capacitor to no avail; I have even tried the calc without it -all stays the same.

Any ideas?

Thanks in advance,


Ciao, Paolo.

Please, allow me to tell you about an experience of mine.

Once (not upon a time...) I bought 'new' N-size batteries and they worked flawlessly for a few days. Then, one day, I turned my HP41 on and it came up OK, but for each number key I pressed the display dimmed and got back. I tried some simple calculations - adding, compute sine - and the calculator simply turned off. I tested each battery and their voltages measured OK, although after tuning the calculator on, the nominal voltage of the set dropped to about 5Vcc (1Vcc below the expected 6Vcc). When pressing a number key the voltage dropped to about 4.2Vcc and after trying any calculation it went to less then that.

As for a conclusion: one of the batteries was defective. It measured about 1.5Vcc but had no current capacity. I'd suggest briefly connecting each of them to a 100ohms resistor while measuring the Vcc in their terminals to check if there is any voltage drop. Try lower resistor values, at least about 33ohms (not less than this, it should give about 50mA), always briefly connecting so to keep the battery life for using in the HP41.

I hope you find a 'bad guy' amongst them. If not, research should go for the IO/battery contacts block.

Cheers. And success!

Luiz (Brazil)

Edited: 23 Aug 2012, 3:46 p.m.


Hi Luiz,

thanks for your prompt reply. I waited a bit to post again, just to test a third set of new batteries. The calc has exactly the same behavior. It seems to drain too much current, so there's obviously some electrical leak. Any other component to check?




One thing you could try is to let the calculator without batteries for a while until the main electrolytic capacitor(s) discharge completely. The HP41 manual states that in some circumstances, the calculator may freeze and not to react to keyboard presses. I had my HP41 with constant beeping after inserting batteries, or beeping on each key press and in some other awkward behavior. Chances are that part of the hardware locks itself in some higher current consumption, or even the SW is in loop somehow. Because the main circuits are constantly fed for as long as the batteries are inserted and they usually have a very low quiescent current, it might take a few days so the main electrolytic capacitors discharge completely.

I guess the best explanation for that is fast current variation while inserting the batteries after the calculator is left off for a while. When inserting the battery pack, contacts may momentarily fail in many points of the link: at least eight battery terminals are shaking, being pressed or sliding as the battery pack is inserted. Because internal capacitors are discharged - your calculator was stored for a while, right? - initial current is higher then normal prior to the battery set gets to final position. Meanwhile, circuits are under current and voltage variations, and final behavior may not be as expected.

If you could open it, carefully shorting the electrolytic capacitors would accelerate the process, but if your calculator has never been opened, then leave it as it is. Again mentioning manual contents, continuously pressing the ON key while the calculator is without batteries would also accelerate the capacitors discharge.

I guess I wrote too much.


Luiz (Brazil)

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