The Museum of HP Calculators

HP Forum Archive 06

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Message #1 Posted by Paul Vollbracht on 2 Nov 2001, 2:16 a.m.

Dear all,

I own an HP41C (one of first). I think I bought it in 1980. Afterwards I also bought the Card Reader and the RAM Memory (Quad Memory) on it. Because I use the Card Reader I also have (had, some time ago it broke down) the rechargeable battery pack in it. I replaced it by the normal battery pack and suddenly the calculator did not function anymore (no reading on the LCD). I already tried with new batteries but without any result. Knowing the batteries are OK I tried the Card Reader and it doesn't work either. The BEEP or TONE functions don't function either. Are these useful indications???

Last week I opened the calculator and measured on the print. I have 6.3 V DC on it.

My questions to you:

If the LCD for instance has broken down; can it be possible that the Card Reader doesn't function anymore either. It gets it's supply from the calculator? Do you know on what tension the Card Reader functions? BEEP and TONE don't work either can it also be related to the fact that the LCD is broken? Is it possible to obtain electronic schemes or repair manuals so that I can measure voltages or test for signals? Someone told me a DC/DC converter is on it to change the 6.3V to 18V for the Card Reader and the LCD. Apparently old LCD function on 18V DC. and sometimes the transistor on this DC/DC converter breaks down. This converter consists of a coil and a transistor.

I would appreciate it if you could give my any information so that I can repair my HP41C.

Best regards & many thanks in advance,

Paul Vollbracht Belgium

Re: HP41C
Message #2 Posted by Vieira, Luiz C. (Brazil) on 2 Nov 2001, 8:30 a.m.,
in response to message #1 by Paul Vollbracht


I am not sure, but I do not remember measuring 18Vcc (or Vac) at any point in the 41C/CV mainboard. By looking at the schematics (try this link: "". There is a PDF in here for the earlier 41's. It is not applicable for most 41's models).

I did not understand one thing: is the display broken, I mean, fragmented in pieces? Or it is in one piece but operating? And how did you test the card reader? Have you tried to pass a card through it?

It seems to me that you have a bad contact between the IO/Batteries assembly and the mother board. But you mentioned: "I opened the calculator and measured on the print. I have 6.3 V DC on it." In which point were the 6.3Vdc measured? The first 41's models (fullnut) had a pair of connectors made by golden plated spheres plus springs built in the AC adapterīs tunnel. Through this square tunnel the user could insert the AC for the rechargeable pack OR a DC PSU to drive the machine directly. I have never seen such unit, but I have used these connectors for testing purposes. This was the easiest way I found for measuring inside voltages with the calculator ON or OFF.

I really would like to help, but I did not figure out some points. Would you post some more information?


Re: HP41C
Message #3 Posted by Paul Vollbracht on 5 Nov 2001, 3:13 a.m.,
in response to message #2 by Vieira, Luiz C. (Brazil)

Thank you for your reaction.

Your message can be very helpful. I downloaded the HP41-scheme. I will try to measure some voltages again.

In reply to you, the LCD is not broken. My HP41C looks brand-new. For years I use the rechargeable battery pack on it. I had to replace it by normal batteries because the rechargeable pack is not available any more. When the normal batteries were in it it functioned again. No problem. But after a while it suddenly gave no reading any more on the LCD. I took the batteries out for a check but they were installed OK. But them back in but without any result. No reading anymore on the LCD. So I tried the card reader by putting a magnetic card in it and it did not function either. BEEP or TONE don't work either. Because of that I wondered if the malfunction of the LCD could be related with the fact that the card reader or BEEP or TONE did not work either. Normally they should work because they have nothing to do with the display. But it is also possible that if the LCD does not function all other functions are cut off also.

So I opened it and put the batteries in and started to measure. Between pin 3 and 11 on the main print I have 6.3 V. Because I have no scheme of the calculator I only followed on the print the 6.3 V track because I thought it had a bad contact somewhere (no power supply) because the card reader did not function.

I indeed have the gold plated spring at the side of the calculator for plug-in of the AC-adaptor. My 41C is one made in USA and is one of the first.

Best regards & thanks for your help

Paul Vollbracht

Re: HP41C
Message #4 Posted by Vieira, Luiz C. (Brazil) on 5 Nov 2001, 6:23 a.m.,
in response to message #3 by Paul Vollbracht


Just a small point: BEEP, TONE, as any other features in the 41 System, will work only when the calculator is ON. I have seen some 41īs that would generate sound without any reading in the display, but they were switched to ON and the display was malfunctioning. In other situation, a failing 41 just reacted as an endless buzzer at the very moment the batteries where flipped inside. Crazy fella!

But there is a straight relationship between TONE, BEEP and LCD, as the LCD is the primary source of information that the calculator is working fine. O.K., keyboard too; but I had an HP41CX that generated almost no sound at all; the beeper was failure.


Re: HP41C
Message #5 Posted by Tony Duell (UK) on 2 Nov 2001, 1:55 p.m.,
in response to message #1 by Paul Vollbracht

Yes, there is a DC-DC converter _in the card reader_. I am not sure what the output voltage is, but even if it's failed, the rest of the machine should work. I would start by trying to fix the 41C on its own (without any peripherals or modules plugged in). Then, when that works, fix the add-ons, if they need fixing. OK, most problems with HP41s are bad contacts inside. Take the machine apart (4 screws under the feet), lift off the back cover, undo the nuts holding the logic PCB in place (if present), remove the logic PCB and its connector. Clean the contact pads on the logic PCB and keyboard PCB. Clean the logic PCB connector. Clean the flexible PCB contacts on the I/O assembly inside the lower case. Squeeze the sides of the flexible PCB at the keyboard connector area to improve the contact there. Propan-2-ol (isopropyl alcohol) is about the best stuff to use for cleaning these parts. Then put it all back together. Most likely it'll work again. Sometimes an old rechargeable pack will leak and damage the flexible PCB. It's possible to carefully solder wires across the damage, but it's not easy to do this. It's better to look arouns for a 'parts unit' with a good I/O assembly. Of course the HP41 is almost entirely CMOS logic, so you must take anti-static precautions (ground yourself with a wrist strap, and so on). You don't want to damage the ICs.

Re: HP41C
Message #6 Posted by David Smith on 2 Nov 2001, 5:48 p.m.,
in response to message #1 by Paul Vollbracht

A very common cause of an HP41 working on the rechargeable pack and not the alkaline pack is related to the center two battery connection posts on the flex circuit battery connector in the calculator.

These two posts are just connected together. They connect the center two battery contacts together. In the rechargeable packs the center two batteries are hard wired together. If your alkaline pack does not connect properly to these two center posts then the machine will get no power. You can check this by using a piece of foil across the center two battery posts.

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