Repairing a HP41C Calculator Battery Case

This HP41C calculator battery case has corrosion on 3 of the four springs. These springs serves to hold the N-size batteries in place as well as provide electrical connection between the four batteries to provide the 6V DC needed. Two springs each is actually made from one single wire. So you actually have only two separate springs. Each pair of springs connect two N-size batteries together to create 3V. The battery contacts on the HP41C caculator itself  connect these 2 3V supplies into create 6V.

Because the springs is very hard to find ( since most battery spings are single spring not paired like this ) I decided to clean off the corrosion and recoated the ends of the spring with solder. The springs are made of metal with a high copper content, most likely a cupro-nickel alloy so soldering is quite easy.

These pictures shows the before and after of recoating the springs with solder. A check with a ohmeter shows good electrical contact ( measuring 0.5 ohm - mostly like due to probe contact resistance and meter tolerances )

HP41C Battery Case

Note that the two thin flat springy metal clips that clips to the inner walls( one from the left outside and one from the right outside wall ) are missing. The intent of these thin flat strips of springs  is supposed to prevent the N size battery from slipping out of the battery case when tilting the battery case to install into the HP41C calculator. However on closer introspection, these may not be needed since you do not need to tilt the battery case too much in order to install it properly into the calculator. Once installed the battery springs push against the battery contacts in the calculator and provide good electrical contact as well as hold the  battery case securely in the HP41C calculator.

HP41C Card Reader Mug Shots

A HP 41C Calculator Card Reader

This is a card reader taken from a HP41C calculator. It is in good mechanical condition, no scratches, cracks. However I am not sure if it still works as I have no resources to test it. This card reader is once use to read magnetic strip cards that store programming instructions for the HP41C programmable calculator.

Fitting AA Size Batteries Into The HP 41C Battery Holder

Why HP chose not to use AA size battery in the HP 41C calculator is a mystery.
Here are some AA size alkaline batteries next to the HP 41C calculator battery holder. Lengthwise, the AA battery is longer ( about 20mm ), width wise there seems to be some hope in camouflaging the buldge. Actually, the little increase in size of the calculator is not as critical as say the increase in size of a hearing aid.

AA size battery are cheaper than N size battery and their capacity is also higher. They are cheaper because there are more appliances using them than N size battery.

HP 41C Calculator Battery, Battery Pack, Battery Holder

The HP 41C uses a N size battery. Earlier version use the N size NiCd rechargeable battery while later versions used the N size alkaline battery.

The N size battery is almost the same in external diameter as a normal AA size battery and about 20mm shorter.

The normal operating current of the HP 41C is about 20mA without the other peripherals like printer or card reader. The standy current is very minimal. Thus a normal alkaline AA size battery should last quite a while.

Even then the reason for the choice of N size battery may be know to the people in HP who design the HP 41C calculator. Would not the use of a normal standard AA size battery makes the HP 41C more a joy and cheaper to own? Would a extra 20mm in length of the calculator do that much damage to its aethetics?

Another unique feature of the HP 41C is the use of special springs for the battery contacts in the battery holder. A normal batery holder uses a spring that has a flat coil linked to another conical coil. The HP 41C battery holder uses a spring with 2 conical coils. This is arises as a result of using the springs to hold the battery holder secure in the casing. ( These springy battery contacts press against the printed circuit foil battery contacts on the IO connector ). It is a clever design, aimed more at manufacturing simplicity I believe. However the special springs are very hard to replace if they became corroded from leaking batteries.
( A less elegant replacement solution is to use four standard spring battery contacts and overlap one conical end with the flat coil end of another )

In restrospect, HP could well have ensure the long term survial of the HP41 if somehow they could have designed it to be use with AA size batteries then. ( Set aside the problems cause by the crack screw posts and the resulting poor contacts in the IO connectors and the CPU PCB to keypad connector )

However N size battery are still available today, either as alkaline or rechargeable battery.

A good option is to use the newer rechargeable NiMH type rather than the obsoleted NiCd type. A NiMH battery provides 1.2V. Four will give 4.8V which is well with the minimum operating voltage of 4.5V of the HP 41C. NiMH are less prone to leaks. Use an external charger to charge the battery.

With this and some luck you may still be able to use your HP 41C for another 30 years.

Why The Screw Posts In The HP-41C Calculator Cracks

HP has ingenously design the HP-41 for very speedy production line assembly. The four screws that are use to fastern the two halfs of the case together also serves to ensure that the IO connector and the CPU PCBA make good contact with the keypad PCB. The printing wiring on the keypad PCB serves the all important task of connecting the IO connector, CPU and keypad together.

It is a well know fact that broken or crack screw posts that holds these four screws are very common in HP-41C calculators.

I just taken apart a HP41C with this problem. The reasons why the screw posts cracked like they do are very obvious. Due to manufacturing tolerence in moulding, it is quite difficult to ensure the the screw posts on each half of the casing mate properly. The screw posts that fasten the CPU board to the keypad has actually to press down on the CPU PCA in order to do this.

The simple solution is to use fiber or rubber washers at the right places around the screw posts as these pictures shows. Actually a HP service manual published in July 1979 mentioned the use of washers. But this calculator was made in 1982 and no washers were found. Could the washers have been left out during repairs?

Unfortunately I do not have access to a original unrepaired 1982 HP-41C or a updated manual to find out. But current owners of HP-41C could well be aware of the issue and the cure and take the appropriate steps could see their HP-41C giving more years of enjoyment.

In passing, here we have a fine piece of computing device brought down with a simple mechanical oversight, much like the O-rings that crashes the Challenger space shuttle.

Inside The Hewlett Parkard HP-41C Calculator

The HP-41 is easily opened by first removeing the four rubber feets on the back cover.

Then removed the four screws. These four screws are very important. They hold the HP-41C together plus ensure there are good electrical contacts between the modules inside. This is a very clever design, unfortunately the mechanical strength of the four screw posts frequently cracked and leads to failure of an otherwise well design machine, functionality and form wise.

As show in the picture above the screw posts on the back cover at the top end of the calculator where the IO connector has broke off. This is likely the result of overtightening or the use of too short screws. On hindsight the use of some washers between the post and the IO connectors may have solved this issue. Then again these may have been lost or misplace during servicing.

It seems that the design choice for the IO connector and the contact pads for the CPU PCA was constrained by the need to make the calculator thin. Use of connectors would have raise the height of the calculator as plastic injection moulding precision in the late 1970s and early 1980s may not have been precise enough to make close tolerance and miniature connectors. No doubt using connectors and plug would have make for a more reliable calculator ( considering that the majority of failures out there seems to be cracked screw posts which leads to contact failture between the electronics modules.

Hewlett Packard HP-41C Programmable Calculator Mug Shots

Here are some mug shots of the HP-41C. The most obvious feature is the wedge-shape design.

This is a clever design to make the calculator slim and at the same time able to handle plugin modules. The resulting slant upwards toward the LCD display makes reading the LCD display much more easier when place on the desk.

Note the moulded serial number on the back of the lower cover. I am wondering if ever a cover is not use. This is probably a hallmark of HP just in time manufacturing practice where materials are made available just in time for use with the resulting low inventory overheads. Moulding the serial number on the casing does reduce manufacturing time and eventually cost making the production line more efficient. But it needs careful planning to prevent wastage. A smaller manufacturing outfit that lacks the resouces for such planning will probably use serial numbers on stickers as overruns in this case is less expensive( just discard the excess stickers ). But the extra time needed on the production line is a cost to be considered.

This is the physical look of the HP-41C calculator. The reason the HP-41C is so popular is the functionality which is defined by the CPU and the software thats runs it.

Note the plugs for the plugin modules which can be anything from memory to printers to card readers.

One of the port for the plugin modules is covered. The rest are plugin modules that are missing from this specimen.

Hewlett Packard HP-21 Calculator - Improvements On Hindsight

In restrospect, the HP-21 has been designed with a lot of attention to quality especially so in the functionality of the calculator, but more design effort in the following areas would make the calculator survive the odds of a battery leakage.

1. Improve battery compartment that is battery leak proof.

2. Sealing the LED display from the environment by sealing the gap between the magnifying lens and the printed circuit board.

3. 100% sealing of the keypad against dust/spillage etc.

The reasons that these are not done is probably the fact that though these ideas sound simple enough on paper, implementation in a manufacturing enviroment can be costly in terms of materials, time and eventually money. Weighed against the need for a affordable calculator with good funtionality and a battle harden calculator, it is understandable why this is not done.

However, for collectors it is still not too late to battle harden their HP-21.

The LED display can be sealed with instant glue ( just be careful not to allow the glue to slip into the LED IC chips.

Lining the battery compartment with absorbent paper could prevent damage from leaking batteries ( that is if you still want to use a NiCd ) else switch to more modern leak-proof battery.

Not much can be done about the keypad, as the use of heat stakes to fastened the faceplate to the printed circuit keypad wiring make this a more skill instensive activity.

Hewlett Packard HP-21 Calculator Battery Charger

The battery charger is just a simple AC step down transformer that output about 10VAC when unloaded. This particular model has a switch to select between 110VAC or 230VAC.

It seems like HP is very determined to cut the cost of manufacturing down by incorporating a part of the AC 110/230V AC selection switch into the printed circuit board. It is not a very reliable switch in terms of dust and contact resistant, but you normally dont switch it too often, perhaps only once when you first use it to adjust to local power supply voltage. However the assembly of the movable switch and the charger does call for a some dexterity from the factory operators which they found in abundance from the young workers in Singapore.

For the HP-21 calculator this is half rectified and feed to the circuits via a resistor. A half wave rectified AC of 9.9VAC will get about 4V DC, with the resistor this would have reduce the voltage further to operate the HP-21 or to charge the battery.

Interesting point to note is that electrical safety standard in the 1970s do not seems to mandate the use of a over current fuse or over temperature fuse in the AC step down transformer.

This charger have some intermittent connection problem. I disassembled it to check. There was cracks in the solder joint where the AC power pins connect to the transfromer AC input pins. This probably results from stress when the AC power pins where move too much. It seems that the power pins are not soldered at the grommets to probably anchored them. This causes rocking of the pins then the charger is plug in or remove from the wall power socket.

The screw posts of the charger case has also been cracked as a result of over tightening. So I have use epoxy to stabilised the cracks and at the same time epoxy glue to casing halves together. Interestingly HP may have intended the charger to be repairable so they fastened the casing together with screws instead of just thermal fusing the two halves together like what is done in todays battery charger, a testimony to the cheap labour available compare to material costs in the 1970s.

A Very Unique Charger Plug.

The charging terminals on the HP21C is at the back of the calculator. This could have influence the design of a low profile charger output plug. This allows the caculator to lay flat on a surface when charging is in progress.

Hewlett Packard HP-41C Programmable Scientific Calculator

This HP-41C calculator has the battery terminals and the springs in the battery pack corroded. The memory modules are missing. Applying 6V to the battery terminal and turning on the calculator, the LCD display shows "MEMORY LOST"; look like a pretty intelligent calculator.

A little while later the display when blank and the power supply ammeter indicates a internal short circuit somewhere as excessive current where being drawn.

The serial number of this calculator is 2143S13449. This indicates that this calculator is made in week 43( around October) of 1981. And this is the 13449th calculator to come off the production line.

This calculator suffers corrosion to the battery/modules connector plug. The internal modules where not corroded.

The LCD display seems to be a little discolored. This is a common problem with LCD display stored for a long period of time especially in hot climates. But the LCD can stil display as was discoverd on initial power up.

Hewlett Packard HP-21 Calculator LED Display Module

The 7 segment LED display module plugs into the main logic PCB via gold-plated pin mounted on the main logic PCB. This particular display is also badly corroded by battery leaks. The printed circuit wiring are silver plated. Each digit is actually very tiny and consists of 7 LED segments each lit up individually to form a digit, hence the magnifying lens built over the digits.

Note that the LED digits silicon chip are directly bonded to the printed circuit board. If you look carefully you can see tiny gold wires leading out from the chips to the printed circuit wiring.
Note the size of each LED digit compared to a refill from a ball point pen. The ability to make a LED display from such small dimensions is an engineering marvel. Because of its small size each individual segment needs to be carefully lit with the righ amount of brightness so as not to overhelm the rest of the segments. On hindsight if HP is to make the HP-21 again they would have better display technology to work with now, like OLED. In fact later generations HP calculators use LCD display which can have larger digits.

The gaps where the plastic magnifying lens meets the PCB are not sealed. This probably allows corrosive substances to seeps in and eat away almost all the material in the LED chips. However sealing the gap will probably drive up the manufacturing complexity and cost and even turn around time. It could be also no easy manufacturing solution can be found which would seal the gap. Although the same resin use to coat the logic PCB may used.

This this the foil side of the LED display module. Note the Made In Singapore imprint. Back in the 1960s to 1970s Singapore is in an early industrialisation and manufacturing cost are low there. HP has long since move manufacturing out of Singapore.