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-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.

Hewlett Packard HP-21 Calculator Keypad Assembly

These pictures show the keypad assembly of the HP-21 calculator. Corrosion from leaked Nickel cadmium battery fluid is very evident.


The switches for each key assembled onto a glass-fiber printed circuit board.

The electrical connections on the PCB are all gold plated. The flexible thin pieces of metal are what caused the switch to close when the key tops on the keypad is pressed.


Note the small thin piece of plastic. This is intended to protect the key switch from dust or spill drinks that may seep in from the key tops. It sits between the keypads and the PCB switches. However it does not seals the key switches 100%. This is because tiny holes have to make to allow the heat stakes from the faceplate to pass through to reach the PCB. The heat stakes are used to securely fastened the key switch PCB to the faceplate where the calculator key tops are placed.



This is the faceplate with key tops removed.


On hindsight, using the heat stakes may have prevented a more complete sealing of the key switch PCB against dust and liquid spillage. The heat stakes replaces screws that are needed to fasten the key switch PCB onto the faceplate and hold them against the key tops.

I believe that the corrosive liquid from the battery did not actually eat away the gold plating but rather the copper below the gold plating via the thin edges where the copper meets the PCB board itself. Without the copper backing the gold plating just fall off.

HP-21 Hewlett Packard Calculator Interior Side Views

These views shows the clever package to create a compact handheld calculator.

HP-21 Hewlett Packard Calculator Interior

The bottom cover of the HP-21 is easily removed by just removing 2 screws. The reveals the logic board
plug onto the keypad PCB.


The two power supply terminal is clearly visible and is silver coated.

This two terminals together with the spring in the battery pack keeps the battery pack in place and form a very sturdy retainer for the battery pack, avoiding the use of flimsy plastic catch.


The bluish tinge on the PCB is common with corrosion caused by NiCd batteries leaking.



Here is the 3 PCBs unplugged.
The keypad, the logic board and the LED display module.

Note the corrosion eated varnish on the resistor on the right of the logic PCB.

From the look of it this HP-21 calculator may have been exposed to the elements besides receiving damage from leaking batteries.

HP-21 Hewlett Packard Calculator Battery Pack

This is the underside of the HP 21 calculator. Notice the 2-cell AA size nickel cadmium battery pack. HP has cleverly design the battery pack as a cover to the battery compartment as well. It is expected that the user will have to buy a new battery pack from HP should the rechargeable battery becomes unchargeable. A more knowledgeable use will simply remove the old batteries and either put in normal AA size dry cells or another pair of AA size nickel cadmium cells.

The battery compartment or battery pack looks like the weakest link in this handsomely designed calculator. Nickel cadmium batteries has a tendency to leak badly when left in uncharged state.

This particular calculator is extensively damaged by the corrosive battery fluid, as the following pictures will show.

The NiCd battery actually has a vent hole. This somehow can be a source of leakage of corrosive battery fluids especially when in the discharge state.

Given the high quality components and material use in the rest of the calculator, its seems a pity that not much though has gone into making the battery compartment more battery leak proof.

Hewlett Packard HP-21 Scientific Calculator

This HP 21 scientific calculator was damaged by leaking NiCd batteries. The calculator has tiny LED display that consume quite a bit of current, hence the design of a calculator with a rechargeable battery. The power specs for the charger( found on a small sticker on the back casing ) says 2.5V 500mA, this is huge with compare to todays calculators.

I intend to clean it out and power it up to see if it still works. The liquid from the leaked NiCd battery pack really has done extensive damage to the PCB and the gold plated circuit on the keypad. Look out for the detail pictures.

The serial number moulded to the back of the casing is 1603S10198. This means that it is manufactured in 1976 Jan ( week 3 ). This is probably the 10198th calculator make in the Singapore plant since the start of 1976. That seems to be alot of HP-21 calculators, supposed most are not in landfills or incinerated with the garbage. It is a pity, since a calculator functions is timeless, they could have been resused if not damaged like the one here by corrosive battery leaks.