What the listing stated:

This auction is for a used cased Sinclair Cambridge Memory pocket calculator and original case. The item is in very good cosmetic condition as is the case which still has its instruction sheet. The item is powered with 4 x AAA batteries (not supplied) and does work although 1 of the digits is faulty and does not display (see pictures) plus the number 5 digit is not working. Please refer to the pictures and description provided before bidding.

EBay

So it does work, but it doesn’t? This calculator is a model one memory calculator, that dates from around July 1973, ( Actually May 1975 see photos below) and is one of the earliest available mass produced electronic calculators available in the UK at the time. And it was produced in collaboration with a guy called (Sir) Clive Sinclair, who in the following decade would become synonymous with tech development in the UK. It retailed at £29:95GBP, and given the rate of inflation, its cost today in 2025 would be a staggering £463GBP. Wow!

I love collecting old calculators, I couldn’t afford one back in the day when they arrived on the scene as I was only a child and probably only on about 20 pence a week pocket money, and savings and investments were not even known to me at this period of my life. The thought of saving that precious 20p a week for the next 150 weeks wouldn’t have even remotely crossed my mind. What no sweeties?

But I can buy them now, so no big issue!

So this one has become available, and I’ve been tracking it for a week or so, there were nine other people watching but I secured it for a total including postage of £14:49GBP, and I’m happy with that, it’s a piece of retro history for a very good price. Even if it remains faulty, or should I say working but not working?

This unit obviously has its problems, the button number 5 doesn’t work and one of the led digits is also not functioning. Hopefully I can get these issues sorted and soon have the calculator back up and working as it should. That would be nice. I’m looking forward to this little project.

Assessment:

It’s arrived and it’s a lot smaller than I anticipated. It has a separate hard protective case, which is a nice touch and a small info sheet on its operation. Cosmetically it’s in a good condition with just minor signs that are age related. There are no gouges or scars so it has been treated well, though it’s not pristine.

Batteries go in ok but, I believe old style AAA batteries were a little wider than those used today and would sit a bit more snuggly in the battery compartment. As you can see there is a little wriggle room here, and springs at both ends need adjusting to help prevent this. I may have to use some spacers so the batteries sit tighter in place.

The switch is a bit temperamental and can be seen quite plainly from the battery compartment. It looks strangely out of place with no batteries in place.

It is such a basic design solely relying on tension of a small metal plate to short across the connection points. Should be a simple enough issue to sort.

There is a little battery contamination on one of the battery contacts, again this shouldn’t be too much of an issue and should clean up ok with some IPA.

It was originally reported that there was one unresponsive button this being the number “5”, there is also another unresponsive button, the multiplication “X” button. There is also one LED indicator, the 4th one in from the left hand side that is not operating. Add to this the issue with the On/Off switch and the contamination, and the original faults reported in the original sales pitch have now doubled. I just wish people would spend more time going over the issues and then give actual accurate feedback as to what the real faults are, it would make for a far more pleasant buying experience. Rant over.

There doesn’t seem to be a single screw holding the body together, I just hope it isn’t all heat welded.

Let’s try to get inside.

Repair:

Well it cracked open quite nicely with no issues with just a plastic flat prise tool. The main board just sat comfortably in the unit, secure, and not a screw in sight. Strange as time moves on some of the games units I come across have best part of fifty of the little blighters to remove before you get anywhere. sometimes the old way is good.

The dismantling of the keyboard is a little complex and you have to take time and make sure you know how it’s going to go back together, it’s just a bit fiddly. The board is quite straightforward and as soon as I see some of the IC’s it dates the unit perfectly. The chips are dated May 1975, and that is about 18 months younger than what I originally thought, it’s quite informative to get inside and learn occasionally and this is just as good as having a birth certificate presented to you. All good stuff.

I’ve proved that there isn’t a problem with the missing digit on the display as using my multimeter in diode mode I am able to prove that this LED is working fine.

The picture shows just one part of the display range on this particular digit, I can assure you all other sections of the display are also working.

Regarding the case with the buttons not working. I have checked this out for continuity and both digits go through the same portion of the main IC and there doesn’t appear to be any broken traces. It’s a strange one but I have also found some really poor solder joints that are either cold joints or just poorly soldered from the start, there are a couple of resistors that need re soldering. It may be nothing at all, but it needs attention, a full reflow wouldn’t go amiss or take too much time.

I’ve reflowed the entire board due to there being a few cold solder joints.

On top of this I have taken off two old capacitors and tested them out of circuit, and both were out of their operable range of +/- 10%. As a result of this I have replaced the offending components with comparable new ones.

Even with all these extra tasks being undertaken there is absolutely no change in the way it operates. Nothing has gotten worse, the faults that were originally there still remain. I have done some research on line and carried out some further tests and checked expected voltages, most are within range except one that appears to be less than its expected value. After testing everything on this board, every component I can only surmise that one of the three chips has failed, I suspect very much that this, the main chip, a CZL550 integrated circuit. Otherwise known as “Calculator on a chip” is the one that is at fault.

To be quite honest these chips are fairly rare and command a price far in excess of what I paid for the original unit, and I don’t really want to do that. I think I’ll wait around to see if I can secure another faulty unit to complete this repair, so in the meantime, and until I can secure such a unit I will put this repair on hold.

Result:

Well, it’s not what I wanted but sometimes you just can’t win with some of these old projects. In no way am I walking away from it, it’s just that the parts are so difficult to get hold of that you really do have to just wait until a sufficiently faulty one comes up for sale. And that could be days, it could be weeks or months. So for now i admit defeat, but it will not be going to trash. It will remain in my ever expanding “To do” box, for me to pick up on at a later date. And when I am in a position to move this project on, I’ll pick it up in a continuation of this post.

Many thanks for passing by. It’s always appreciated.