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Texas Instruments TI59 – Repair of my old calculator

Surely one or the other may still remember his calculator in school. Especially the older ones among us, who still carried a real piece of technology around in their school bag. They may not have been as elegant and feature-packed back then, but they are a piece of technology history. There was no color screen back then, let alone touch screen. There were also no formulas or fractions displayed. At that time, you still had to be a little bit familiar with the subject and, above all, you had to master the basic skills of mathematics.

I can already hear the years before us, who still had to work with slide rules and logarithm tables. To be honest, I am glad that this was already history back then. It wasn’t necessarily anything special in school to own such a calculator, but inexpensive this pleasure wasn’t back then. I can no longer say exactly how expensive it was, but it was a good 400 to 500 DM. It’s a mystery to me that parents get upset today because a calculator is required in the upper school that costs around 80 euros today.

But back to the actual topic. After I had unpacked the calculator and made it halfway clean, I noticed the battery. The long storage time had played along with it and it slowly started to leak. Fortunately, the calculator was lying with the battery down, so that the actual calculator had not been affected.

The first thing to do was to free the calculator from its case. That went surprisingly well, only 2 screws had to be loosened. After that, the backside could be removed without any problems. Then 4 screws had to be removed from the magnetic card reader unit, and the circuit board could be gently removed.

For the fact that the board has a good 40 years on the hump, everything is still very well preserved. First of all, I cleaned everything a bit and did a visual inspection.

At least visually there is nothing to complain about. The leaking battery didn’t cause any damage and since the computer was stored very dry, there is no other damage either. So we first connected a multimeter and checked if a capacitor had a short circuit.

Again, there was nothing special to measure at first. Looked good at least on the first block.

The battery consisted of 3 NiCd cells, each with 1.2V. So in total 3,6V. Since I didn’t have a corresponding battery anymore, the first thing I did was connect my lab power supply.

I downloaded the service manual from the Internet, hoping to find the expected current consumption there. Unfortunately in vain. Well, first of all I slowly increased the current to 200mA. At least the motor of the magnetic card reader started to run. Although actually also not good, because it should not run by itself.

The service manual says that the computer works with 3 voltages. Once the 3.6V of the battery, and then two negative voltages of -10V and -16V. These can be measured quite easily at the module shaft.

But there were only -7V and -9V. So something seems to be defective. Fortunately there is a schematic in the service manual, but unfortunately not of good quality. Fortunately I found the document TI59 HW Annotations in the internet. There someone had redrawn the schematic. Especially the power module, which was almost unrecognizable in the service manual, I could now read reasonably.

So I first desoldered the power module.

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A small fine module, which generates the two negative voltages from the 3.6V of the battery. This again connected to the lab power supply, yielded the same result. What is visible on the picture, the broken resistor, was not visible at the first moment. Because on the right side the 3 transistors are missing, which I had unsoldered for the test. Since on this small board it is quite narrow, the transistors had hidden the actual problem.

So I replaced the resistor, connected the lab power supply again and measured the voltages.

Now it looked good, there was -15,5V at Vgg and 9,8V at Vdd. All within the specifications, which are according to TI from -15.3V to 16.3V at Vgg and for Vdd from 9.5V to 10.5V. So I soldered the module back in and repeated the test from earlier.

Unfortunately negative again, and this time the power supply shows a short circuit. Somehow it was too easy.

So I unsoldered the power module and checked again. There are 3 capacitors on the module, two of them are tantalum capacitors, which have their 40 years behind them. So first out with it and checked. And the second, on the picture the brown tantalum with 33µF had a short circuit. So I replaced it and the whole procedure from the beginning.

Now it looked much better.

However, an error can be seen in the display. Two segments of the 3rd digit are missing. Well, I’ll probably have to live with that. I’ll try to find a replacement display somewhere, but I’m not too hopeful. At least the good piece is now running again. Somehow it’s a nice feeling to be able to revive such an old device. And it’s amazing that the technology survived the decades without any major damage, especially since it wasn’t always treated with care during the school years.

For those days, this was already a great calculator. And it was not only programmable, but had different possibilities to store the programs. On the one hand, there was a so-called CROM built in from the factory, which already contained various small programs and even “games”. I write games in quotation marks, because they were not comparable with today’s games. They were rather small games, like for example “guess a number”, etc..

More interesting were programs for interest calculations or scientific calculations. But additionally the calculator was expandable with ready to buy memory modules.

On the back of the calculator there was a module slot. In this one could insert then the bought modules. These modules were available for different areas and applications. Personally, I didn’t really need any of them. I can’t even say if this module was already included or if I got it sometime.

Now this calculator was programmable. So you could write small programs that could make your life easier. Especially recurring procedures were always quickly retrievable. And so that one could call different programs, or also appropriate data to it, again and again, these programs were available not only in the buffered RAM, but one could store these on small magnetic stripe cards.

So you always had your most important programs and data at hand, even if the battery was empty again. To call up the program, you only had to insert the card sideways into the magnetic reader. The card was then pulled through by a motor and came out on the other side.

For that time, and we are talking about the end of the 70’s, beginning of the 80’s, this was a great piece of technology. Surely HP was much more advanced and even better equipped at that time, but in terms of price, it was in a completely different class. One didn’t even dare to dream of that.

Next, I’ll try to bring the computer up to scratch visually and give it a new battery. Since they are normal AA cells, this is not a major problem. The battery cage was full of acid residues, but fortunately it didn’t take any damage otherwise. So I only need to install 3 new cells and the calculator is almost like new.

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