Years ago I packed my old school calculator, the TI-59 from Texas Instruments, carelessly into a box. There it lay for several decades, until a few days ago I met this little treasure chest, and there all kinds of treasures came to light. Among other things also the mentioned TI-59. The battery had leaked meanwhile and also otherwise it gave no more Mucks of itself. Since I found that such an old sweetheart does not belong on the garbage, I went to the repair.
Surely one or the other may still remember his pocket calculator at school. Especially the older ones among us who still carried a real piece of technology around in their school bags. They may not have been so elegant and full of features back then, but they are a piece of technology history. There wasn't a color screen then, let alone a touch screen. Nor were there any formulas or fractions. At that time you had to deal a little with the topic and above all you had to know the basics of mathematics.
I can already hear the years before us, which still had to work with slide rules and logarithm tables. To be honest, I am glad that this was already history at that time. It wasn't necessarily something special at school to own such a calculator, but it wasn't cheap at the time. I can no longer say exactly how expensive it was, but it had already been a good 400 to 500 DM. It's a mystery to me that parents get upset today because a pocket calculator is required in high school that costs around 80 euros today.
But back to the actual topic. After I unpacked the calculator and cleaned it halfway, the battery caught my eye. It had played along with the long storage time and it slowly began to leak out. Fortunately, the calculator was lying with the battery down, so that the actual calculator hadn't gotten anything.
First the calculator had to be removed from its case. This went surprisingly well, as 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, and the board could be gently removed.
For the fact that the circuit board has a good 40 years on the hump, everything is still very well preserved. First everything has been cleaned a little and a visual inspection has been done.
At least optically there is nothing to complain about. The leaking battery didn't cause any damage and since the computer was stored very dry, there are also no other damages to be heard. So first connect a multimeter and see if a capacitor has a short-circuit.
Again, there was nothing special to measure. It looked good at least on the first block.
The battery consisted of 3 NiCd cells, each with 1,2V. Thus altogether 3.6V. Since I didn't have a corresponding battery any more, I connected my laboratory power supply first.
I downloaded the Service Manual from the Internet in the hope to find the expected power consumption there. Unfortunately for free. Well well, so first the current is slowly increased to 200mA. At least the motor of the magnetic card reader started to run. Although not good, because it shouldn't do that by itself.
In the service manual it says that the computer works with 3 voltages. First the 3.6V of the battery, and then two negative voltages of -10V and -16V. These can be easily measured on the module shaft.
But there were only -7V and -9V. So something seems to be defective. In the service manual is fortunately a circuit diagram, although unfortunately not of good quality. Fortunately I found the document TI59 HW Annotations on the internet. There someone had redrawn the circuit diagram. Especially the power module, which was almost unrecognizable in the service manual, I could now read properly.
So first the power module soldered out.
A small fine module which generates the two negative voltages from the 3.6V of the battery. This module was connected to the laboratory power supply and produced the same result. What you can see on the picture, the broken resistance, was not visible at first. Because on the right side the 3 transistors, which I had soldered out for the test, are missing. As things were quite tight on this small platinum, the transistors had hidden the real problem.
So I exchanged the resistor, connected the lab power supply again and measured the voltages.
Now it looked good, on the one hand -15,5V at Vgg and 9,8V at Vdd. All within the specifications, which according to TI are Vgg from -15.3V to 16.3V and Vdd from 9.5V to 10.5V. So I re-soldered the module and repeated the test I did earlier.
Unfortunately again negative, and this time the power supply shows a short circuit. It was also somehow too easy.
So the power module was desoldered and checked again. There are 3 capacitors on the module, two of them are tantalum capacitors, which have 40 years behind them. So first of all get it out and check it. And the second one, on the picture the brown tantalum with 33µF had a short circuit. So this also still exchanged and the whole procedure from the front.
Now it looked much better.
However, an error is detected in the display. Two segments of the 3rd digit are missing. Well, I'll probably have to live with that. I will try to find a replacement display somewhere, but don't give me too much hope. After all, the good piece is now running again. Somehow a nice feeling to be able to revive such an old device. And it's astonishing that the technology actually survived the decades without any major damage, especially since he wasn't always treated with care during his school days.
By the standards of the time, this was already a great calculator. And it was not only programmable, but had different possibilities to store the programs. On the one hand a so-called CROM was built in by default, which already contained various small programs and even "games". I write games in quotation marks, because they were not comparable to today's games. They were rather small games, like "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.
A corresponding module shaft was located on the rear side. The purchased modules could then be inserted into this slot. These modules were available for different areas and applications. Personally I did not really need anything of it. I can't even say if this module was already there or if I got it at some point.
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 up different programs, or also corresponding data to it, again and again, these programs were not only available in the buffered RAM, but one could store these on small magnetic stripe cards.
So one had his most important programs and data always ready to hand, even if the battery was empty again. To call up the program, you only had to insert it laterally into the magnetic reader provided for this purpose. The card was then pulled through by a motor and came out again on the other side.
By then, and we're talking here about the end of the 70s, beginning of the 80s, it 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 a completely different class. You didn't even dare to dream of it.
Next, I'll try to get the computer a little better and give it a new battery. Since these are normal AA cells, this is not a big problem. The battery cage was full of acid residues, but luckily it didn't get damaged otherwise. So I only need to install 3 new cells, and the computer is almost like new.
Translated with www.DeepL.com/Translator