Elektronika MS-1103 (Электроника МС-1103) Manual

Probably because of my RPN Nixie calculator project, I got interested in Soviet-era calculators. There are some MC-1103 calculators on eBay, so I bought two. The first one came without a manual, so I decided to open it and reverse-engineer it.

As you can see, it works. Interestingly, it truncated pi. If it rounded, it should end in a 3. I plugged it in to a 110-to-220 converter. However, US AC voltage is 120 which means that I actually get 240 VAC out. This would result in increased AC voltages inside, but the DC voltages should not be affected if they were regulated.

The above image shows the power supply removed from the case. There's a big transformer on right, and what appears to be four series pass transistors. Each such section also had a multi-turn potentiometer, presumably for fine adjust of voltage, and a chip. There were actually two types of chips: кпен2г (KPEN2G) and кп142ен1г (KP142EN1G).

From cpu-ukraine's article on Soviet chip identification, I learned that Soviet chips were all marked in a nation-wide standardized way. KP meant commercial use, plastic package. 142 was the chip family, EN was the type, and 1G was the model.

I also learned, by piecing together statements on various forums and Russian chip resellers that KPEN2G was shorthand for KP142EN2G (so they are the same chip, different models), that they were linear adjustable voltage regulators, that 1 meant Vin max 20v, Vout 3-12v and 2 meant Vin max 40v, Vout 12-30v, and G meant 0.2% regulation. I also found that they were equivalent to the LM723 regulator, but with a different pinout.

The pass transistors were КТ817А and КТ817Б (KT817B). These are NPN power transistors, A meaning 25v max and B meaning 45v max, and they were equivalent to the BD175 transistor.

Putting all this together, I was able to draw a schematic. The power supply provides regulated 5v, 15v, -15v, and -27v.

Anyway, as I was poking around with my multimeter with the power on, I heard a hissing sound and quickly cut the power. One of the capacitors had sprung a leak! Further investigation showed that it had taken its voltage regulator and series pass transistor with it. Maybe I shorted something? In any case, I had to replace these, and I used an LM317, a three-terminal adjustable voltage regulator which combined the chip and the pass transistor.

After a really messy install, the power supply worked again.

Then I got the second calculator, and it came with a manual. And the manual included schematics. Aside from the missing connection between the transformer's terminal 12 and the other end of the VD21, I got most of it right.

Anyway, here's the scan of the user manual I put up on the Internet Archive. Enjoy!