ELKA-50m Desktop Calculator Refurbish

Something ancient and very Soviet-Union arrived: An Elka-50m (Элка 50М) desktop calculator from 1978 Bulgaria. It has a beautiful shape, but the colour, wow, the colour:

The above pictures are full shots of the model I purchased – mine was actually, quite uglier, and had some very stubborn glue marks above the Elka logo. I made a video about the take-apart on YouTube, so before I made too many changes to it, you can see what it exactly looked like:

Seeing as it couldn’t get any worse, and also seeing as Elka-50s once were made in quite pretty choices, I thought that complete disassembly and repainting wouldn’t be something beyond my skills and tools.

What colours could this calculator originally be had in? Pretty lively stuff, especially considering Bulgaria was a USSR satellite state. Its time for another episode of the Retrotech colour guide:

I actually think that the “Bucharest” colour was actually “Artic Outpost” originally, but the ravages of time have not been kind.

Before the story of the fix, a bit of tech history:

Once upon a time, ELKA wasn’t an Eastern European internet shopping term for Christmas trees….

ELKA (ЕЛКА) was a brand of Bulgarian electronic calculator, developed by the Central Institute for Computation Technologies and built at the Elektronika plant in Sofia. ЕЛКА is a contraction of ЕЛектронен КАлкулатор (electronic calculator), and the word “elka” just became the name for any garden-variety calculator. The first calculator that Bulgaria produced was the very big, and very Imperial-looking Elka 6521:

Then things got smaller.

From 1974, the Orgtechnika factory in Silistra begun producing and exporting the Elka series of calculators, of sizes more modest than the 6521, and in colours more spiffy.

Interestingly, these calculators were developed using a combination of imported parts: semiconductors and chips from Rockwell Semiconductors (A part of the now defunct Rockwell International) and vacuum-fluorescent tubes from Japan.

Seeing as we are reminiscing in the past, here is Rockwell’s awesomely sick logo:

Starting with the Elka 50, the series evolved over the years, eventually using Russian VFD tubes, with the Elka 50m and 51 models Bulgarian-made chips. It’s not hard to imagine that a lot of reverse engineering of these two important technologies occurred, driven by the Soviet Union’s ideology of self-sufficiency.

The Elka-50 soon had relatives, in the form of the simpler 8-digit Elka 1300, and the bigger 16-digit Elka 53, 53a, as well as a displayless Elka 55 with a impact printer.

Older versions show internals with quite a big difference compared with what I saw in my one: a combination of newer, plastic-encased transistors in something like TO-92 but with gray plastic and no clear markings, along with some older canned transistors, and a single central chip that looks much closer the 1978 than my Elka’s three chips and its set of only TO-92 transistors. The original calc chip apparently was the Rockwell A4350EB.

Older versions

As can be seen, my newer one uses more boring-looking chips, not made by Rockwell.

How to use the Elka-50

What follows is my simple description of what I have observed, and there are a few details that I am a bit in the dark about, especially the story with rounding, so what I say here should be taken with a tiny pinch of salt. If you want the info from the horse’s mouth, be my guest, and squint over this badly photocopied manual for the Elka-50, in Bulgarian.

When powered on (by 220V mains, incidentally, no batteries here), the calc defaults round all numbers to integers, but calculations can be done with a fixed point decimal: so to set the number of decimal places, you start by pressing the [DP] button and then number of decimal places: [0]… [9] or [,] for 10 decimal places.

So for example, I pressed [DP], then [5], and I got this:


It can be changed in the middle of work, so you could have something on the display like 25.5500, and pressing [DP] and [2] will shave it to 25.55. I could imagine that before one starts dividing, it might be wise to extend the decimal places to catch the remainders.

But it then would be logical to wonder: does it round if we lop off decimals and the middle digits are 5? Unfortunately, it doesn’t round up or down. So switching 25.5500 to one decimal place leaves us with 25.5 on the display.

Oh well. It wouldn’t be 1970 tech if it worked better, I suppose.

There are a pair of switches on the bottom left:

Initially I had no idea how they are used, but later I figured out that they have to do with rounding and where your answer gets stored. Σ stores and updates the answer in memory, and 0 does not store your answer to memory.

As for normal arithmetic, the calculator counts in mixed prefix-postfix logic, and initially its a bit weird, but once you get the hang of it, its quite satisfying: say we want to add 50 to 50 (50 + 50). W start with 0 on the display. We type [5][0][+], and the moment we press the [+] operation, the previous number is operated on. 50 is now on the display. [5][0][+] adds another 50, and the moment [+] is pressed, the display switches to 100. Minus works in the same way.

Another example, according to the Russian Wikipedia entry for the ELKA-50: To calculate 2 + 3 – 5, you need to press the buttons: [2] [+] [3] [+] [5] [-]

Multiplication and division works as normal, so you’d press the divide or multiply symbol, and then the number, and then the [=] button. I suspect that accountants and bean-pushers would like the quick add/subtract style, as you don’t need to press the equal sign all the time. Nice.

When an error occurs, a “C” appears on the far left, and like HAL9000, refuses to do anything else until you press…..C? Oh. Thank you, benevolent little number crunching robot.

The Russian Wikipedia article alleged a minus symbol also appears, but this is lies and deception. Incidentally, at the same position, we get a comma symbol [,] when there is something stored in the memory.

So, what constitutes an error? I guess the memory being overflowed by multiplying by something too big?

(tries to multiply something big by something big…) Yup. Up pops the C symbol.

(divides by zero…) C symbol again.

Okay, I’m bored now. Onto the good stuff, the guts of this thing. And the moment you open it up, we see….

The Weirdest Keypad

During the takeapart, I was surprised to find that ELKA-50 uses reed switches (I was today year’s old when I learnt of the word “hercon” – “hermetically sealed contact”) for each of the keys, with a small but beautifully-shaped permanent magnet for each key. Apparently, this makes the device more efficient and durable.

I initially found one of the magnets was loose – nothing a drop of epoxy didn’t fix – but on closer inspection it became obvious that there were other little issues. One of the reed switches was a bit bent out of shape and was getting ground against by its sliding magnet, another was bent too far away from its magnet to be triggered properly (you hear a very faint click when the reed switch is magnetized).

Before I marched in and rectified them, I imagined the almost-inaudible-but-pristine-and-delicate-tinkle-sound-of-reed-magnet-shattering as future Paul stupidly broke it while trying to set matters straight. This would be a less than ideal outcome, so I used two pairs of long-nose pliers to grip the entire thing by both sides of the wires and shift it into position, hoping this wouldn’t put it under too much strain.

This caused no problems, and the offending keys worked perfectly fine afterwards.

The Other Naughty Bits of an Elka-50

Power supply, nothing too interesting….

VFD tubes (these ones are not the original Japanese ones, I believe, but are the . Very interesting. Mesmerizing even.

These are not USSR IV-3s as we know them, as these are 9-segmented with a decimal point. A Cyrillic search for ИВ-3 seems to reveal a whole bunch of variants, some 7-segment, some 9-segment. Luckily for me, I don’t actually need to repair anything in my ELKA, as the VFDs here are all healthy and happy. Thinking about it further, I realize that I’ve never actually seen those extra two segments ever lit up on any of the tubes. After a search, I discover that the Russian wikipedia entry for the Elka-50 does actually mention this detail.

Once done with the tour of the insides, the next issue was the paint job for the case, and the spring levered stand that seemed to have lost its springiness.

Fresh Paint

Stripping off every part, sanding away bumpy bits, and carefully prying out the front label and screen panel and miscellaneous stickers, I went for a fresh blue on top, and a bright red on the bottom. Before and after….

A thin gauze protected the air vent, but it was now saturated with human skin, a few dead insects, and a strand of long-dead grass. I used what was available, some mozzie-shutter repair panel, but once I was finished, I realized it wasn’t the best option. I was in a hurry though, and part of me knew it wasn’t ideal. In the big scheme of things, its not a huge detail, but I learnt something for next time.

The spring-loaded stand seemed to have lost a pin and a spring-steel plate that was friction-welded into place. So I made a new pin, and cannibalizing an old floppy disk that had gotten destroyed, I used the metal cover to act as a new spring.

I used a combination of epoxy and hot-glue to ensure it will never come out again. It doesn’t look great, but who will see it? The stand now works perfectly, springing open and closed when used. Satisfying.

And of course, the final detail – that label panel on the front. Using dark magic, I scanned and remade the logo. Details, details, details…

And with that, it was done. I am delighted with the rich colours, and in so many ways, this device’s look and feel makes me quite happy. I’m toying with the idea of selling this one, buying another old Elka and repeating, but I am an emotional animal, and this particular device has become quite meaningful to me.

Plus, it would look schweeeet on my desk.

Well, if my desk, um, had any….er….space. Maybe I should just sell it…


But one thing disturbs me still. Actually, a dozen things disturb me. They keep me awake at night.

These leftover parts.

I didn’t forget anything, did I?…

DID I?….

Paul Hoets is a freelance maker who lives in South Korea. If you liked this article and would like to contribute to his empire of dirt, silicon and tech. education, buy him a coffee!