Elektronika EPOS-73A stuck with fixed display

[carlitosbala]

Hi,

I'm hoping I can get some ideas on what to check in this old (1978) soviet-era electronic calculator I got from a flea market, the Elektronika EPOS-73A.

Got it home and plugged it in. It worked just fine at first, I used it a few times for a few minutes each time, "kid with new toy" style.

Then I disassembled it to clean the plastic case and the keys. I couldn't separate the lower half of the case from the board because cables are tied to it, so during the cleanup one of the cables going from the power board to the main board detached, and a leg from one of the capacitors broke.

I re-attached the cable, replaced the capacitor, and now the calculator doesn't work anymore. The display shows "111" in the first few digits instead of "0", with other digits not turning on or doing so dimly. Button input doesn't change what's shown.

It has a reset button, when I press it it sometimes shows nothing, other times shows "999" or "ººº".

I noticed when taking photos for this post that if I press (slightly bend) the board in different places I get some of the digits to be brighter, and that one of the K264YM2 (which according to google translate is a "three display amplifier") is getting hot.

Regardless of the thing with the cable and the cap, I think something worked just enough when I turned it on the first time, and now has decided it's old enough to retire

Next time I check on it I'll reflow the connections in the back in case it's a bad solder joint.

I would like some general advice on, for example, how to properly measure the power board, and if someone has worked with this kind of old device, maybe some more specific recommendations?

Photos:
Working

Power board

main chip

Display as it is now when plugged-in

[stj]

Re: Elektronika EPOS-73A stuck with fixed display

the heater voltage/current is too high - you will kill the display if you keep running it.
the wires shouldnt glow like that - check the psu caps.

as it's soviet/communist there should be schematics available.

[carlitosbala]

I had forgotten about this thread

Thank you for your reply, I ended up taking the calculator to an electronics repair shop for diagnostic to avoid causing more damage while testing. The guy at least confirmed the power supply board looks OK, but apart from that was unable to find a fault and just told me that IC is bad and I should try to get a replacement.

I'm tempted to try removing it from the board and powering it, in my mind the best case scenario is the calculator should work and just have one of the digits not showing. If the IC is shorting the circuit to ground then removing it should also remove the excess current. Especially if I replace it with a resistor to compensate for the chip's voltage drop. However, I'm also afraid that might be disastrous so I don't think I'll try it, at least not until I try finding that chip for some time.

[stj]

find the schematics for it - then address the heater voltage

[redwire]

The VFD looks just like an IV-18, I used one in an Icetube Clock.

[carlitosbala]

Just an update: I got schematics, for the calculator itself (there is an emulator project at SourceForge, https://sourceforge.net/projects/emulator-epos-73/), the display controller chip, and the processor. However, their usefulness is limited by the fact I don't understand Russian and Google Translate doesn't help that much because the PDFs are images and not actual text.

As for further testing, I played with the potentiometers in the power and main boards, and there is a position where the display shows a "0" and the wires don't glow as much. But the calculator is still unresponsive, nothing changes when I hit buttons. Unplugging and plugging it again not always gets it again in that "0" state.

Took the "faulty" display chip out but nothing changed either. I noticed that the chip doesn't get as hot depending on what is being displayed, so I guess it is not faulty after all and is just getting hot because is being overdriven and always on.

At one hand, I'm thinking if maybe some of the orange components (which I assume are transistors) might be faulty and not giving the processor the right initialization signals. But I need to first confirm they are transistors, and then read enough of the schematics to identify proper replacements in case some are actually faulty.

Finally, what if the problem is that this thing wants ~220V and is just not liking the current ~240V mains voltage? It would be problematic because it would mean those first few times when I plugged it and it worked I was in fact causing a failure, but in any case I dont have equipment to help me test that theory, unfortunately

[stj]

attach the schematic

[carlitosbala]

The most complete documentation is what is there in SourceForge, really. Or at least that is the best I can find. It's 37MB, I'd assume attaching it is not possible

[stj]

i'll take a look