Elektronika MK-52: The MK-61 big brother?

[Adalin]

Hi

Yes I know how fragile the PCB in these machines are, I replaced a defective VFD in a MK-54 a while ago and it turned out alright.

Seems that the VFD display is prone to failure after a while also, the solder contacts are only glued in place on the glass to a painted on conductive path and can fracture the bound and create a high Ohmic contact if you handle the calculator rough, making the display dim or loose numbers.

It seems they used recycled wood and mud from the swamps in the taiga to make the PCB, it hisses and steams when you solder the pads, but if you are quick and steady it works. but yes cutting the legs of the chip is a good way to do it.

I have tried to save programs to different addresses in the eeprom and it seems to save parts of the program on some of them, but it fails to save all steps and creates its own steps and numbers so the program is useless, But I will try to replace the eeprom and see If that helps

[Adalin]

I got another mk-52 to see if it was me or if it was the calculator which was faulty, and the eeprom in this new one works as it should.

One interesting thing about the mk-52 seems to be what kind of plastic they used for the different details, this one is from 1987 and the lid is the same plastic as the screen cover, dark transparent green, and some of the buttons is a light brown/gray color rather than the light gray color they should be.

The lid on the 1991 one is dark grey/black

there was a lot of different color variants apparently, maybe they got some plastic over and didn't want to throw it away so they made lids of it instead.

the coloring of the function printing differ also, much lighter blue color than the 1991 one and easier to read.

I still haven't got the replacement eeproms but I will try to replace it when it arrives.

[Adalin]

So I finally got the eeproms, and started the procedure immediately



I cut the legs off





I accidentally sucked off two solder pads with my solder sucker, that was not a wise move but I didn't want to use a de-solder wire because I didn't want to heat up the pads too long, it really is a crappy pcb, extremely fragile.



I cleaned up the solder and soldered the new eeprom in place and fixed the broken pad with a piece of wire



I wrote a small program and saved it and turned off the calculator for a couple of minutes before I restarted it and loaded the program and executed it. It worked!



there we go, another one works as it should do.

I bought another mk-61 last week also, and it had a strange fault when I started it, the vfd would show every number and point lit and scramble all the numbers then turn off completely for a minute or so before the calculator jumped to life and worked, it looked like a capacitor problem so I opened it up and found one 50v 20µF capacitor in the power supply pcb, I changed it and the problem went away.

I put the capacitor in my esr meter and it showed 20 Ohm internal resistance and vloss of 30%. compared to the new one I put in that measured 0,13 Ohm.

So if you have problems with your mk-XX calculator, replace the capacitors first.

[d b]

Cool. Great info Adalin. I hope my 52 and 61 never need it though.

[jebem]

(01-20-2016 08:06 PM)Adalin Wrote:  So I finally got the eeproms, and started the procedure immediately

Well done. Thanks for sharing your findings!
By the way, where did you get those EEPROMs?
I might order a few, just in case, if the price is right.

(01-20-2016 08:06 PM)Adalin Wrote:  I accidentally sucked off two solder pads with my solder sucker, that was not a wise move but I didn't want to use a de-solder wire because I didn't want to heat up the pads too long, it really is a crappy pcb, extremely fragile.

Tell me about it... no matter how careful one is, at least one pad will be striped no matter what.
These PCB's reminds me of my good old teenager days when we use to make our own PCB's using chemical etching, I think we used mostly iron perchloride and a home sink.
Fiberglass material was out of question due to the high cost, so we ended up using phenol resin (I think) like this one used in the MK's.

(01-20-2016 08:06 PM)Adalin Wrote:  I bought another mk-61 last week also, and it had a strange fault when I started it, the vfd would show every number and point lit and scramble all the numbers (...)

I put the capacitor in my esr meter and it showed 20 Ohm internal resistance and vloss of 30%. compared to the new one I put in that measured 0,13 Ohm.

So if you have problems with your mk-XX calculator, replace the capacitors first.

Yes, that's my findings too (reported in this thread somewhere in one of these posts).

Just for clarification of those less enlightened in electronics:
A DC resistance of 0.13 Ohm means the capacitor is bad (in short-circuit) and needs to be replaced; however your reported reading relates to AC impedance and its value should be as low as possible.
I believe that not everyone here has access to a ESR meter, so most of us end up using a ohmeter that will give you just DC readings.

[Adalin]

Hello

I ordered the eeproms from the same seller you found earlier in the thread, I have ordered many times from him in the past.

yes there is a difference between DC resistance and AC Impedance and you are right one should differ between them. lower ESR is better than higher in capacitors, but low DC resistance over a capacitor is nothing you want.

everyone that does repairs on electronic stuff should own a ESR meter,
its as valuable as a multimeter to have.

[walter b]

Please excuse my ignorance but which kind of ESR do you mean? Looking for this abbreviation didn't really help - more than one meaningful solution.

d:-?

[ElectroDuende]

(01-26-2016 07:45 AM)walter b Wrote:  Please excuse my ignorance but which kind of ESR do you mean? Looking for this abbreviation didn't really help - more than one meaningful solution.

d:-?

Hi Walter,

They are refering to this value:

https://en.wikipedia.org/wiki/Equivalent...resistance

Checking a capacitor with just a "capacitance" meter is not enough, as the ESR may rise with age, making them useless, even if capacitance is OK.

[MK52]

Hi,

There is a story that the MK-52 was used as a backup for the main computer in some soviet spaceflights. (Like the HP's were used in the US space program).

Does anyone know if the MK-52 used the astro-navigation/celestial navigation ROM or a custom spaceflight ROM?

[Didier Lachieze]

Quote:...and there was a ROM module with astronomical programs that the cosmonauts in Mir used... of course it was military/restricted, and no one ever saw it!

From the Museum of Soviet Calculators on the Web.

[MK52]

Thanks Didier,

Yes thats the story I was referring to. It would be fascinating to learn if there really is a special ROM for the cosmonauts.

[jebem]

(07-13-2016 12:34 PM)MK52 Wrote:  It would be fascinating to learn if there really is a special ROM for the cosmonauts.

See it here:
BRP-Astro: Astronavigation module (not mass produced)

Sergei Frolov is well informed on these Soviet machines, you may try to contact him here for additional details.

[Hlib]

We all want to believe in fairy tales. But the reality is that of the 20 calculators MK-61-52 I have a healthy just one. I think in space this shit didn't send.
I saw how the control of ballistic missiles. MK-52-61 - it's a beautiful story :-)

[MK52]

There is a space flown MK-52 on ebay. Ebay item 272058501564

I assume that the astronavigation ROM mentioned by an earlier post was used for what is called celestial navigation by sailors and others. I.e. using a sextant etc....maybe cosmonauts could use the same celestial navigation programs....if anyone knows it would be great to hear from you.

[MK52]

I found a Russian chap has dumped the MK-52 ROMS.

https://translate.google.com.au/translat...t=&act=url

[linuxslate.com]

OK, bumping a really old thread here, but I just learned about these, and purchased one through "a certain popular auction site".

It seems to generally work, but it does have 2 issues that I am contemplating on attempting to repair:

1. The keyboard foam has gone flat. The keys all work fine, but at rest, all are crooked, and the keyboard has no "feel". I purchased some thin insulation that is meant for insulation around a window air conditioner installation. I will cut to size, poke holes, and try it. If anyone has a better suggestion of what to use, I am certainly willing to hear it.

2. There is something wrong with the EEPROM. If I save a program, and then recall it, it is generally correct, but functions that are represented by larger code values change. For Exampe: 4A X->N(a) may become 5B when the program is re-read from EEPROM. (not sure it is exactly those values. I'm too lazy to determine exacty what bit(s) are flipping)

I figure the 3 most likely causes are: -- Improper Write Voltage from the power supply -- Bad solder or broken path(s) near the EEPROM -- Bad EEPROM chip.

I'm pretty sure the bit errors are always the same, so I guess this reduces the likelyhood that it is a Write Voltage problem.

It has the seal on the screw, but to fix the above, I guess I will have to remove the seal.

I expirienced at replacing old caps, since I also work on vintage tube audio equipment -- which pretty much means -- I replace bad caps.

The Transformer in the AC adapter does seem to have another primary tap. Does anyone know if this can be used to make it work with 110V - 120V?

If not, I plan to cut the wire off of the AC adapter, and attach a USB connector to the adapter end of the wire, thus making a USB power cable.

Lastly, I have written a simple proggie to roll a dice using a super simple implementation of a "Middle Squares" psuedo random number generator, which creates seeds for the internal psuedo random number generator.

Willing to share here if there is any interest.

[linuxslate.com]

Ignore part of above about the flash memory. Mine now seems fine.

Appearently, if you don't properly clear the flash before writing, it basically OR's the old data with the new data.

Also (and yes, a little geek bragging), I've owned several other (now vintage) scientific/programmable calculators over many years, including:

The Radio Shack labled version of the Sinclair Cambridge (white with purple LED display)
Commodore (I think P-50) It was brown with brown, tan, white keys and a red LED display
Radio Shack Pocket Computer 1 and 3.
HP28C and 28S.

[Daniel.]

Hi there, I'm new here and I live in France.

I just received this calculator that I bought as NOS but won't light up.
I've removed the condensers and checked the VFD disdplay. As you can
see, I hope , in the left corner some parts of the VFD is missing and the
white color on both ends isn't a good sign.

I bought new condensers and VFD of course, and now I wait for this new
parts.

[cruff]

It looks like the left most position only includes a minus sign and is not a complete 7 segment digit. Is that what you are talking about? Or are you saying the glass envelope of the display is broken? Or are you talking about the partial third digit position from the right?

[Daniel.]

Hi there, yes, the glass envelope of the display is broken.



Have a nice day.