Vintage Russian Alarm Clock Repair

Last week I obtained a very interesting treasure :-) an old vintage russian (USSR made) digital alarm clock ELEKTRONIKA 4. These clocks were qite popular and often used during 80's in eastern countries. The accurate digital clock can be understood as a sign of modern electronic era beginning. Up to now, there is not difficult to find these clocks, but after many years, there will be not much still operable.

My type has ELEKTRONIKA 4.13 from 1986 printed on the case. Nice presentation of similar clock can be found on perfect Czech private website, in English and Czech, but the internal circuit isn't the same! Probably, there are many modifications exist.

http://danyk.cz/stare_digi_hodiny_12_en.html 

You can also find more info on russian clocks on the website:

http://danyk.cz/stare_digi_hodiny_en.html 

Many thanks to the author of this website and congratulations to his collection.

Now to the actual repair. There were no visual damages both outside and inside, only dust layer corresponding to the age. After drying the clock properly, I tried to plug it into the power. A number 00:00 showed on the display, but the brightness was approximately half than shloud be. Any of setting and function keys didn't respond and I hear a little noise form the display. Control signals for the display had changing randomly apparently and the control circuit had crashing just after power up reset. So I disconnected the power quickly.

The power supply must have the first attention. The state described above is probably caused by failed DC filter capacitor. The big metal can in the picture, 100 uF / 50 V from November 1984. The power supply itself is very simple. Small transformer produces one branch for electronics and one for display heating (filament) + anode/cathode voltage. The main branch contains 4 diode rectifier (diodes in plastic cases) and simple voltage regulator with transistor and three zener diodes in series (nice metal cases).

 

As expected, the main capacitor was bad. It's sealant changed volume during decades and made almost invisible cracks at the capacitor case and leads. The electrolyte from the capacitor evaporated which leaded to capacity decrease and internal resistance increase. The power supply main branch output voltage was -25 V instead -27 V with approx. 7 V AC on it !!!

 

After replacing the capacitor the colck working properly. All fuctionalities are OK, Display is not worn out, alarm working and power consumption is 3,15 W for 230 V.

 

 

Finally, all parts were cleaned carefully and the clock was assembled. For better safety, I suggest to cover all 230 V wires inside by additional insulation tube, the old standards in USSR was less strict than we know today.