The repair of a C1-94

Going through the basement a few years ago to clean up and to throw out old crap, I ran into some stuff that I actually wanted to try and salvage. One of these items is an old C1-94 (spoken as S1-94) oscilloscope from soviet origin. (I think it was designed in Russia and not one of the member states, but i’m not sure on that). From what I can gather from Russian and German sources is that this particular model was designed and built in 1983, so this is well within the soviet era, which explains the tiny CCCP mark on the top right.

Picture courtesy of EEVBlog forums where I found this one. This is a placeholder for when I finally snap my own.

When I plugged it in however, it didn’t work, which was a damn shame. No lights came on and a quick inspection of the fuse also didn’t bring a solution as it was still good. Of course I then tried to fix it with the standard soviet percussive analysis and repair procedure (beat the shit out of it until it works) but even that did not make it pop back to life. Oh well, seeing that it was well within the 2010’s when I found the thing, it would be a good bet that the caps were gone. But I left the C1-94 oscilloscope to figure out for later.

But first a word to the wise. If you happen to have one of these laying around and you want to fix them, be sure that you know what you are doing! I am an amateur electrician / electronics enthusiast and this thing tickled me. This scope is stepping up your plain 220 volts up to a nice and crispy 2000 volts for the display tube! And that cathode tube, when it’s on, can give you a nasty zap too! Now the cathode tube zap is probably not enough to kill you when you touch it, but it will wake you up alright! Take it from me, I was stupid enough to do just that and I got a quick reminder to keep my head at what I’m doing. Now the 2000 volt circuit, that one can plain kill you. You have been warned, do this at your own risk!

Now it’s 2016 and I ran into it again. Having learned a trick or two I felt that I was ready to start and salvage some old technology. Sure there is so much more and better oscilloscopes out there these days, but there is some charm of old school tech that I just find hard to resist. So, there we go, opened the thing up and the first thing that I noticed was that the thing was built far more simplistic than I thought it would be. Which is hopeful of course! Maybe I actually can fix this thing myself.

Now, searching around, there was not much information available about this scope in English, and I don’t read Russian, but I did manage to find some sources in a German blog (link here!) that was helpful. All these sites also indicated what I originally thought to be the case: the caps were old and gave out and that’s why it’s no longer working. However, on mine the power light didn’t even come on. And this bugged me a bit. Because if the thing has power and caps are rotten enough to let the high voltage circuit fail, then the power light still would have to come on, which it didn’t.

Well, seeing that the caps should be replaced anyway since I don’t have too much faith in those old Soviet produced capacitors, I decided to do some swapping of those that were most likely to have failed, and created quite some space on the board while I was at it. Those old caps are huge compared to what you get nowadays.

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Capacitors, soviet style!

Now the caps that need to be changed are in the high voltage circuit so make sure you unplugged this thing long enough for the caps that do still work to be drained. It is not known how long this takes but in my case I found that a good 30 minutes was enough to not zap myself when touching the board. On the schematic you can see the caps, and they are C1 to C6, which are 20uF50V for C1, C2 and C4. And 1uF100V for C3, C5 and C6.

I could not get the 20uF50V caps, but as I understood, the values of these caps are not that strict, so the safe way to go about this is to replace them with caps that are at least those values. I replaced them with 22uF63V caps and it works just fine. But I suspect anything will do as long as your replacements are at least 20uF50V in value.

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All caps were now changed that were on the ‘advisory list’ to change and… Still nothing. Plugged it in and still no damn light coming on. Time to break out the schematic I suppose, after fixing some ‘solder work’ from the previous owner that for some reason decided to put on too much tin with a too hot iron. After cleaning that up it looked much better.

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Yeah I know, it still looks very crusty, like it was sitting in a basement for 20 years.

The internet nowadays is a wonderful technology and it didn’t take me long to find the original schematic of this little oscilloscope. An old magazine photo from Hungary however had a much clearer schematic which eventually led me to the solution.

c1-94

Reading this schematic was easier than I expected, if I just kept in mind that the Russian letter V (for Volt) is a B, and the F (for Farad) is a ф. For the rest the symbols were exactly what I expected them to be, apart for some oddball ones, which are from what I made up from the schematic, linear rectifiers which were shown as a diode symbol in a diamond. Makes sense to do it this way instead of drawing down 4 separate diodes. Saves space and everyone with some technical knowledge knows what it is and how to hook it up. +1 for the Russians in this case I guess.

At any rate, after verifying that everything inside the oscilloscope was just fine and dandy now, I started looking for simpler things, such as, is the electricity actually getting into the oscilloscope at all? A quick measure of the electrical cord gave me that epic facepalm moment. There was a break in the cord. Oh man, I might have saved myself some trouble if I started there. But hey, I guess that is how you learn, right? And on the other hand, those caps would go sooner or later so changing them out is never a bad idea.

So I changed the cord out and, there you go buddy. There is image. So I put it in test mode and straightened everything out and now I have myself a nice old school working oscilloscope that I know will give me some pleasure later on since I am planning on getting myself a HAM license.

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it’s ALIVE! Another working C1-94!

But as you can see, sitting in the basement for probably 20 or more years didn’t do much for it’s looks. I will have to seriously clean this baby up, maybe make a new face plate to get rid of the Russian on it, and then when I’m done with that, I will post an update with the new picture of what it became.

I hope you enjoyed this little adventure and if you have questions, comments, feel free to leave them below! Oh, and total costs for this fix? 40 cents or thereabout. Well worth it!

 

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