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Broken Scope Update: Transistor testing? Replace blown ones with commonly available equivalents? or BER?

So in a previous question, https://www.instructables.com/answers/My-oscilloscope-died-How/ I have stated that my oscilloscope died, and listed the symptoms. Since then, I have popped the cover off, had a look inside, and was I found inside what certainly appears like a "mod," it goes off to the 3 by 3 connector I have mentioned also. I forgot to mention a small blueish purplish wire was next to it, similarly just sticking out the scope. I figure it is a MOD because it is:


A) precariously mounted to the transformer with only one small nut on the transformer

B) Has so many bodge wires crawling around on it, that it looks more like a cobweb (and w/o silastic or hot glue for vibrations/stress)

C) Sloppy soldering and flux residue left behind indicating hand soldering

D) The PCB has a very different appearance than the rest of the boards inside; No solder mask, greyish white, and no silkscreened values, parts, or numbers.


I traced both the wire and the connector and ribbon cable to the same mod board, which clearly got toasted. There was a small orange wire tangling off that board, it it broke off before I even realised it. Luckily the pictures show roughly where it was connected. Tracing the blue wire back to the board, I discovered it is connected directly to one of the cooked resistors, and I think the other cooked one was in series with it.

The only connection found to the main board is the orange wire, and there are ceramic capacitors in series on that crappy looking PCB, indicating it is capacitively coupled. I was Hoping that what had happened was the blue wire came into contact something it was not supposed to, and shorted the output of that chip causing it to catastrophically fail, knocking out power to some of the boards. Unfortunately did not appear to the the case, and the supply voltages seem OK.

Nothing appears obviously burned out on any of the other boards from what I can tell. However, just today I have discovered that some driver transistors, which are mounted to heat sinks, are getting unconfortably hot to touch. I suspect they may be blown, by the blackening and discoloration around them on the PCB.They themselfs do not appear to have any physical damage. I will desolder them and connect them to a transistor tester I have to see if they register as operational.

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As for some additional questions, can I replace those weird looking obscure transistors with standard ones? I am not sure of their exact specs, the only datasheets I can find for "2SA818" and "2SC1628" are some scanned PDF datasheets that are not in english, and I certainly could not find replacements online for them. I have the gut feeling radioshack is not going to carry them either. :P

I hope this is not Beyond Economical Repair (BER), but used analog scopes can be bought used for around $30-$70, and of course I am on the verge of getting something better than that.

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You know, fascinating though it is, it really is BER, especially since you know you are upgrading.

But if you are screwing around with Tesla coils GET AN ISOLATION AMP....so you don't blow the next one up.

-max- (author)  steveastrouk2 years ago
Well if I can replace a cheap op amp and A few transistors, I would say it is worth it, but my lack of understanding of the schematic topology make it hard to know where to look. You're probably right, it is BER considering I got it for $30.

An oscillation amp? Never new there was such a thing. I do now though :(. I was thinking about making something that would serve that purpose for my phones audio output as an improvised function gen.

My "Tesla coil" was not really a Tesla coil, just a large slayer exciter powered with a isolated 24V supply. I doubt that is what had happened, I would expect it to have knocked out the channel A and B amp...

It DID pain me to say BER, believe me, but its a tech we shall not see again.

-max- (author)  steveastrouk2 years ago

Interesting thing I found, is that there is actually a 555 timer in the scope doing the trace I think, certainly some for of timing!!!! It is a "RC555" specifically, and to verify that, I popped in a jellybean NE555 motorola part, and it worked just the same! There was no immediately clear difference in performance from what I could tell, although I am not too sure. I put the RC555 back in just in case there is some noticable difference if I get it working again.

BTW, what do you think could have caused the trace to not go the whole screen on all the ranges? That is what that first picture shows. It only shows the last 3/4 of the trace w/o trigger. If I stick a wire in the BNC connector, it will trigger and the trace will draw somewhat correctly.

-max- (author)  steveastrouk2 years ago

I am beginning to think even if I did did figure out what transistors are blown, I would not be able to find replacement parts, or spend way too long searching. :'(

I assume it is not possible to replace the old transistors with general purpose modern ones like 2n3904's? I think that would cause the performance characteristics to change hugely if there are a lot of 'matched pairs' and stuff.

I do not have money to get a new digital scope yet, as I am currently in need of making a major purchase on a new computer for school.

-max- (author) 2 years ago

UPDATE: The "power" transistors in the last picture seem to be fine. I have desoldered them and tested them on a simple transistor tester (the testing circuit is nothing more than a basic oscillator with a small transformer of which the larger coil is connected to an LED lamp). :(

I am once again at a loss as to what could be wrong. It could be any number of parts.