Leader lbo 508 oscilloscope help
Good day folks. Some of you may remember that a while back I posted about this scope, how I found it in our junk pile at work and decided to have a play with it. Well in the meantime, my "colleague" took on the challenge himself and wouldn't let me go near it, so I don't know what he's done to it or how it actually behaves, because I haven't plugged it in.
Now, it has landed on my table at last and he told me "it's got poor sensitivity"...is that a thing with scopes ? "Low sensitivity" ? Now I don't know sh!t about scopes TBH, so it's a learning experience for me too.
I took a peek inside and noticed two varistors with cracks in them on the board with the large caps on it (see the pics). I swear, I suck at this CRT stuff
In the schematic I attached, on page 37, they're D116 and D117. 102K would imply they're 1000v varistors, which the schematic indeed confirms. I couldn't find the exact datasheet, but a fairly close one hinted that I was right: 1000v. They read completely open with my meter. Now why would they pop ? From what I can assume: the transformer on the left (PT101) chucks out 930v. It goes through the large C117 "oil capacitor" and into the string of diodes which rectifies it in negative, so only the negative peaks go through. For those to pop, it would mean the voltage went higher than 2000v, which is what the little label tag TP.5 tells us it's supposed to be on that point, so perhaps it was a simple overvoltage scenario, since I don't see any regulation to prevent it: it relies solely on the stability of the mains input. Any fluctuations on the mains will show up on the output on that X-former, thereby affecting the point after the diodes too. If it got a surge on the primary hard enough, the secondary voltage could've ramped up....just a theory.
Aside from that, since that's certainly not the problem (I'm betting it would work without the varistors entirely) what should I look out for in a generic sort of way ? Any specific parts on old scopes that go bad faster and easier than others ? It's only a speculative discussion at this point since without plugging it in and showing you chaps what it actually does, it's hard to pinpoint. Photos are pretty random too, since there's a lot of space to cover and I didn't catch everything. I can provide better ones if needed. Cheers guys. Thank you.
Now, it has landed on my table at last and he told me "it's got poor sensitivity"...is that a thing with scopes ? "Low sensitivity" ? Now I don't know sh!t about scopes TBH, so it's a learning experience for me too.
I took a peek inside and noticed two varistors with cracks in them on the board with the large caps on it (see the pics). I swear, I suck at this CRT stuff
In the schematic I attached, on page 37, they're D116 and D117. 102K would imply they're 1000v varistors, which the schematic indeed confirms. I couldn't find the exact datasheet, but a fairly close one hinted that I was right: 1000v. They read completely open with my meter. Now why would they pop ? From what I can assume: the transformer on the left (PT101) chucks out 930v. It goes through the large C117 "oil capacitor" and into the string of diodes which rectifies it in negative, so only the negative peaks go through. For those to pop, it would mean the voltage went higher than 2000v, which is what the little label tag TP.5 tells us it's supposed to be on that point, so perhaps it was a simple overvoltage scenario, since I don't see any regulation to prevent it: it relies solely on the stability of the mains input. Any fluctuations on the mains will show up on the output on that X-former, thereby affecting the point after the diodes too. If it got a surge on the primary hard enough, the secondary voltage could've ramped up....just a theory.Aside from that, since that's certainly not the problem (I'm betting it would work without the varistors entirely) what should I look out for in a generic sort of way ? Any specific parts on old scopes that go bad faster and easier than others ? It's only a speculative discussion at this point since without plugging it in and showing you chaps what it actually does, it's hard to pinpoint. Photos are pretty random too, since there's a lot of space to cover and I didn't catch everything. I can provide better ones if needed. Cheers guys. Thank you.
Attached Files
Comment