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What's up with my oscilloscope?

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chris414

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I'm using an old 20Mhz oscilloscope and have been having some problems recently. On both channels, when I turn the time/div dial up past about 10Mhz, the trace becomes very faint and then dissappears altogether around 15Mhz. What could be the cause of this? Can I fix it myself or should I take it into a repair shop?
 
I have a few old scopes that do that - I've always assumed the gun/screen is starting to wear out and the faster scan-rate of the smaller time divisions is too quick for them. I guess you've already tried turning the intensity up?
 
Yeah intensity is maxed out... if the problem is as you suggest is there any fix for it or am i wasting my time/money? I only want this oscilloscope to work for another year or so until I can buy myself a nice digital one.
 
Not that I know of - you might be able to up the intensity somehow internally, but I've never tried and you'd probably fry the tube anyway...

Just use it for lower frequency stuff, I suppose - it might be worth asking somebody who repairs CRTs about it...
 
I have been using some brand new BK CRT 30MHz scopes for a few years now and they do the same thing. The problem is not that the oscilloscope is old or wearing out, it's that the speed at which the electron beam is moving across the phospherus screen has to go up with the frequency. It needs to be at leas twice as fast as the frequency you have it set to. The reason it's dimming is because it's moving so fast that with the normal human eye you can't see the extreemly dim, quickly moving light. The simple fix is to turn up the intensity of the trace.
 
I have been using some brand new BK CRT 30MHz scopes for a few years now and they do the same thing. The problem is not that the oscilloscope is old or wearing out, it's that the speed at which the electron beam is moving across the phospherus screen has to go up with the frequency. It needs to be at leas twice as fast as the frequency you have it set to. The reason it's dimming is because it's moving so fast that with the normal human eye you can't see the extreemly dim, quickly moving light. The simple fix is to turn up the intensity of the trace.

my intensity is as high as it will go and i still can't see it?
 
As antknee says, turning the lights down will help.

Also a 6-8" cardboard tube made out of black construction paper and taped over the tube will help greatly at normal ambient light levels.
 
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You might want to check the CRT filament supply for a leaky cap and/or a pot to crank it up a few tenths of a volt. It does sound like low cathode emmission.
 
When the old CRT TVs developed low cathode emission (which would show up as silvery looking highlights in a B&W picture) they would sometimes add a small filament autotransformer (particularly designed for this purpose) that plugged into the CRT, and which boosted the filament voltage by a volt or so. The higher filament current and temperature from this would increase the emission and make the CRT usable again. But the increased voltage would also shorten the filament life and it often wouldn't last more than a year or so after adding the transformer.
 
Here's an idea...if you actually have a waveform to sync to, the trace becomes more visible. If you're just turning knobs, the high speed trace doesn't hold still and keep reprinting on the same place.
 
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