Test Equipment: Oscillocopes

[TinselKoala]

I'd like to see a bit more data, like the second channel trace, before you start taking it apart. If you take the case off that might give you a longer test time before it starts glitching. Also, turn down the intensity! The trace should not "bloom" under normal conditions, it should just make a nice readable line without illuminating more of the phosphor than needed for visibility. Don't worry at all about not seeing the rising and falling edges from the Calibrator, that is normal at this horizontal resolution and indicates that the calibrator circuit is giving you nice fast rise and fall times, a good thing.

Instead of using the scope's calibrator as a signal source, you could also try connecting the GenRad oscillator to get a sine waveform instead of the square wave. For low frequency testing you don't really need to worry about fancy connectors or precise impedance matching, just use clipleads to connect the GenRad's ground to the scope's BNC grounds and another cliplead to connect the center pin of the GenRad connectors to the center pin of the scope's BNC channel inputs. Set the GenRad to the same frequency as your calibrator output. The sine wave will not have "invisible" portions like the square wave from the calibrator does. Start with the output of the GenRad set to the lowest possible value, then gradually turn it up until you have an acceptable trace amplitude on the scope.

When you do open it up and start prowling around in there, I would suggest that the very first thing you do is to check the power supply voltages, using a DMM and the information in the SM. You have a handful of low-voltage DC supplies to check, starting with the +55V supply.  Since you have a good beam and at least acceptable traces until it starts glitching, I wouldn't even worry about the HV supply at this point. But off-voltage LV supplies can cause all kinds of weird problems, and since these use old and maybe flaky capacitors, and the problem is heat- or at least on-time related, that's where to start looking.

The LV power supply test points and voltage ranges are on section 6-8 in the SM. You'll have to look at the board drawings to find the locations of the numbered test points. Don't worry about testing the HV supply. And don't try adjusting anything yet! Just let me know what the voltages are at the test points. Be sure to set the scope's controls as required for the PS testing. The voltages are referenced to chassis ground, so use a nice sturdy cliplead to "permanently" connect the DMM's Minus input to the chassis. Then carefully probe the TPs with the DMM's pointy red lead. Be very careful because obviously you don't want to short anything out or hit a HV point by mistake. But this is actually really an easy testing to do once you have located the correct testpoints on the boards.

[TinselKoala]

that is an effect of the camera.  the brightness was only up just high enough to where the vertical traces are slightly visible.  the illumination was also down all the way.  I don't know why the camera does that.  maybe I have it on low light setting or something.

Turn the intensity down! The vertical parts of the traces from the calibrator on the settings you are using are _supposed_ to be nearly or even completely invisible. The important part is the horizontal parts of the traces and these should not "bloom" excessively beyond the trace itself.

Of course if you really _need_ to see the rise and fall edges for some reason you turn up the intensity. But normally you would run at a lower brightness level.

Note that in the "normal" pic below the horizontal portions do not "bloom".

[MarkE]

that is an effect of the camera.  the brightness was only up just high enough to where the vertical traces are slightly visible.  the illumination was also down all the way.  I don't know why the camera does that.  maybe I have it on low light setting or something.

The traces were blooming heavily.  If you don't turn it down, you are going to burn the screen.

[MileHigh]

Well, I am going to repeat my cautionary note again.  I get the impression that Brian has no electronics experience at all and finding bad components and changing them inside an oscilloscope is not a trivial matter even if you have the service manual.

As an example:  Brian, if you switched off the power to the scope but left it plugged into the wall socket, did you know that if you touched a component with a regular soldering iron things could "blow up?"  If you didn't know that then that shows you should be careful.

Here is a way of looking at it:  From what I can see from your clips the problem is annoying but tolerable.  The scope is still working and showing you waveforms.  The suggestion is to just use it like that for now.  In six months if you truly are interested in this stuff and are learning the whole time, then perhaps try to fix it.  However, there is a chance that in six months you will have moved on and dropped the whole thing.  Then fixing the scope becomes a moot point because you will not be using it.

MileHigh

[TinselKoala]

Oh ye of little faith.