Re: How to troubleshoot a faulty 7B50 timebase


Harvey White
 

Replies Interleaved....

On 3/6/2021 5:21 PM, Thierry Delaitre via groups.io wrote:
When using beam finder, I can see a single vertical bar of 3cm height on the two thirds of the right hand side of the screen
Which points back at the plugin, saying that the trace is deflected all the way to the right.  Normal location for an inactive trace is the left side of the screen.

The 7B50 does not have XY mode.
It would have been nice to feed in a signal and bypass the sweep generator.

I tried to put the timebase into an input slot and I can also see horizontal bar when using beam finder
Not sure that you should be seeing a horizontal bar, but that's more or less moot for now.

The 7B50 appears to work again when the plugin was cold. The beam then disappears on the 7B50 timebase after 15/20 seconds or so after the scope is switched-on
Ok, what I suspect is that one of two things can be going on.

1) the sweep is still active, but the driver transistor that couples to the horizontal amplifier is open/leaky, or you have a capacitor that is making that transistor behave like that, thermally related.  Upsets the bias on that output stage and I think it will be turning off.

suggestions:

    if the sweep pickoff is in the plugin, then look at that.  If not, then use a separate scope to see what is hung up.  A can of freeze spray may reverse the symptoms long enough for you to figure out what part is bad.

2) the sweep itself is not working, however, the triggered light indicates (to me) that the sweep is working, since I think it's frequently driven by the sweep gate itself.

suggestion:  remove side panel from scope, remove cover from plugin, insert into far right slot, and use another scope to see what's going on.


Now, given that you do have a 7000 series scope (you'll like this part), instead of a separate scope:

put your working sweep plugin in slot #3, put the scope on chopped horizontal mode, and use the working vertical channel and sweep to debug the faulty one.

Harvey




Thanks

Thierry




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