Author Topic: More scope shenanigans  (Read 1122 times)

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Offline Zenwizard

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More scope shenanigans
« on: November 02, 2018, 02:30:30 am »
I have mostly finished up a 7603 mainframe scope for the bench and got new probes in for it today. It is nitpicking but it has an odd quirk when the readout is on it looks like there is static on the trace. Readout is rock stable. I think I have isolated the fault to the readout board. Attached are some shots from my other scope. The weird thing is when the readout is on the clock on U2126 is no where near stable. When readout is OFF. trace is smooth as glass no static. The service manual is not to clear on this or I am just not totally understanding what it is trying to tell me.

Thanks for the help.
 

Online tggzzz

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Re: More scope shenanigans
« Reply #1 on: November 02, 2018, 07:52:24 am »
I have mostly finished up a 7603 mainframe scope for the bench and got new probes in for it today. It is nitpicking but it has an odd quirk when the readout is on it looks like there is static on the trace.

That statement would be a lot more useful with a picture of the "static", whatever that is (e.g. X/Y/Z axes). Is it at all sweep rates?

Consider that the electron beam has to draw the characters, and during that time it cannot be displaying the trace. That can cause trace intensity variations at some sweep speeds.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline Zenwizard

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Re: More scope shenanigans
« Reply #2 on: November 03, 2018, 03:23:12 am »
It is a Z  axis fault. Attached are 2 photos one is with the readout off the other is with the fault present. It is almost like the blanking circuit is firing randomly. The fault is the dark spots on the trace. Also they are not stable they scroll through the trace but they do not exist on the readout display.
 

Offline drussell

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Re: More scope shenanigans
« Reply #3 on: November 03, 2018, 03:51:47 am »
It is almost like the blanking circuit is firing randomly. The fault is the dark spots on the trace. Also they are not stable they scroll through the trace but they do not exist on the readout display.

Like tggzzz said, are you sure it isn't just artifacts from when the scope draws the on-screen characters?  Does it change when you vary the sweep speed?   

Presumably the timing for the on screen display of characters is not timed to the sweep speed so will not always occur at retrace time or anything...
 

Offline duak

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Re: More scope shenanigans
« Reply #4 on: November 03, 2018, 10:04:05 pm »
I'm pretty sure those are artifacts from the readout.  The beam has to refresh the readout at a minimum rate and it becomes visible at certain sweep speeds.  At slow speeds, the gaps are too small to see while at fast speeds, the readout refresh is interleaved between sweeps.




 

Offline David Hess

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Re: More scope shenanigans
« Reply #5 on: November 06, 2018, 12:20:08 am »
The beam is multiplexed to generate the readout so at some sweep repetition rates when the readout is free running, the multiplexing becomes visible in the trace and this is completely normal.

There is a switch on the readout board which controls when the readout multiplexing takes place.  In GATE TRIG'D mode which is suitable for photography, the readout is generated at the end of the sweep but in this mode, there is no readout unless the sweep is present.  Some of the other 7000 mainframes have this control on the front panel.
 


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