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Agilent 54835A scope (4 channel 1GHz / 4Gs/s) repair & uphack
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Tony_G:
I'm at a loss sorry. Interested though in what you find if you do end up finding the cause.

TonyG
nctnico:
I suspect this is a fault on the PCI interface board. There is a connection between the video card and the PCI interface board. This is likely an interface to project an overlay onto the output of the videocard. When the PCI interface board is generating the image, something goed wrong. Likely an address line is stuck. Better retest with a triangle wave which filles the entire screen. This should give a clue about what information shows up where.
analogRF:
that's my suspicion too but why everything is fine when the graphical display is enabled?
actually if you look at the pictures i posted, when the graphic display is disabled and waveform is distorted, clearly the graticule lines are not evenly spaced anymore neither the small ticks on the horizontal axis. they are all misplaced. I just looked at it on the scope too. I will post new pics with triangular wave in a few hours. i have to run now....

oh, btw, i put in 4 sticks of ECC memory exactly as stated in the manual of the AMI motherboard and enabled ECC check in BIOS with no effect on anything...everything remains the same...
nctnico:
The memory on the motherboard has nothing to do with the graphics. I guess when the graphical display is enabled the CPU is used to draw the signal (and other graphics) while without the graphics the overlay generator on the interface is used. Did you already try with a triangle waveform? And how about different time/div settings?
analogRF:
here is triangle waveform. this happens on all time bases. It's not just the waveform that is distorted. the whole graticule, the whole screen is actually distorted. Even with no signal fed to the scope (just the base line) still the graticules are distorted in that mode.

yes, I can see what you are saying about an address stuck in the video memory. But if the VGA memory on that board is bad, it should show up somewhere else, no? The two memory chips on the VGA board (NOT the interface board) are KM416C254DJ-5 which are 512KB (256K x 16 bit) chips and windows 98 reports 1MB video memory

Is it possible that someone moved the boards on PCI slots and I need to change the IRQ in the BIOS or something? You see, in the Agilent manual it says change the IRQ9 to ISA (from default PCI) but I cannot see what IRQ9 is doing. but I can see IRQ10 is given to the VGA board.
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