Hi all,
I have a HP8757A scalar network analyzer with a display problem which has appeared after some 6 months of no use. It was previously working just fine, with a HP8350B sweep generator and the 10MHz - 20GHz plug-in.
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What happens is that on switch-on, it goes thru the self-test and then reverts to a display which seems to contain some correct information such as frequency span, key button names (at the right), and a 'noisy' scan waveform (as expected) - BUT the display refresh is very slow and is well off-centre - it is clearly cycling through the 4 'display channels sources' but it is nearly unreadable (certainly unusable).
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I have another 8757A which I use with another sweeper which works well, and by using this as a 'test reference' I can determine which buttons to the right of the screen to use to perform some tests. In this way, I have called up the test pattern display (SYSTEM -> MORE -> SERVICE -> DISPLAY -> TEST PATTERN) and this generates the wanted test pattern BUT this is also well off-centre and refreshed slowly.
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I have checked the error-code LEDs (removed the top cover of course) and they are all off (zero) after switch-on or after "PRESET" which indicates no error found, and also checked the +15V, -15V and +5V on the small power board with the fuses. The +105V rail there is also correct.
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This problem is independent of whether the 8350B sweeper is on or off, and if it is on the PRESET button initializes both the 8757 and the 8350 as it should. I have compared the problem unit with the working unit and the basic functionality would appear to be present. I am wondering if the battery on the CPU board (backing up some constants?) could be a reason, but I doubt it - I can replace this tomorrow if I think it might be.
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I know that the refresh frequency of the display will be lower with the more channels that are being scanned (A, B, C, R) but this does not appear to change the problem on the unit that I am dealing with.
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For what it's worth, I have a comprehensive workshop and lab at home and I am used to working with both microprocessor designs and (more so) with microwave / ATV designs. I have read the service manual and operating manual but with so many pages and different references I am not sure where I should be looking.
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Would anyone here have a good idea of where the problem could be?
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My sincere thanks in advance.
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Trevor Gale