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calibrating a 54111D

Question asked by alexw on Aug 25, 2009
Latest reply on Aug 31, 2009 by ko2054
   I've got a couple of 54111D's which I'm trying to calibrate. Wonderful boatanchors, remarkable design for their time, thoughtful software and interface, and still very useful. I've read all the Service and Extended Service manuals, and the articles in the HP Journal; learning how some of this works has taught me a great deal. However, there are the usual problems. I've adjusted the DC power supply, CLOCK and GAP, GAIN and FLAT, and CAL, and done the vertical self-cal, etc. Relays in some of the input attenuators on some of the ranges are sticking; will try to solve with disassembly and cleaning.

    1) The frequency response isn't good, even after adjusting GAIN and FLAT; I am checking this with a borrowed N9310A (another beautiful instrument). The 3dB point is around 250 MHz instead of above 500. I think the input curve should be Gaussian, so I think I should be seeing e.g. 95% to 200MHz, 88% @ 300MHz? Is GAIN and FLAT the key adjustment that affects the high frequency behavior?  I don't have a Tek PG 506 scope calibrator, so I'm using a slower pulse generator (my 8082A has the fast edges, but doesn't settle quite fast enough, so I've borrowed a Phillips which settles to within a few mV in 10 ns.) I've added a bunch of broadband attenuators to mock-up the cleanest rising-edge signal I can get; it's now only 50mV amplitude or so, but I gather from the service manual that that's not really important. What else can I try? The modules still show 50 ohms, so I hope they have not been damaged.

    2) I suspect the input attenuator divider compensation capacitors do need to be adjusted, despite what the manual suggests; response is worse on higher ranges (but this should not affect the divide-by-1 path, no?) Of course, I don't have the special adjustment tool HP #8710-1515 ... but I found the trimmer capacitors on the module, and they look a lot like a Johanson Thin-Trim 9702 (SL-1?), which calls for a 0.025 in square-driver, which Newark does still carry. Is this correct?

    3) Probe tip cal is a little mysterious. The manual specifies that the vertical specifications will only be met with the probe that the channel was calibrated with. When you do probe-tip-cal, does the 54111D observe and store correction parameters that it later "convolves" with the input before display? I am not interested in probes, only 50-ohm cabling, so could this compensation create problems, and if so, can I zero it?

   In general, the calibration procedures talk a lot about low-frequency and DC adjustments (and later triggering), but not how to correct the high-frequency behavior. I hope this is possible; I really like these scopes, for all their age, and I need them for research. Much thanks for providing this forum.