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Unstable trace on Tektronix T922

Discussion in 'Troubleshooting and Repair' started by Redwood, Jan 14, 2013.

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  1. Redwood


    Jan 14, 2013
    I've been taking on simple electronics repairs and projects for years but am just lately trying to get more serious about it. I've had a Tek T922 for over a decade and only used it occasionally so my skills with it are not high level to say the least, but I'm generally able to apply a signal and track it through basic tube amp and guitar stomp box circuits.

    Recently I was working on an Ampeg V4 with pretty high B+ voltages and I must have put the probe to a point that was more than the scope could handle. Now I have a couple of problems with it that are probably related.

    First off, when I find a trace and center it on the graticule, with each click of the V/div knob the trace moves up or down. As I go to finer and finer resolution, the trace moves up and off the screen, and after a few clicks it's out of range and I can't adjust the position enough to bring it back on screen at all.

    Which leads to the second issue, which is that I'm now very limited in the range of signals that I can see at all. With a tube amp having several gain stages, this means I can only see the first couple of them, after which even going to 10x on the probe I can't find them at all.

    I should mention that even working at voltages where I can get a very good trace of my signal, the image is never static but always blinking.

    The T922 has a probe calibration feature where you touch the probe tip to a node on the front of the scope and it generates a square wave. Supposedly then adjusting the screw on the probe then causes the top and bottom of the square wave to deflect one way or the other, the objective being to find the setting that leaves it flat and symmetrical. Also, if you align the top of the square wave to the centerline while set to DC, it should move up so the whole wave is centered over the centerline after switching to AC.

    On my scope, neither of these things appear to work. I get the square wave, but the adjustment screw on the probe does nothing, and it does not move when switching between AC and DC. I just purchased a pair of brand new probes to make sure my old probe wasn't the issue.

    I think I probably popped a cap and/or resistor in the scope, either associated with the probe inputs or the AC/DC switch. Since I get the same behavior on both channels it would seem the latter is more likely.

    I have searched but can't find discussion of any similar issues here or elsewhere. I'd be very grateful for any pointers in tracking down the source of my problem.

    Thanks very much,
    Last edited: Jan 14, 2013
  2. quantumtangles


    Dec 19, 2012
    Hope this helps. Entitled 'How not to blow up your oscilloscope', this Youtube tutorial from a reputable electronics expert will save me a lot of typing :D

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