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Tektronix 465M HV problem

Discussion in 'Electronic Repair' started by Steve Turner, Feb 18, 2004.

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  1. Steve Turner

    Steve Turner Guest


    I recently acquired a nonworking Tek 465M to serve as a source of
    spare parts for my main scope (also a 465M). After receiving the dead
    scope, I thought it would be fun and educational and maybe even
    worthwhile to try to fix it, so I would have a spare scope instead of
    just spare parts. Fortunately, I have a full set of schematics, which
    partially make up for my complete lack of formal electronic

    The scope has no trace or HV. After removing the horizontal and
    vertical modules and the CRT, I discovered a blown 0.25A fuse on the
    mainboard. This fuse supplies the HV oscillator with +32v (unreg),
    and I verified that, yep, it blows instantly on powerup. All other
    supply voltages are right on the money.

    I have checked the transistors, diodes, and electrolytics in the HV
    oscillator and nearby HV regulator circuitry, and there are no obvious
    shorted (or open) junctions. I did discover that applying the nominal
    +3.9v to the base of Q548, which is part of the HV regulator, prevents
    the fuse from blowing and allows the HV oscillator to be powered.
    However, it still refuses to oscillate, meaning no drive for the
    transformer and HV module.

    At this point, I'm kinda stuck. I think I've done my Google homework
    and found lots of interesting things, such as the fact that there are
    a number of Tek experts on this forum, but I was unable to find the
    solution to my problem. I'd be most appreciative of any pointers.

    I'll try to post the relevant schematic to


    Steve Turner
  2. Jamie

    Jamie Guest

    by you applying Biasing on the regulator you may have done nothing more
    than shut down the Oscillator..
    try to vary that voltage and see if its it gradually coming on and
    your getting excessive current build up as it attempts to drive the
    output module? if this is the case then you most likely have a shorted
    high voltage Transformer or High Voltage tripler shorted or in many
    cases the high voltage unit which has both the coil and tripler build in..
  3. Steve Turner

    Steve Turner Guest

    Thanks for your reply.

    I did as you suggested, and found that lower bias voltage on Q548
    results in a larger current through the fuse. Current is around 50 mA
    at a 3.5v bias level. At this point I probed around a bit with the
    scope and ... lo and behold ... the HV oscillator was oscillating
    after all -!! I don't know why the difference from my original
    observation. The waveforms look a bit funny though. I'm out of time
    tonight but will play with this some more tomorrow.

    Thanks again for your feedback.

    Steve Turner
  4. Jim Yanik

    Jim Yanik Guest

    Did you read the circuit description in the service manual?
  5. Steve Turner

    Steve Turner Guest

    Yes, several times, until I sort of understood it in a general way.
    After Jamie's feedback I played with the "artificial" bias on the HV
    regulator transistor and found that this level 1) dramatically affects
    the current through the HV oscillator, and 2) prohibits oscillation
    entirely if above a certain level. Maybe this is obvious to you but
    'taint to me. At least I can get some scope waveforms now, although
    they are not good waveforms and bear little resemblance to the ones in
    the service manual.

    Even with the HV oscillator oscillating, the other end of the
    transformer doesn't show +95v like it's supposed to. Maybe the
    transformer is "bad"?

    As much as anything, I was wondering if anyone else had had a similar
    experience with this model scope. If so, I wasn't able to find it
    with Google.

    I appreciate all comments. Although I'm familiar with most electronic
    fundamentals in a textbook sense (having waded through Horowitz & Hill
    on my own), I have zero practical experience.

    Steve Turner
  6. Steve Turner

    Steve Turner Guest

    Forgot to mention one thing ... I'd be happy to transcribe the circuit
    description here if anyone wants. (Maybe not as quickly as I'd like
    though, since I have in-laws arriving for the weekend...)

    Steve Turner
  7. Guest



    BTW, I also got a non-working Tek 465M 2 days ago from eBay and managed to
    get it working 2 hours ago thanks to the schematic you've posted earlier :).

    I have similar problem like the one your describing but without the blown
    fuse for the +32V unregulated line. All the +5, -5 and +32V are good. I did
    check the +95V testpoint and got ~ +13V. I also checked all the HV related
    components in-circuit and the +95V line for any shorts, all appeared to be
    OK. I decided to take the Q552 off circuit and noticed that the B-E and B-C
    forward voltage was a bit off the 0.5-0.6V range. Bought the replacement
    part (2SC2527) this afternoon, soldered the transistor after dinner and
    viola! it's working.

    Yours might have a different component/s thats faulty. Try to check the
    components off circuit.



    Note: my email is valid once the extra 't' is removed.
  8. TekMan

    TekMan Guest


    Either transformer bad or too much load. Latter
    might be in the +95V path Or too muc hload due to partly damaged HV multiplier.

    yepp, I had once a bad multiplier, causing same sympthoms.

  9. Guest

    I mean

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  10. Steve Turner

    Steve Turner Guest

    That is very interesting information. Your symptoms sound similar
    except for the +32v fuse blowing. I may try this solution, if I can
    figure out how to get underneath the mainboard without breaking
    anything. Doesn't look like a trivial process.
    I guess the problem is how to do this when the mainboard is still
    screwed down in the chassis. I have isolated a couple of
    electrolytics (w/axial leads) by snipping one of the leads on the top
    surface of the board. This can later be soldered back into place.
    I'm not sure I like this approach. It's easier than dismounting the
    component from the board, but I dislike connections held together by
    nothing but solder...

    Thanks much for the feedback.

    Steve Turner
  11. Steve Turner

    Steve Turner Guest

    That's what I'm afraid of. However, although the HV oscillator is
    oscillating in some fashion, the waveforms are quite a bit different
    than the ones shown in the service manual. So I'm not sure if the low
    voltage means that the transformer or HV multiplier is faulty, or if
    the transformer simply isn't being driven properly. I need to do some
    more looking at this, but have house guests at the moment...
    Do you recall if you were able to find a replacement?

    Thanks much for the comments.

    Steve Turner
  12. Jo

    Jo Guest

    A shorted Q552 obviously will cause a blown up +32V unreg fuse. Besides a
    transistor is more likely to fail than a transformer. Accessing the main board is
    just the matter of locating the hidden screws. Two on the rear below the CRT neck
    and two on the middle near the four screws that holds the CRT itself. The rest of
    the five or so are visible and of the same typeas the other four.

    Good luck!

  13. Steve Turner

    Steve Turner Guest

    It looked to me at the outset that Q552 was the most likely culprit
    for the symptoms I was seeing, so it was one of the first parts that I
    "tested" (to my limited ability). It does not appear to be shorted
    outright. However, I'm willing to believe that my testing is not
    revealing the fault, so I think I'll go ahead and replace it with the
    2SC2527 replacement device that you specified earlier. I don't have
    much hope of locating a replacement transformer or HV module, so it
    will probably be that or nothing anyway.

    Thanks also for the tips on mainboard removal.

    Steve Turner
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