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Tek 465B sweep problem

Discussion in 'Electronic Repair' started by bg, Dec 5, 2003.

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  1. bg

    bg Guest

    I've got a Tek 465B that has lost the B sweep from the 50 microsecond range
    to the 1 microsecond range. Above or below that range all looks and works
    Suspecting the timing switch board, I have checked every component and cam
    switch contact, all were good. After reassembly the sweep came back for a
    few minutes and then went out again. The B Sweep start gate signal is
    present at P4541 pin 1, but there is no output at Q4570. Any ideas??
  2. Jim Yanik

    Jim Yanik Guest

    Perhaps a bad B timing cap for that group of ranges? The schematic for the
    timing switch should show what caps are used in those ranges.(I don't have
    any manuals).It could also be holdoff related.
  3. bg

    bg Guest

    I've got it solved, thanks for your attention Jim.

    Jim Yanik wrote in message ...
  4. bg

    bg Guest

    The collector of the output stage was shorting to ground on the bad ranges.
    The short was comming from the timing switch board. I had pulled this board
    several times and couldn't find any bad components or switch contacts.
    There's about 3 dozen contacts.
    I traced the short through the switch contacts and followed a trace that
    went under several large timing caps. When I pulled the caps to see where
    the trace went, the short dissappeared! So there must have been some kind of
    crud or solder or something lodged under those caps.It was one of those "now
    you see it now you don't" problems.
    James Sweet wrote in message ...
  5. Jim Yanik

    Jim Yanik Guest

    More on this;Is the B sweep stuck on the left side of the screen,or the
    right.If on the left,it's not yet ramped (down IIRC),so it's not gated
    ON,or in holdoff,or the sweep generator is dead(not making a ramp),if on
    the right,it's ramped down and the sweep is not resetting(gate stuck ON,or
    End-of-swp comparator not working)

    The manual has a detailed circuit description section which is valuable in
    knowing how the sweep circuits work.
  6. bg

    bg Guest

    conductive spider?
  7. Jim,
    You seem very knowledgeable about Tektronix scopes. I have a 475 that the
    sweep only works in "A INTEN" mode. I do have the technical, maintenance
    and calibration manuals. Any ideas where to start looking?
    Carey Fisher
  8. James Sweet

    James Sweet Guest

    So what was it?
  9. Jim Yanik

    Jim Yanik Guest

    There's a chart in the circuit description that detains which sweep gen
    runs for each mode;A only,A inten,B swp,etc. The circuit description is
    VERY valuable in knowing how this scope works.

    You may need another scope to troubleshoot.You first need to know if the A
    sweep is running,and if the problem is a bad Z-axis circuit or a weak CRT.
    In A inten mode,the B sweep adds some extra current to the Z-axis amp for
    brightening the portion of the A sweep.(A INTENsified by B)
  10. Jim,
    Thank you VERY much. I now know where to start looking. I'll post my
    results when I find the problem. I do have another scope I can use (a 465)
    to look at the sweep circuits.
    Thanks again!
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