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tektronix 2246 intensity stuck high

Discussion in 'Electronic Repair' started by TekMan, Apr 26, 2004.

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  1. TekMan

    TekMan Guest

    Hi folks,

    this time I am looking for your your help:

    I have a tektronix 2246 model A (the mil. version of the 2246).
    Everything is working fine, except brightness control: Both (A and b
    intensity) are stuck a highest brightness.

    I already checked the intensity pots (going smoothly -2.5 to
    +2.5Volts),
    all voltages from the LV power supply (-7+ 7.5 -/+ 15V, 130 Vdc,
    130Vac ): all fine, low riple.

    High voltage seems ok, since I can focus the beam very well and
    readout is clearly visible.

    So, has anybody some voltages of the HV supply in mind or on schematic
    -- then I can compare it with my readings.

    A partly scanned schematic of the HV/crt part would be welcomed too.
    and_de AT gmx.de for scans.

    Has anyone an idea where to continue with fault search? Jim, any known
    problems in this area to you?

    Thanks for any help and ideas in advance!

    Andreas
     
  2. Jim Yanik

    Jim Yanik Guest

    (TekMan) wrote in
    First thing I'd do is replace the DC restorer diodes(shifts the Z-axis
    waveform to HV levels).TEK scope DC restorer diodes often would break down
    under HV,yet test OK with a DMM.(CRT arcs could cause this)
    The only way to be sure is to test with a curve tracer.
    It could also be a bad HV cap.(same problem;HV breakdown)
     
  3. TekMan

    TekMan Guest


    thanks Jim, terribly sorry to tell that the HV diodes are working
    fine. I tested all of them with a tek577, all good. And replaced with
    known new ones. Both led to no change.
    Caps tested and changed too, still have the same problem.

    It seems that the signals from the (microprozessor intensity control
    ?) Measuring shows that intensity pot-information are not "routed" to
    the CRT:. Even before the DC restorer diodes, I can't measure a change
    in voltage(with another scope) when turning the intensity.

    Grid bias pot helps to reduce the overall intensty, so to avoid
    burning parts of the screen phosphor. Thias pot is located direct
    nearthe restorer diodes/hv caps.

    So it seems that there are no bad parts in the near-CRT area.

    Are there any known faults in the circuit-pathes before the DC
    restorer parts?
    If yes, where to check next?

    regards
    Andreas
     
  4. Jim Yanik

    Jim Yanik Guest

    (TekMan) wrote in
    Well,I don't remember enough about 2245/6 series to help you here.I guess
    you will have to dig out the manual,read the circuit description to find
    out how the intensity control works,and then TS down to where you lose the
    signal,or control switching.That's what I would be doing,if I were working
    on it.
     
  5. Asimov

    Asimov Guest

    "TekMan" bravely wrote to "All" (27 Apr 04 04:18:56)
    --- on the heady topic of "Re: tektronix 2246 intensity stuck high"

    Te> From: (TekMan)

    Te> thanks Jim, terribly sorry to tell that the HV diodes are working
    Te> fine. I tested all of them with a tek577, all good. And replaced with
    Te> known new ones. Both led to no change.
    Te> Caps tested and changed too, still have the same problem.

    Te> It seems that the signals from the (microprozessor intensity control
    Te> ?) Measuring shows that intensity pot-information are not "routed" to
    Te> the CRT:. Even before the DC restorer diodes, I can't measure a change
    Te> in voltage(with another scope) when turning the intensity.

    Te> Grid bias pot helps to reduce the overall intensty, so to avoid
    Te> burning parts of the screen phosphor. Thias pot is located direct
    Te> nearthe restorer diodes/hv caps.

    Te> So it seems that there are no bad parts in the near-CRT area.

    Te> Are there any known faults in the circuit-pathes before the DC
    Te> restorer parts?
    Te> If yes, where to check next?

    Te> regards
    Te> Andreas

    Sorry to jump in but if the intensity voltage doesn't change perhaps
    there is a missing ground path. Perhaps a bad transistor switch used
    to boost brightness for photo purposes? Check the beam intensity
    modulation input circuit (on rear?). Auto intensity circuitry?

    .... Letterman of Borg - "Ok, Top 10 reasons why resistance is futile:"
     
  6. TekMan

    TekMan Guest



    Hi Tektronix wizards,

    we were all very close to solve the problem, but no cigar :)

    All components show "good" in the curve tracer. No change in intensity
    wiht the rear input for "Z-axis".

    BUT: The reason why the intensity is stuck was so stupid simple (once
    discovered).
    By accident I noticed with magnifying glass a small crack in one of
    the several wire jumpers (Jim probably knows what I want to explain:
    The small wires on the mainboard, in the middle of each wire is a
    white plastic bead. There are used frequently on the main board to
    route the signals).

    I don't know is someone else ( it is not my personal 2246) cut the
    wire. Through the magnifying glass,the crack looked more like
    "material fatique".

    I scanned the whole mainboard for more suspect wires, and found a
    secound one.

    Okay, I soldered two new wires in, switched on: intensity now
    perfectly adjustable. scope works like new ;-)

    Thanks for all the help in the group, was fruitful discussion. anyway
    If someone has a scan of the 2245/6 schematics, I'd be still
    interested.

    Jim, your opinion please: Did you see any of these/similar problems
    in the 2245/5/7 boards?


    Andreas
     
  7. Jim Yanik

    Jim Yanik Guest

    (TekMan) wrote in
    No,I think it's very rare for a jumper to crack,and there should not be any
    mechanical fatigue possible,unless those jumpers are between two PCBs.
    You need movement for mech. fatigue.
    OTOH,It could have been damaged by the insertion machine,and maybe
    vibration/temp cycling finished the job.

    Most of the failures I recall in this series were TEK-made ICs.(155 and 234
    prefixes)
     
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