Tek 7904A rapid ticking - no HT...

Discussion in 'Electronic Repair' started by Mike Deblis, Jan 2, 2006.

  1. Mike Deblis

    Mike Deblis Guest

    Hi,

    Bad day at the office (shack) - two failure in one day ;-(

    I recently turned on my trusty 7904A after perhaps 2 years of non-use...

    It briefly displayed both traces (linup is 2 x 7A26, 7B92A, 7B53A), and
    then the traces dissapeared after about 10 seconds and the scope is
    issuing a rapid "ticking" at about 200 Hz - more of a sharp-edged buzz
    really. Now when I turn it on, I only get the buzz/tick.

    Removing all the plugins makes no difference...

    I'm not a HT person. Ideas? If it helps, I'm in the Uk.

    Many thanks
     
    Mike Deblis, Jan 2, 2006
    #1
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  2. Mike Deblis

    Guest

    Sounds like bad filter caps in pwr supply
     
    , Jan 2, 2006
    #2
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  3. Mike Deblis

    Steven Swift Guest

    "Mike Deblis" <> writes:

    >Hi,


    >Bad day at the office (shack) - two failure in one day ;-(


    >I recently turned on my trusty 7904A after perhaps 2 years of non-use...


    >It briefly displayed both traces (linup is 2 x 7A26, 7B92A, 7B53A), and
    >then the traces dissapeared after about 10 seconds and the scope is
    >issuing a rapid "ticking" at about 200 Hz - more of a sharp-edged buzz
    >really. Now when I turn it on, I only get the buzz/tick.


    >Removing all the plugins makes no difference...


    >I'm not a HT person. Ideas? If it helps, I'm in the Uk.


    >Many thanks


    The 7904 has an overcurrent protection circuit, which will cause the
    ticking. This is a good thing.

    First step, remove all the plug-ins. Still get the ticking? If yes, you
    can isolate boards by removing connectors, one board at a time.

    I had one of these fail that turned out to be a bad diode on the Z-axis
    board.

    Most likely problem: a bad cap on one of the boards. The tantalums go
    bad and Tek used them by the truck load.

    Good luck.

    Steve.
    --
    Steven D. Swift, , http://www.novatech-instr.com
    NOVATECH INSTRUMENTS, INC. P.O. Box 55997
    206.301.8986, fax 206.363.4367 Seattle, Washington 98155 USA
     
    Steven Swift, Jan 3, 2006
    #3
  4. Mike Deblis

    Keyser Soze Guest

    "Mike Deblis" <> wrote in message news:dpbrju$7s$-infra.bt.com...
    > Hi,
    >
    > Bad day at the office (shack) - two failure in one day ;-(
    >
    > I recently turned on my trusty 7904A after perhaps 2 years of non-use...
    >
    > It briefly displayed both traces (linup is 2 x 7A26, 7B92A, 7B53A), and
    > then the traces disappeared after about 10 seconds and the scope is
    > issuing a rapid "ticking" at about 200 Hz - more of a sharp-edged buzz
    > really. Now when I turn it on, I only get the buzz/tick.
    >
    > Removing all the plugins makes no difference...
    >
    > I'm not a HT person. Ideas? If it helps, I'm in the Uk.
    >
    > Many thanks
    >
    >><> wrote in message news:...
    >> Sounds like bad filter caps in pwr supply
    >>


    It could also be that one of the high voltage power supplies arching to ground.

    Check the HV wires for breaks in the insulation.

    The post deflection accelerator is very high voltage so try not to get killed.
     
    Keyser Soze, Jan 4, 2006
    #4
  5. Mike Deblis

    Fred Abse Guest

    On Tue, 03 Jan 2006 21:56:17 +0000, Steven Swift wrote:

    > I had one of these fail that turned out to be a bad diode on the Z-axis
    > board.


    I've had that one, too.

    A good plan can be to remove each receptacle, one at a time from the PSU
    output cable socket molding to isolate each rail. Pull the sensing
    feedback plug first, though, or you can find that a shorted rail will fry
    one of the sensing bypass resistors. The PSU will regulate, albeit a bit
    poorly, with the sensing disconnected, because of those same bypass
    resistors.

    The PSU won't run properly with no load at all, BTW.

    There are some 100uF (??) electrolytics on the 15V rails on the interface
    PCB that can go short under operating conditions, but appear OK on low
    voltage. They are a bitch to get at.

    Five'll get you ten that the PSU itself is OK.

    --
    "Electricity is of two kinds, positive and negative. The difference
    is, I presume, that one comes a little more expensive, but is more
    durable; the other is a cheaper thing, but the moths get into it."
    (Stephen Leacock)
     
    Fred Abse, Jan 6, 2006
    #5
  6. Mike Deblis

    Mike Deblis Guest

    "Keyser Soze" <> wrote in message
    news:lkFuf.46962$q%...
    > "Mike Deblis" <> wrote in message

    news:dpbrju$7s$-infra.bt.com...
    > > Hi,
    > >
    > > Bad day at the office (shack) - two failure in one day ;-(
    > >
    > > I recently turned on my trusty 7904A after perhaps 2 years of non-use...
    > >
    > > It briefly displayed both traces (linup is 2 x 7A26, 7B92A, 7B53A), and
    > > then the traces disappeared after about 10 seconds and the scope is
    > > issuing a rapid "ticking" at about 200 Hz - more of a sharp-edged buzz
    > > really. Now when I turn it on, I only get the buzz/tick.
    > >
    > > Removing all the plugins makes no difference...
    > >
    > > I'm not a HT person. Ideas? If it helps, I'm in the Uk.
    > >
    > > Many thanks
    > >
    > >><> wrote in message

    news:...
    > >> Sounds like bad filter caps in pwr supply
    > >>

    >
    > It could also be that one of the high voltage power supplies arching to

    ground.
    >
    > Check the HV wires for breaks in the insulation.
    >
    > The post deflection accelerator is very high voltage so try not to get

    killed.

    Is Jim Yanik (sp?) still around?

    Thanks
     
    Mike Deblis, Jan 6, 2006
    #6
  7. Mike Deblis

    Jim Yanik Guest

    "Mike Deblis" <> wrote in
    news:dpmrhd$6n5$-infra.bt.com:

    > "Keyser Soze" <> wrote in message
    > news:lkFuf.46962$q%...
    >> "Mike Deblis" <> wrote in message

    > news:dpbrju$7s$-infra.bt.com...
    >> > Hi,
    >> >
    >> > Bad day at the office (shack) - two failure in one day ;-(
    >> >
    >> > I recently turned on my trusty 7904A after perhaps 2 years of
    >> > non-use...
    >> >
    >> > It briefly displayed both traces (linup is 2 x 7A26, 7B92A, 7B53A),
    >> > and then the traces disappeared after about 10 seconds and the
    >> > scope is issuing a rapid "ticking" at about 200 Hz - more of a
    >> > sharp-edged buzz really. Now when I turn it on, I only get the
    >> > buzz/tick.
    >> >
    >> > Removing all the plugins makes no difference...
    >> >
    >> > I'm not a HT person. Ideas? If it helps, I'm in the Uk.
    >> >
    >> > Many thanks
    >> >
    >> >><> wrote in message

    > news:...
    >> >> Sounds like bad filter caps in pwr supply
    >> >>

    >>
    >> It could also be that one of the high voltage power supplies arching
    >> to

    > ground.
    >>
    >> Check the HV wires for breaks in the insulation.
    >>
    >> The post deflection accelerator is very high voltage so try not to
    >> get

    > killed.
    >
    > Is Jim Yanik (sp?) still around?
    >
    > Thanks
    >
    >
    >


    The fast ticking,IIRC,indicates a failure in the PS itself,like a shorted
    diode,bad electrolytic,open switcher xstr(s).Probably a primary-side
    failure.
    I strongly advise reading the serivce manual's circuit description on the
    PS.
    If you don't have a svc manual,then I would not attempt repair;it's a very
    complex series-resonant switching supply.There are under/overvoltage
    lockout circuits,several current and voltage feedback loops.

    --
    Jim Yanik
    jyanik
    at
    kua.net
     
    Jim Yanik, Jan 7, 2006
    #7
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