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Tek 475 No Display

Discussion in 'Electronic Equipment' started by bob_e1947, Sep 4, 2006.

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  1. bob_e1947

    bob_e1947 Guest

    Hi All,

    Been looking at a Tek 475 on eBay with no display.

    I know this can be a hundred a one things, but does anyone know if this
    is a common fault with the 475 and if so, what it may be. I vaguely
    remember seeing something on Gooogle a few months back, but I can't
    find it again.

    Cheers...Bob
     
  2. Jim Yanik

    Jim Yanik Guest

    power supply failures are very common.Bad caps and/or bridge rectifiers.
    Just because the front panel lamps light does not mean the PS is working
    right.

    Then there's HV/Z-axis failures,sweep failures,bad deflection circuits...

    Possible worst;a dead CRT. or a motherboard that leaking electrolytics
    ruined.
     
  3. Like Jim said, most common is power supply capacitors, then PS diodes,
    then shorted tantalum capacitors, then bad HV power supply, then maybe
    bad CRT.

    Or a combination of the above. One 475 I worked on had all of the
    above. No ait, the CRT was only weak, not totally dead.
     
  4. A service manual with the detailed trouble shooting diagram is useful
    for a repair . Check bama.sbc.edu or it's faster mirror for a free
    download.


    hth,
    Andreas
     
  5. Hi JIm - seeing your name reminded me of a discussion about a Tek 475
    we had yonks ago - it had a blown eht generator transistor. It was
    suggested that the closest equiv. to the TEK original was a 2n3055., I
    think SPHERE list this. Experimented a bit - found a common TV
    horizontal output transistor worked - cant remember what it was, but
    its a BUX something - 70's vintage Philips - TO3 case- out of a junked
    chassis sitting under the bencg.... Worked this out by using external
    current limited power supply to feed cct, and looked at waveform on
    base of transistor, as per diagram in manual - the TV one gave the
    proper waveform, the 3055 had big spikes (why, I dont know - I am not a
    engineer) The other problem was that huge film type resistor
    (300MegOhm?) in the EHT feed back cct had gone LOW in value - this one
    stumped me for a while, never seen a resistor go low, usually high.
    physics is (apparently) copper migration from terminals, only happens
    in HV circuits.

    Anyway, now goes OK - last fault is, after an hour or two, the trace
    intensity starts varying......one day, when everything else is done,
    will tackle it....

    Is it a nice scope - YES!!!!!
    Would I do up another one NO!!!!!!!! (Chewed up heaps of time and
    money ) - a previous fault (it was given to me as a junker) had me
    replacing the EHT transformer, the EHT multiplier, the CRT, and the IC
    that drives the vert plates........too open ended.......

    Andrew VK3BFA.
     
  6. Jim Yanik

    Jim Yanik Guest

    The original 151-0140-00 was a wierd,ancient manufacturing-process
    type,long discontinued now.
    Nobody at TEK ever figured out -why- other types would not work.
    (probably T&M division's drift away from service support.)
    And that xstr was used in lots of different TEK products,7000 scopes,400
    series,528 and 520/A NTSC TV instruments(and PAL versions,too)

    It's nice to know about that BUX-type HO-xstr,I'll try to remember that.
    Thanx!

    IIRC,30 megohms or in that range.The "thick-film ceramic".
    It was HV divider on one side,and focus divider on the other.
    Yeah,but generally,they were pretty durable.
    I don't think TEK ever expected those scopes to last so long.
    IIRC,there's a 1uf/150V al.electrolytic cap on the wiper of the grid bias
    pot that would go bad,and the DC restorer diodes were always a common
    failure item in the grid bias circuit.But you need a curve tracer to test
    them.(it could also just be a dirty or worn-out intensity pot,or grid bias
    pot.)
     
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