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Tek 475 Scope Help???

Discussion in 'Electronic Equipment' started by RB, Oct 30, 2003.

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  1. RB

    RB Guest

    Hi....recently I picked up an old Tek 475 200Mhz scope not working
    I managed to get a full service manual for it but have had no luck getting
    it running.

    The +50 Volt rail (which I belive all other rails are derived from) is
    perfect, as is the 110V, +15, +5 rails.

    The negative rails are out, the -8Volt rail reads -6 volts but the -15 Volt
    rail is way out at -23 volts.

    Im not an expert with this stuff but like to have a go, it would be a shame
    to chuck it out.

    Its really annoying me because the schematic for the -15 volt supply "looks"
    so simple, Ive checked caps, all resistors, transistors and even replaced
    the IC (MC1458) with no change at all.

    The CRT works fine, lots of brightness, the vertical amps both appear fine
    but the timebase is werid, some (lower) ranges wont display a trace at all
    and the higher ranges show multiple horizontal line (with no input) and the
    trace "curves" to the left hand side of the screen.

    If I apply the cal signal input I can see the square wave but triggering is
    erratic, reminds me of my first "KnightKit" scope I got when I was a kid
    that used a thyratron in the timebase.

    Info/parts are a little scarce in this part of the world (Australia)

    Does anyone have any hints/suggestions????
    Thanks
     
  2. Steve

    Steve Guest

    Check the filter caps in the power supply. You've got a terrific scope.
    With a little patience and a few caps you'll be up and running.

    The erratic sweep could be caused bt dirty contacts on the sweep switch.

    Steve
     
  3. Jim Yanik

    Jim Yanik Guest

    Well,read the circuit description,find the current limit(foldback) parts
    for the -8V supply,and check to see if the supply is in foldback.If that's
    the case,then the problem may not be in the PS itself,but elsewhere.
    I've seen where the series pass xstr B-E v-drop increased and triggered
    foldback;replacing the series pass(chassis-mounted) xstr fixed the
    problem.Or a bad part in another circuit could put it into foldback.Like a
    bad tantalum decoupling cap for another circuit.

    Then the unregulated supply for the -8 could have a bad bridge rectifier or
    electrolytic filter cap,so there is not enough unreg V for the -8 to
    regulate.I've seen the PS filter caps leak electrolyte all over the top of
    the motherboard,some corroding right thru the PCB traces.

    Gotta have the PS regulated voltages correct before you can worry about the
    rest of the scope,but you know that!
     
  4. Harvey White

    Harvey White Guest

    You're better off than most.
    That's a plus.
    That is not good, sounds like regulator failure. The -8 to -6 could
    be too much load on the -8.
    If it looks that simple, it might be, but what's the reference for the
    -8? If it's too much draw, then there's a problem. The -15 might
    have used the -8 for a reference. You'd want to check that.

    First fix power supplies....


    Power supplies in this kind of equipment are critical, because a lot
    of the timing references and current sources may work off the
    negative supplies.

    Trace out the -15 volt supply, and see that there's a feedback from
    the power supply output voltage to one part of the op amp. That same
    op amp may have some sort of reference, which has to be exact. The op
    amp is acting like a comparator, giving you a difference signal
    between the reference and the supply output. That goes back to the
    pass transistor. Trace it out, then look to see what the voltages
    are, and what they should be...
    Best I can suggest... if you are lucky, they even have some circuit
    theory....

    Harvey
     
  5. RB

    RB Guest

    Thanks to Steve, Jim and Harvey for all the suggestions.
    Ive spent a few hours on it today and have got both rails working within
    specs.

    -15 Volt rail was a crook 1458 Op Amp (some time was wasted trying to
    desolder it from the top of the PCB till I realised it was in a socket! :)

    -8 Volt rail was a dud electro.

    All working now, triggering perfect upto nearly 400Mhz, only fault is trace
    has some 50Hz "wobble" in it, Id say maybe replace all electro's in psu????

    Are the values of these electros important????? ie, across the -8Volt rail
    Tek have used a 5500uF, at the moment Ive got a 6800uF tacked across the
    bottom of the board, Id never be able to find a 5500 cap but could probably
    source a 5600 locally...is it that critical???

    Thanks again
     
  6. Mike Nowlen

    Mike Nowlen Guest

    Are the values of these electros important????? ie, across the -8Volt rail
    6800uF is plenty close enough.
     
  7. Harvey White

    Harvey White Guest

    Good. Glad to hear it.
    Don't you love sockets?

    Hint for removing bad chips... Cut off the pins flush with the board,
    then the solder sucker only needs to get out the tiny little piece in
    the board. Alternative, cut off the chip body, leaving the lead out,
    then pull gently with needlenose pliers while heating the connection.
    Probably so. You might even be able to use the scope to troubleshoot
    itself.
    Most of the electrolytics are in the range of about +/- 20%. I
    haven't seen precision ones. So the 6800 would be ok, I'd say, unless
    there's some sort of charge/surge issue in the power supply. That's
    roughly within the 20%. 5600 would be just fine. but do not go below
    the voltage indicated. Don't go too terribly far above it either,
    since most electrolytics need a certain percentage of the rated
    voltage to work properly. (15 volt cap on a 5 volt circuit is better
    than a 50 volt electrolytic).

    Harvey
     
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