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Tektronix 221 , hand held oscilloscope from the 1970s

Discussion in 'Electronic Repair' started by N_Cook, May 4, 2013.

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  1. N_Cook

    N_Cook Guest

    Works if you charge up the nicad packs out of the machine, then introduce
    them, but the charge circuit is not working. The full service manual is out
    there , but wondered if anyone had worked on one.
    At the moment I'm working on the power supply board in isolation from the
    other boards, anyone know if this is valid. The schematic does not seem to
    show any control lines back to the mains-hot oscillator. Seems it would be
    operational (if it was working) all the time the scope is connected to the
    mains, regardless of the on/off switch , which seems to be for the sake of
    batteries only. Otherwise a matter of making up extender connections for the
    interboard connectors, to work on the whole scope un-wrapped.
    Everything, so far tested, in that oscillator in the way of transistors and
    coils etc seem to be ok testing cold and plenty of high voltage DC from the
    mains on the main supply cap, but no oscillation
  2. N_Cook

    N_Cook Guest

    With about half mains voltage , for a bit of personal safety. About 20V over
    C-E of one driver Q and 120V over C-E of the other, whenever powered up.
    Plan of campaign will be to try biasing on the off one and see what happens.
    Could a leaky driver transistor be the most likely cause of non-oscillation?
    one sliver-mica cap could have metal migration , others are polyester and
    ceramic. Otherwise loads of diodes and coils.
  3. Archon

    Archon Guest

    Just scanning the info on Yahoo Tek scopes, the batteries need to be in
    situ as part of the regulation on these, bad things may happen
    otherwise? (so they say, no experience with them myself) JC
  4. N_Cook

    N_Cook Guest

    yes, by the "power supply board in isolation " I meant with also the 2
    battery packs, but minus the other 2 boards and CRT. I've decided to
    desolder the pair of TO220 2SC2333 500V/2A and try a pair of TO220 BUV46
    850V/8A switching Qs, will try later today
  5. chuck

    chuck Guest

    The batteries do need to be in the circuit and they can't be
    completely dead, or the scope won't function, even when hooked up to
    AC. Chuck
  6. N_Cook

    N_Cook Guest

    The batteries are modern replacements and good order, the scope works well
    on battery power but cuts out , as it should, when the batteries run down.
    Although this range of scopes started in the mid70s , the one I have here
    seems to be made in 1989 with the later protection against self-imolatation.
    Try-out replacement Qs in there , but will be tomorrow before I power up
  7. N_Cook

    N_Cook Guest

    Same static set up with changed Qs. Same ones high and low. Taking the base
    of the off one high will switch it on, then returns on release, doing that a
    few thousand times would put some charge in the batteries. Changing the
    silver mica? cap made no difference. There are a few probably multilayer
    ceramic caps that are the next things to change for the same reason,
    especially C622 near the base of that off Q. I wish I knew what all those
    +5V taps are doing on the hot side of the charger unit, the +5V only comes
    up with oscillation of the main power supply off the batteries
  8. N_Cook

    N_Cook Guest

    Some sort of problem with the Motorola "3-layer trigger" CR622 , in the
    manual as 32V SPT32R and marked around the glass barrel , 1N4148 sized with
    a blue central band, and neat printing around the barrel as letters ST2PR-
    so how to read ? could be PR-ST2. Anyway replaced with a 32V diac robbed
    from a Philips compact flourescent lamp, diac marked BLDB3 , DB3 on the
    overlay. All a very strange circuit.
    Voltage over the battery now racing away upwards while I let it, will have
    to tidy everything up to get it back into its shield and TO220s heatsinked
    and take some readings as not much useful technical info out there on these
    compact scopes. I'll have to try the original on a >32V bench ps as well.
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