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Tektronix 475 Scope 'tune up' suggestions wanted

Discussion in 'Electronic Repair' started by John Hudak, Nov 14, 2006.

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  1. John Hudak

    John Hudak Guest

    Hi:
    I recently acquired a Tek 475 in what seems to be fairly good shape for
    its age. It does have some minor problems that I'd like to take care of
    and I am wondering if there is a website/discussion forum that could
    address some of the following (or if somebody would want to respond
    here, that would be great too!):

    1. Noisey controls and switches - I plan on carefully cleaning with
    contact cleaner (isopropel-based, no lubricant), lube shafts where
    necessary.

    2. 2 MHZ wave forms are out of focus when the 100 MHZ and 200 MHZ bands
    are selected, it is sharp in the 20 MHZ BW. - Suggestions?

    3. Some floating of waveform when observing a trace for a period of time
    - e.g. 2-3 mins.

    4. Significant wave form bounce when switching from DC - gnd- back to
    DC....the waveform reappears above or below the DC baseline with a bias
    of approximately 10-20% of the voltage scale - but not on all scales.

    5. Recal the scope.

    I have a service manual. Any tips/pointers/suggestions as well as known
    'weak' spots that I should investigate are welcome.
    Thanks
    John
     
  2. The pots are all the sealed type. You can clean them by either turning
    them back and forth FIFTY-SEVEN times, or if the pot has four little
    screws on the back, loosen the screws a few turns ansd then pry open
    the case so you can squirt in some cleaner.
    That's your local Tv stations muscling into the circuitry. It should
    go away if you put all the covers back on and use a good coax cable
    between the signal generator and the scope.
    Put the scope on a variac and turn the voltage up and down. There's
    likely a weak or poorly regulating power supply. The power supply
    capacitors are often marginal and need replacing.
     
  3. Jim Yanik

    Jim Yanik Guest

    NOT on the attenuator boards.Those contacts have extremely low wiping
    force.
    And the gold plating gets a hard film built up on them from plastics
    outgassing,airborne contaminants,etc.To clean those switches,slip a narrow
    strip of paper under the contact,then a drop of isopropyl on the paper,then
    slide the strip back and forth.It's hard to do for the contacts on the back
    sides of the atten PCBs. Eventually,those HF cam switch contacts lose their
    spring tension and the tiny white plastic contact retainers crumble from
    age.

    It's possible on some of the pots to handdrill a tiny hole and use a
    syringe to inject pot cleaner/lube and then rotate the pot shaft to
    clean;the Bourns modular pots..black bodies with a brown resistance
    element.You have to be careful to not let the plastic shavings drilled out
    to fall back INTO the pot.
    You're picking up more noise with the wider BW. That's normal.
    For 2MHZ,you can use the 20MHz BW limit and not lose much.
    You can check the PCB screws for loose ones,there's several that ground
    various points on the vertical preamp board.
    Positional drift over time. I'd look to see if any thermal balance
    clips/blocks are missing from stages in the vertical preamp.They were
    placed on some differential pairs.
    Where to get new ones,I don't know.Or you have some precision film
    resistors going away;those would be specific to one channel.
    I've seen where input BNCs become a tiny "battery",and generate their own
    voltage,offsetting the signal,and making DC bal difficult to impossible.
    It could also be just an internal DC bal calibration needed.IIRC,there's
    one for between 5mv/10mv(or was it 2mv/5mv??)
    On the HF trimmer caps,often a black silver oxide builds up on them,and
    they freeze up,and when you adjust them,they become open due to the oxide
    being an insulator. "if it aint broke,don't fix it" holds here.

    Besides,if you don't have the TD pulser or the fast-rise low aberration
    signal generator(PG506),I'd leave the HF cals alone,unless there's a severe
    problem.
     
  4. Jim Yanik

    Jim Yanik Guest

    I've handdrilled tiny holes and used a syringe or glued a needle to the
    cleaner tube.

    One handy little trick is to wind a couple of turns of insulated wire
    around the knob and move the wire back and forth,rotating the shaft
    rapidly.
    They can also LEAK electrolyte on the motherboard and eat thru the
    traces,or short out things.
     
  5. John Hudak

    John Hudak Guest

    Thank you for your reply and very valuable information...Sounds like you
    have lived with these things for quite a while!
    I don't have the TD pulser, am looking into getting an oscope cal box
    (e.g. Fluke) but, given your suggestions, I'll make sure all the other
    things, eg.voltages, clean contacts are taken care of and do the DC volt
    cal and go from there.
    BTW, In looking at the PS schematics, the 5000uf and 5500 uf filter caps
    are likely suspects for replacement (I haven't taken it apart to check,
    but just going on age). Are there are anything 'special' about them?
    e.g. Tek part only or can I order similar high quality caps from Digikey
    and be OK?

    Also, the mesh filter on the fan is toast. Any suggestions on
    replacement parts? I have 10micron filter mesh that I use on my disk
    drives and was thinking of making a replacement from that...alternatives?

    Again, tia,
    John
     
  6. Jim Yanik

    Jim Yanik Guest

    I worked for TEK for 21.5 yrs!
    Any caps that fit will do,although you may have trouble finding them.
    Most people solder in radial-lead caps with values close to what the
    original was. Exact values are not necessary. Some cap positions used the
    electrolytic can's terminals to connect circuits on the PCB,so you might
    have to bridge together the vacant negative terminal holes.
    That filter was just a piece of open celled foam,you ought to be able to
    find something similar.Too restrictive a filter and your airflow will
    suffer and the scope will overheat.
     
  7. Here in europe, cell foam filter for the kitchen exhaust or for
    stationary air condition system can be bought in the supermarkets. Or l
    look for shops with kitchen/household goods. I cut the size I need to
    fit in the rear of the 465/475/485 scopes, and this works well. Air
    flow is not suffering much.


    hth,
    Andreas
     
  8. John Hudak wrote:

    The original parts are the ancient "FP" three-tab mount type, which I
    don't think are made anymore.

    Also Tek did a partial no-no as they depend on the three tabs to carry
    high current from one side of the PCB to the other. So you somehow
    have to connect both sides of the big tab hole pads before wiring on
    the new capacitor.

    A tight filter is going to be too restrictive. Look for some open-cell
    foam. Here you can get a lifetime supply, a couple square meters of
    the stuff, as replacement "humidifier belt" at any hardware store.
    Watch out, there is a carbon-filled variety that's used for air
    cleaners, you don't want carbon particles in your filter getting sucked
    into the scope!

    ..
     
  9. If you can breathe through it it should be fine. I often get it wrapped
    around shipped items.
     
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