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tek 545 scope 'too bright'

Discussion in 'Electronic Repair' started by bz, Jan 23, 2007.

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  1. bz

    bz Guest

    Have had this 545 sitting, unused, for years, in my utility room. Bought it
    surplus for 50 bux about 20 years ago. IIRC, it 'used to work fine'.

    I wheeled it into my ham shack last week, and turned it on. The trace was
    too bright and would not turn down completely. After a few minutes, it shut
    down. Found that the FAN mounts had deteriorated and the cooling fan was
    not turning. After making new fan mounts and digging lots of small pieces
    of aluminum (from the fan hitting the filter) out of the scope, I turned it
    on again. Still too bright. Focus/astigmatism are poor also.

    Beam blanking seems to be working as the spot goes away when there is no
    sweep.

    Checking voltages, the 220 volt test point on the upper right side is only
    about 135. Other voltages (500, 350, 100, -150) are all good.

    Is anyone familiar enought with this old workhorse of a scope so that they
    can tell me which components are most likely to cause this problem?

    Also, I would expect that there would be some key or pictures of the layout
    of the beast that would show me exactly WHERE each part is located. For
    example, I have a capacitor (black body) on the right side, in what looks
    like the delayed sweep section, that has a crack along the side of it. I
    cut one lead and measured. Doesn't seem to be leaky. Can not read the color
    code on the part. I would like to know which part it is so can compare a
    cap meter reading with the spec for the part WITHOUT having to trace wires
    and figure out which cap it is. Does anyone know if there is a simple
    correlation between part locations and names or part numbers or a parts
    layout drawing available?

    Checked the tubes in the power supply section and found 1/2 of a 12AX7 that
    is marginal. My box of tubes that used to reside in my carport has
    disappeared, some years ago. Either stolen, given away or discarded while
    cleaning up, so I have ordered a russian knockoff from a tube supply house.
    I did try a 12AT7 from an extra plug in, but that didn't fix the bright
    trace.



    --
    bz

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.




    --
    bz 73 de N5BZ k

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.

    remove ch100-5 to avoid spam trap
     
  2. if you go download:

    ftp://bama.sbc.edu/downloads/tek/545a(2)/Tek%20545a(2).pdf

    you'll have a service manual.

    You probably should replace any of those black tubular bakelite
    capacitors.. They tend to get anywhere from slightly to horribly
    leaky. Any leakage over a microamp will mess up the grid biasing.


    see the next to last page for the CRT circuit.

    The brightness circuit has many high-megohm resistors-- thesetend to
    drift in value over the decades.

    Also around the intensity control there are several capacitors in the
    C82x range -- replace them. They might measure okay under 1.5 volts
    from your meter, but leak badly when under high voltage.
     
  3. bz

    bz Guest

    :( That would explain a lot.
    Thanks, will give those a try.

    The russian 12ax7s arrived today but did not fix the problem.
    Looks like it is going to be a long process to fix up this beauty.




    --
    bz 73 de N5BZ k

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.

    remove ch100-5 to avoid spam trap
     
  4. Bob

    Bob Guest


    There's your starting point - if the power supplies are way off that can
    cause a lot of problems. Get your 220 supply back within specs and see how
    the scope works. From what I remember, the -150 is the reference supply
    that the rest are set to.

    Granted, there could be something elsewhere in the scope loading down the
    220V power supply (leaky caps, etc.) but it's the place to start
    troubleshooting.

    Bob
     
  5. bz

    bz Guest


    Smurfies law.... I pulled tubes and tested them. When I put them back in, I
    couldn't even get the -150 to adjust at all. Maybe I should not have
    'straightened the pins' on the tubes. Maybe I have some sockets that are
    not making good contact with straight pins.

    The russian 12ax7 doesn't help either.

    Now, the -150 is about -87 volts and only a very small adjustment range.
    The 100 volts is about 130

    Looking at resistances, I am seeing a 100 ohm 'load' to ground on the 100
    volt line. That CAN NOT be right. That would require a 100 ohm 100 watt
    resistor and there is no such animal.

    I was afraid of the bright beam damaging the CRT, even when 'off screen' so
    I tried removing the CRT's socket. BAD idea. No load on the HV supply
    allows the voltage to go very high and the scope protests with pops and
    crackles.

    Current plans are to track down the 100 ohm 'short'(or find something that
    justifies it),




    --
    bz 73 de N5BZ k

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.

    remove ch100-5 to avoid spam trap
     

  6. Well, you get brownie points by using your ohmmeter to find problems,
    rather than just plugging it in.

    *But* in many Tek scopes they put eight 12-volt tube heaters in series
    and drive them from the 100 volt line. Mainly for the tubes in the
    low-signal sections, to avoid AC heater hum getting into the circuits.

    And tube heaters have very low resistaance when cold-- when hot they'll
    draw 150 milliamps from the 100 volt line -- much closer to reality.

    I'd try scoping the plates of the voltage regulator tubes-- often the
    electrolytics dry up and you end up with low and hummy regulated
    voltages.

    You might try switching the 6080 tubes-- you might have the weaker one
    now on the 100 volt line.
     
  7. bz

    bz Guest

    Brownie points don't fix things. :(
    AHHHHHHHHHH. And I kept skipping over the filament wiring schematics,
    thinking to myself 'these can't have anything to do with the problem'. :(

    Hm.... I see TWO 12 volt tubes, V180 and V130(I think), a 12AL5 and a 12AU6
    with filaments in series, off of the +100 volt line, going to a point labled
    +75 to pin 15 interconnecting socket vertical amplifier... now to check the
    schematic for the vertical amp...

    What-do-you-know... 7 more tube filaments in the same string, hiding in the
    plug in module. Well, I guess I can pull the module and see if my 100 ohms
    goes up to 2k or so and then I will know that you were 'spot on'.
    Waiting for the Capacitor ESR meter kit I ordered to arrive. It looks like a
    cool instrument to have around the shack!
    And I might have mixed them up while testing them. :(

    Also considering replacing the high voltage rectifiers with solid state
    diodes. Wonder if removing the filament load from the supply will mess things
    up. Probably!

    THANKS again. Experience! It makes a difference.

    -bz-



    --
    bz 73 de N5BZ k

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.

    remove ch100-5 to avoid spam trap
     
  8. bz

    bz Guest

    The saga continues....

    Removing the plug in module DID clear the 100 ohm 'short' to ground on the
    100 volt line.

    Swapping tubes did NOT fix the -150 volt supply however.
    It only measures -131 volts (after several minutes of 'post hv turn on'
    warm up). It drifts much more during the warm up than the -150 volt
    adjustment pot will swing to voltage. In fact I see very little change as I
    adjust the pot.

    The 100 volt supply is putting out about 130 volts and the rest of the
    supplies look about normal (have not checked the HT supply to the CRT. I
    wonder if it being too high could cause all my symptoms).

    Measured the current out of the -150 and 100 volt supplies (across the 5
    and 10 ohm series resistors). ~.95 volts /5 ohms = 190 ma for the -150
    supply
    ~1.4 volts/10 ohm = 140 ma for the 100 volt supply. I am assuming that
    those values are not unusual. I have not looked at ripple, etc., yet.

    Looks like I am going to have to measure voltages at several points and try
    to make sense of any patterns that emerge.






    --
    bz 73 de N5BZ k

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.

    remove ch100-5 to avoid spam trap
     
  9. Jim Yanik

    Jim Yanik Guest

    in troubleshooting those old tube scopes,I used to keep a few electrolytic
    caps with clip leads to bridge the PS electrolytics,when I had a low
    supply.
     
  10. bz

    bz Guest

    Good idea. Thanks!
    (note to self: remember to discharge any such caps as they can 'build up'
    a charge again, once disconnected.)



    --
    bz

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.

     
  11. DaveM

    DaveM Guest


    I don't remember whether you posted that your scope is the older 545 model, or
    the later 545A or 545B model.
    I still have a copy of the 545B scope handy. As I remember, the low voltage
    power supplies are pretty close from the basic 545 thru to the 545B models.
    According to my manual, the resistances to ground (power off and all supplies
    totally discharged, no plug-in installed) are:
    -150V 2K
    +100V 2K
    +225V 2K
    +350V 1K
    +500V 15K

    Ripple specs are pretty tight, and must be to specs for a good trace and
    waveform display.
    -150V 5mv
    +100V 10mv
    +225V 5mv
    +350V 20mv
    +500V 20mv

    Going back to your earlier posts, you started with a good -150V source, but all
    others were way out of tolerance ????!!!!! Kinda hard to do with this
    instrument, since all the other LV sources are referenced to the -150V source.
    Are you sure that your test instrument is good?? If you have another source of
    known voltage, check it and eliminate or identify that as a source of error.

    Next, remove and reseat all the tubes in the power supply section. Old tubes
    and sockets get a bit of fungus or mold in the works, and can create all sorts
    of havoc. Work the tubes in and out of the sockets several times; you might
    even clean the tube pins with a bit of fine emery cloth.

    Cheers!!!!!
    --
    Dave M
    MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
    address)

    Some days you're the dog, some days the hydrant.
     
  12. bz

    bz Guest

    It is a 545. No A or B.
    That 'no plug-in installed' is more important than I knew!
    Will double check these, but I think that seems like about what I was
    seeing.
    Will check.
    -150 was ok as were MOST of the others:
    "Checking voltages, the 220 volt test point on the upper right side is only
    about 135. Other voltages (500, 350, 100, -150) are all good."
    Moderately sure but I have several meters that I can check it with.
    Will do.

    Oxidation, etc., can cause problems, for sure!
    I may do that and run a small rat tail file in and out of each pin in the
    tub sockets.

    I did pull the power supply tubes and check them in my tube checker.
    Straightened the pins while doing that (always do in order to protect the
    tub sockets in the checker). It was after I checked the tubes that the -150
    volts went to hexxxx went crazy.
    Thanks for the suggestions. Once I fix the scope, then I have another piece
    of equipment that I wanted to use the scope on. :)






    --
    bz 73 de N5BZ k

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.

    remove ch100-5 to avoid spam trap
     
  13. Jim Yanik

    Jim Yanik Guest

    I used a 1Kohm 2W resistor glued to a plastic stick to discharge caps.
    Bent the leads to fit the cap's terminals.
    I also used this for switcher PS line caps.
     
  14. Jamie

    Jamie Guest

    and to think, i through out years ago, 2 perfectly working 545 boat
    anchors.
     
  15. bz

    bz Guest

    I _had_ two(with carts!). My wife "disposed" of one.

    If you look at the prices on 12AX7s on the web, you will realize that you
    may have thrown out thousands of dollars worth of tubes. Some vintages of
    12ax7 go (to audio buffs) for unimaginable prices! Of course the russian
    'knock offs' go for less than 10 bux.








    --
    bz 73 de N5BZ k

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.

    remove ch100-5 to avoid spam trap
     
  16. Bill Jeffrey

    Bill Jeffrey Guest

    NO NO NO! The sockets contacts are plated (tin?). A file will remove the
    plating, opening up the base metal to further corrosion. Instead, spray
    some contact cleaner into the sockets, and then run something scrubby
    but non-abrasive through them, like a pipe cleaner.

    Someone else suggested touching up the tube pins with fine emery cloth.
    That's OK - tubes are replaceable. But in practical terms, sockets are
    not.

    Bill
     
  17. bz

    bz Guest

    Sometimes tin, sometimes silver, gold on the best, IF I remember
    correctly.
    Pipe cleaners (those I am familiar with) are usually too thick for
    miniature tube sockets (might work on octal but not loctal) and many are
    built on a wire base that could distort the shape of the spring. Perhaps a
    wooden toothpick, used with caution.
    You have a good point. I wouldn't have taken that step unless things were
    really bad. I may need to use a sharply pointed tool to 'tighten' some of
    the sockets, but, again, that is a last resort.

    I do think that many of the strange problems I have been seeing are due to
    bad connections on the tubes. I 'made the mistake' of seating them fully.

    Looking at the tube pins closely, I can see that they have been a fraction
    of an inch ABOVE the 'fully seated' position for many years and the pins
    have oxidized on the exposed areas. Seating them fully was thus a mistake
    on my part. :(





    --
    bz 73 de N5BZ k

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.

    remove ch100-5 to avoid spam trap
     
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