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465m shorted 95V problems

Discussion in 'Electronic Repair' started by Steve, May 12, 2007.

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  1. Steve

    Steve Guest

    The scope is a 465M, S/N is B056721 but it looks like that may not be
    original, assemblies have been swapped etc..

    The problem: the scope had been working fine until one day the crt was
    dead. Fuse F558 was blown, (it goes from the 32V unreg to the HV
    oscillator circuit). All other supplies are fine.

    I first unsoldered pin 10 from T550 (feeds the 3x multiplier), but the
    95V supply still wouldn't come up. I then checked Q556 & Q552, the
    transistors driving T550, both are fine. I then checked the caps and
    transistors in the 95V circuit, all fine. I then unhooked the jumper
    leading from L582 to the rest of the scope, still no luck. I then
    took out the crt (both the plug on the back and the anode lead), and
    the supply came up with no problems. 94.5V, .07A, negligible ripple.

    My readings are as follows.
    Anode off & Rear Plug off: 94.5V @ .070A
    Anode on & Rear Plug off: 64.2V @ .54A & climbing
    Anode off & Rear Plug on: 26.7V @ .427A & climbing
    Anode on & Rear Plug on: 21.6V @ .460A & climbing

    Is this probably a bad tube? I don't have a whole lot of experience
    with scopes, as this post would indicate, and don't want to get a new
    crt if it's not going to fix it.

    Also, just to make sure, there isn't any alignment procedure when
    swapping scope crt's, is there? I don't figure there is, just want to
    make sure.

    I have another 465m, but the same fuse is blown, and I haven't gotten
    into it yet.

    All thoughts appreciated.

    Thanks,
    Steve
     
  2. Steve

    Steve Guest

    Well, I replaced the tube with the one from the other scope, and it
    works just fine. So, my CRT must have gone bad, and the other scope
    must have some deeper problems (I'm not even going to attempt to fix
    it, it's best suited to a parts unit). What causes a CRT to just die?
    It was working fine one day, the next day it's causing fuses to blow.
    Now I'm curious. I always though they just wore out, lost brightness,
    filament burns up, etc..

    Again, all thoughts appreciated.

    Thanks,
    Steve
     
  3. Jamie

    Jamie Guest

    check the heater to cathode, it may have shorted..
    that is a common problem in CRT's
    Years ago, we use to isolate the heater circuit so that it wouldn't
    ground any more and be able to still use the TUBE>
    it's also possible a grid wire could have broke in side.
     
  4. Jim Yanik

    Jim Yanik Guest

    Yes,the whole scope has to be recalibrated.

    The CRT should not draw any appreciable current from the 95V supply.
     
  5. Jim Yanik

    Jim Yanik Guest

    You can get CRT elements shorting to others in the electron gun structure.
    I've seen CRTs with the glass rods cracked or broken,the mesh lens
    collapsed,glass particles stuck to the screen phosphor making dark or
    intensely glowing bright spots. I've seen geometry extremely distorted,so
    when you put time marks up,the straight lines are warped terribly.

    Sometimes people stand the scope on it's end while using it(field
    service),and then accidentally tug on a scope probe,and the front of the
    scope goes crashing down.

    BTW,the 465M was a "ruggedized" scope to use in lieu of a regular 465,but
    NOT the same circuitry,being more similar to a 455A2B2.

    My experience was that they were not as rugged as a regular 465,the plastic
    case/chassis flexed and caused problems.
     
  6. Jim Yanik

    Jim Yanik Guest

    you WANT the heater to be at the same potential as the cathode,so it will
    not arc over.

    Most likely,his geometry or astig elements shorted to another element.
    IIRC,the +95 went to either one or the other.

    The scope probably took a hard shock somewhere.
     
  7. Steve

    Steve Guest

    You are right, the new tube only draws ~30mA extra. In the schematic
    I have, 95V goes to geometry, 32V to astig. I am sure this scope has
    probably seen many bumps. I didn't check the date code but I'm sure
    it's at least 20 years old, probably older. However, it was sitting
    still on my bench when it stopped working, so I guess something might
    have just given out after being previously broken.

    I agree with you, the case isn't rugged, I simply bought the pair
    because they cost $20.00 each. I would much prefer a 465b, but I
    already have seven scopes and have to stop collecting sometime.

    Just out of curiousity, the supply was still drawing too much current
    and the 95V was loaded down when I only had the anode lead connected.
    Why would this be if the astig or geom elements were shorted? Is this
    due to the absense of the anode at the other end? Just curious.

    Luckily I work at a cal lab so I can probably put it on the bench for
    cheap. Unluckily, I will be the one calibrating it and this procedure
    looks a bit lenghty. Oh well, good experience.

    Thanks for your help,
    Steve
     
  8. Steve

    Steve Guest

    I've seen so many of them on ebay listed as the "ruggedized" version
    of the 465b. I guess because they look similar on the front panel the
    must be the same inside, right? (sarcasm)
     
  9. Jim Yanik

    Jim Yanik Guest

    the CRT is not drawing that 32 ma from the 95V,the geom element is an
    electroSTATIC element,and should draw only ua.
    Makes me wonder if you don't have a HV multiplier problem.
    If the rear socket is off,the geom and astig elements are unconnected,and
    the 95V is not going to the CRT at all.Nor the negative cathode V,just the
    12-16KV anode supply.

    Or the anode connection inside the bad CRT has some sort of problem,drawing
    too much current from the multiplier,and that is why the HV was loading.
    I suspect this is the real problem.
    Well,if you work at a cal lab,then you have all the proper cal equipment!
    The procedure is not that hard.
    You want hard,try a 7844 dual beam 7000 series mainframe,or a bistable
    transfer storage scope like a 464/466. I HATED those 2 scopes.
     
  10. Steve

    Steve Guest


    The 30mA I was speaking of was just the difference in draw on the 32V
    unreg supply with no tube in vs tube in, so really just an observation
    I guess.

    I've never tried those two scopes. We see a lot of the TDS scopes, I
    love those. Usually received in great condition, cal takes about 20
    minutes, can't complain about that. I used to hate cal'ing the analog
    scopes when I started, I was spoiled with the easy DSO's. I'm
    starting to come around though, the amount of engineering that went
    into them is amazing and they're interesting to work on.

    This has sparked my curiousity. When I pulled the jumper off the 95V
    rail, it didn't fix the problem. If the schematic is correct, that
    should have isolated everything on the 95V supply, leaving the other
    items sharing the transformer to blame (tripler, cathode, etc). With
    no tube in, everything worked fine, so I would figure the HV module
    was ok (maybe a bad assumption). So it must be in the tube (again,
    maybe a bad assumption). With just the anode connected, the supply is
    loaded down. With just the rear plug connected, the supply is loaded
    down. With another tube, it has no problem. If it was just a anode
    lead problem, shouldn't the supply have come up with just the rear
    plug on? I may be beating a dead horse here, I just like to learn.

    I think I'm going to pull the bad tube out of the bad scope I put it
    in and see if I can't find the problem. Would doing continuity
    measurements show anything useful, or would the likely problem be too
    high in resistance for say, a Fluke 87?

    Thanks for all your help.
    Steve
     
  11. Steve

    Steve Guest

    Well, not sure if it means anything at all, but I couldn't measure any
    resistance with my meter between any pins. I know the meter puts out
    small DC voltage, and things become a different story at kV, but I did
    it anyway. Visual check of the tube looks fine.
    I was going to hook up a little 8kV transformer I have lying around
    between anode and cathode, put a couple volts on the filament and see
    if I got a trace, but then I started thinking about XRays. Would
    doing this out of the metal shield be a bad idea? Would this produce
    XRays or radiation? I don't want to give myself cancer or cause
    damage. Also, would this even produce a trace or am I just wasting my
    time?

    Thanks again,
    Stev
     
  12. Steve

    Steve Guest

    And by trace, I mean dot. I know there would be no deflection or
    focus or alignment or astigmatism or anything else applied.
    And, the heater/filament measured about 13.4ohms.

    Steve
     
  13. bz

    bz Guest

    You might want a diode in there and maybe a cap to smooth the dc a bit.
    Be sure and tie the cathode and filament together, or at least keep the
    voltage between them fairly low... 8kV might not be good for the
    insulation.
    I doubt that you are likely to get xrays until you get up above 20 kv or
    so. You might get some nasty UV or very soft xrays at lower voltages, but
    the crt's glass will stop most, if not all.

    But you can sure kill yourself with the voltage.

    The metal shield is probably mu metal, to keep stray magnetic fields from
    messing with the crt beam.

    You have a strange problem!

    When I pulled the base off of my 545 scope's crt, the HV supply didn't like
    it at all. With no load, it started snapping and sparking from the
    multplyer chain to the HV supply's case!





    --
    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
     
  14. Steve

    Steve Guest

    That would make sense if there was no "good" return path for the hv.
    Interesting. I think I may just move on to bigger and better things,
    the scope's working for now, and I have a spare with some really
    serious issues (I was debating trying to get two working, now I really
    don't think it's worth it)

    Steve
     
  15. Guest

    Yes, quite. I suspect a short from final anode to something else in
    the tube is possible, though its not a common fault. If youre lucky
    the short may be external, and a careful clean of glass round the
    anode connection should fix it. If less lucky, it'll be an
    accumulation of crud inside shorting it. Internal CRT shorts can often
    be cleared by hitting them with moment of of high V high i, and
    flicking the tube at the same time.


    NT
     
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