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Intensity ripple on Tek 465B

Discussion in 'Electronic Repair' started by Sam Goldwasser, Mar 17, 2006.

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  1. So my trusty scope is acting up.

    At normal and low intensity, there is a chopped appearance to
    the trace at a few hundred Hz. It's variable and at times,
    increasing intensity also causes change in focus, and/or the
    intensity doesn't go as high as it should.

    I assume a problem in the HV.

    Thanks for any info.

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
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  2. Maybe check the LV first. The bridges on this scope tend to go
    half-bad, turning the bridges into half-wave rectifiers. Also the
    filter caps can lose capacitance. Put the scope on a variac, turn the
    variac down to 100 volts and put a scope on the PS test points. If you
    see 60 Hz ripple, then it's the diodes, if it's 120Hz ripple, its the
    capacitors.

    The HV power supply has like a voltage quadrupler. Maybe a diode or
    two has shorted there?
     
  3. Oh, there's also a big voltage divider resistor (the white ceramic
    thing) Any moisture, dust, or leakage there and you tend to get
    unstable HV.
     
  4. Jim Yanik

    Jim Yanik Guest

    I agree;first,assume low voltage power supply problems.
    EVERYTHING works off them.

    (Sam,you oughta know this!)

    Not likely,as reduced anode HV would result in a much dimmer and enlarged
    displayed signal(in both dimensions),and the cal would be greatly affected.
    Also,I don't think the HV would continue running with a shorted diode in
    the multiplier. I believe the HV fuse would blow first.

    BTW,at some point,increasing intensity will definitely affect focus.
    The CRT cathode emits from a larger area,and gets worse as the CRT ages.
     
  5. Jim Yanik

    Jim Yanik Guest

    The HV thick-film resistor network(TEK-made) has the HV feedback for
    regulation on one side,and the CRT focus divider on the other. Either one
    can go out-of-tolerance,and the metal pins can crack and be
    intermittent.I've never been successful in resoldering them,either.

    I do not think this part would be the cause of his intensity modulation.
     
  6. Followup: The frequency of the chopped line does NOT correlate with
    line frequency.

    And I should note that at low intensity, it is worst going full off. With
    the intensity turned up, it's not visible. There is minimal
    if any geometric distortion of the trace (outside of the focus problem,
    which doesn't always appear) so I doubt any LV power supplies
    are involved that also feed the amps.

    It's almost as if the Z input has some oscillation on it. When I first
    noticed, I thought maybe the chopped blanking was misbehaving, but this
    occurs on all vertical modes.

    It's also related to warmup. At power-on, there is no problem. It takes
    a of couple minutes to show up. The fan is running.

    And even then, the severity varies randomly.

    So, what could affect the Z modulation AND sometimes focus, but not geometry?

    If it was the HV divider I guess a parts unit is the only source for a
    replacement?

    Thanks.

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

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  7. n cook

    n cook Guest

    I only have the manual for the 465 , not B variant.
    But assuming ps similar I would suggest a problem leading to ripple on the
    unreg 120V supply rail (bridge, caps )feeding the EHT oscillator/driver
    circuitry or problem in that oscillator circuitry or of course, wallectomy
    time, problem in the EHT step-up transformer
     
  8. Jim Yanik

    Jim Yanik Guest

    Perhaps HV oscillator freq?
    Then you're down to decoupling caps.
    A thought occurs to me;there's a 1uf/150V(IIRC) electrolytic on the wiper
    of the CRT grid bias pot,to ground;that could be going bad.
    Yes,it was a TEK-made part,from their defunct hybrid/ICO in-house plant.
    Removing a good one from a motherboard without damaging it is not an easy
    task
     
  9. I don't think so. It's order of a few hundred Hz and not constant.
    I'll check that.
    Now it really doesn't like even normal intensity.

    But the geometry (beam stiffness, etc.) is not affected. So, I'm
    leaning toward a problem in the grid circuit, not the HV itself.

    Does anyone know of a free source for decent quality schematics? I have
    several complete manuals downloaded from various places but the resolution
    of the schematics tends to be terrible and only marginally useful.

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  10. sck0006

    sck0006 Guest

    I think we have the 465b manual at work, I'll have to check on Mon.
    If we do, I'll only be able to copy a couple of pages so I'll just be
    able to get the power supply and maybe a few others, but I'll give it
    a shot.
     
  11. Thanks. That would probably be adequate. There's no problem with the text
    of the manuals, just the schematics. And this problem would seem to be
    associated either with the power supplies or the CRT bias/intensity/Z
    input circuits.

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  12. I found the 465B manual from bama.sbc.edu is nice: I can read the
    schematics easily, and quality is alright for my repair purposes. Sam,
    did you try this free source? don't forget about bama mirror at
    edebris, for faster download.

    Only thing you have to do extra is that you have to install the free
    reader, for .djvu format. Even if it is old, it still works adequate.


    hth,
    Andreas
     
  13. That looks much better than the ones I have. Thanks. I knew
    about the bama Web site but never bothered to download the djvu reader
    before.

    Any hints on getting pages to print with reasonable quality?

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  14. JW

    JW Guest

    Check your mail.
     
  15. sck0006

    sck0006 Guest

    I've got the schematics if you still want them. I was only able to
    get horizontal amplifier, crt circuit, power supply, and the power
    supply troubleshooting sections. Let me know...

    Steve
     
  16. Thanks but I think what I was able to get from the bama Web site is
    quite legible, if a pain to print. Printing from within the djvu reader
    results in poor quality, at least with my printer. I now export the
    page to a .bmp file and print it in sections. I think all I should need
    is the CRT circuit and I've printed that.

    I'm just surprised that with all the 465Bs out there, no one has come forward
    to say they've seen this problem before.

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  17. sck0006

    sck0006 Guest

    Glad you got what you needed. It is a strange problem that you have,
    I hope you can track it down. My T-935A has a problem sort of similar
    to what you're describing, but it is by no means periodic noise and
    seems to be much more like a dirty intensity pot. It has a chopped
    appearance, but just looks like dirty pot noise instead of what you're
    describing. I haven't had any desire to take it back apart, too many
    other things to do. Best of luck.

    Steve
     
  18. It may be.

    New followup: I've now got the thing open. Since it appeared heat dependent,
    I used a rubber tube to blow on various components in the HV area.
    Blowing on the thick film resistor network even immeidately after
    power-on and the beam appeared (already messed up) seemed to make a
    big difference causing the symptoms to disappear entirely for awhile.
    This could not have been heat related because it was within 10 seconds of
    powering up so nothing really could get hot. Then, after touching the
    thick film assembly perhaps with a few wiggles thrown in, the problem
    has disappeared entirely and has not reappeared. I've since cleaned
    it with alcohol and I will be running the scope off and on for a few
    hours over the next few days on the bench to see what happens.

    Of course, the instant I put the cover back on and replace the scope in its
    hard-to-reach spot, it will screw up again. :)

    So the working hypothesis is that indeed either contamination or cracked
    joints to this part.

    If cracked joints turns out to be confirmed with the problem reappearing,
    I would probably try conductive silver Epoxy first at each of the
    connections to the thick film itself rather than solder. If that
    didn't last, then what about fashioning a replacement from discrete parts?
    It might be a bit of a challenge to get high voltage resistors to fit in
    the space but seems like it could work. It's only 4 resistors, though
    admittedly 3 of them are high resistance (6.57M, 25.6M, 24.5M, and 550K)
    and at least 2 of them need to withstand almost 3 kV.

    Any comments?

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  19. n cook

    n cook Guest

    Some woven glass sleeving , cut to required lengths and formed into a
    compressable and insulated pad or more pads if you cannot localise better
    and cautiosly compressed by cable tie/s around the thick film ?
    I've only done similar on low V SM ICs that are covered in black epoxy ,
    boards as well, so impossible to re-do the SM solder , to the ceramic
    boards, and is a throw away job otherwise.
     
  20. Jim Yanik

    Jim Yanik Guest

    I've wondered if low melting point solder like what comes in the ChipQuik
    desoldering kit might work on thick-film connections.It wasn't available to
    me at Tek.The conductive silver epoxy may be a better idea. Trouble if you
    have to resolder the thick-film back into the motherboard.

    Alternately,you could fashion a small PCB to fit where the thick film
    goes,and solder your resistors to the PCB. I've not tried this,though.
     
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