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VS5010 revisited

Discussion in 'Electronic Repair' started by JURB6006, Sep 11, 2003.

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  1. JURB6006

    JURB6006 Guest

    It's me again;

    This is not for the faint of heart.

    Now it's blowing the LA7838 in about a minute. Same deal, there is some DC on
    the output, it has leakage to ground, but it does not discharge the boosted
    Vcc. Now I have disconnected the shutdown, and of course the filament supply.
    There was no raster even with the vertical running, but it has the CM kit so
    whatever isn't right is probably triggering the CM ckt..

    Now this is your typical current feedback output compared to a ramp. The ramp
    appears about right and the top of the yoke output appears alright on the
    scope. Any variance in the impedance curve of the yoke load will cause a change
    in the output voltage as the chip tries to match the current to the ramp. Right
    ?

    The output voltage waveform matches the picture on the print except for one
    thing, there is a bowtie waveform superimposed on it. Many times this is
    normal, however I simply can't know everything so I thought, OK the fault is
    not properly bypassing the HF component of the feedback, as such it's screwing
    with the slewing rate of the outputs. I added a 1000pf cap to that pin and the
    bowtie component is still there. In either case it wasn't that big, I'd say the
    HF component peaked at about 15V P-P. Still blew it, no change.

    This time I saw it fail, I made it a point to sit there and watch the scope. It
    simply became a flatline at about 10Vdc (I think). I didnt see a spike or
    anything. Of course at that slow sweep time it might not have been displayable.


    I have also changed the NS PIN transformer to no avail.

    What are the chances of it being a rogue yoke ? To test that theory I think
    I'll run the set with the red and blue yokes, or all of them disconnected from
    the vertical. I think I can manage to turn the plugs around to make it so. Of
    course the filament supply will remain disconnected and the LA7838 will put out
    square waves. If it runs for hours like this, now does that really <u>prove</u>
    that it needs one of the yokes ? Before when I looked into the tubes the sweep
    seemed pretty normal, without the mirror in place I can't say it was absolutely
    correct but there were no glaring abnormalities. I couldn't quite see the top
    of the rasters, but the bottom was not bowed or compressed. Even if one of the
    vertical windings was a dead short it woudn't do this, but if it was an
    intermittent short to the H winding it would.

    As of during the day today, I had replies and both have been dealt with, I
    changed the "boost" diode and the 11V is present on pin #1 even when there is
    no sweep. Thanx for your help, but it didn't work this time. Anyone else
    replied since this afternoon, I'll read it in a few minutes.

    One thing's for sure, if it blows with the yokes disconnected, the problem is
    on board for sure, it should be able to put out those square waves for quite
    some time, right ? Or should I find a yoke somewhere and hook it up ? Maybe
    both ways.

    This thing has good enough tubes and it has been on one of my benches for way
    too long so any ideas are appreciated, what would you do ? Thanx in advance.

    JURB
     
  2. My vote would be to find a yoke or yokes to substitute. That's of course
    the easiest way to narrow it down.

    Could you run with the horizontal yoke windings disconnected to eliminate
    the possibilty of a short causing the part to fail?

    Does the part get hot before it blows? If all inputs and outputs
    look correct but it blows without overheating, then one must suspect
    some outside cause like a shorting yoke.

    This is probably all obvious.....

    --- sam | Sci.Electronics.Repair FAQ Home Page: 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
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    Important: The email address in this message header may no longer work. To
    contact me, please use the Feedback Form at repairfaq.org. Thanks.
     
  3. John Del

    John Del Guest

    Subject: VS5010 revisited
    I don't know if you covered this Jeff, but where are you getting those 7838s
    from? Unless they come directly from MGA, Zenith, Panasonic, etc., I'd bet
    real money the problem is garbage subs. If you are getting them OEM direct,
    then I'm stumped as well.


    John Del
    Wolcott, CT

    "Nothing is so opportune for tyrants as a people tired of its liberty."
    Alan Keyes

    (remove S for email reply)
     
  4. Did you replace D4000 ( 11V Zener) and C920 (10uf 100V) in the power supply?


    --
    ==========================
    Jeff Stielau
    Shoreline Electronics Repair
    344 East Main Street
    Clinton,CT 06413
    860-399-1861
    860-664-3535 (fax)

    ==========================
    "If you push something hard enough it will fall over."
    Fudd's First Law of Opposition - Sir Sidney Fudd
     
  5. JURB6006

    JURB6006 Guest

    Hello;

    Thanx for your time and attention, of course I will reciprocate whenever
    possible. We are, however, still fighting this beast.

    #1
    "Did you carefully look for thin spikes and any is fizzy (noisy)
    waveform where it should be sharp clean thin waveforms? This is what
    I'm dealing with mine (RCA PTK195 using TDA8172)

    Not to scale, just a rough digram: "*" is the HF fuzz exactly (less
    than 1/4 inch wide)"
    -------------
    It uses a saturable reactor NS PIN ckt., as such some hash will crop up on the
    waveform (WF) normally. I checked the WF to the print for the VS5001 and it
    matched, as do all of the onboard Vsweep components I've seen. Even the Vcc
    matches, so I would bet the yokes are electrically compatible at least to the
    vertical ckt..

    #2
    Could you run with the horizontal yoke windings disconnected to eliminate
    the possibilty of a short causing the part to fail?

    Does the part get hot before it blows? If all inputs and outputs
    look correct but it blows without overheating, then one must suspect
    some outside cause like a shorting yoke.
    ----------------------------
    Yes, but even a dead short of one yoke shouldn''t cause this. Yes, the original
    2 ICs got hot, they took around 5 minutes to fail. That changed to one minute,
    and the only thing I'm not sure of is if changing the NS PIN transformer
    resulted in this. It did not cure the problem, so I'll change it back when I
    get back on it, probably tomorrow.

    #3
    I don't know if you covered this Jeff, but where are you getting those 7838s
    from? Unless they come directly from MGA, Zenith, Panasonic, etc., I'd bet
    real money the problem is garbage subs. If you are getting them OEM direct,
    then I'm stumped as well.
    ---------------
    The first two were from RCA I believe. They came in an RCA paper envelope.
    These were the ones that lasted longer. The third one was from a chassis in the
    back. The tab twisting was sure to screw the original up, and it was an
    ECG70XX, a true ECG, not a knockoff. The last couple came from B&D with whom we
    have really had good luck with. Of course there could be a bad batch of them
    around. You might remember when I posted about the bad STK4274s, lot #7027.
    I'll not soon forget that.

    #4

    Did you replace D4000 ( 11V Zener) and C920 (10uf 100V) in the power supply?
    ------------------
    No, but I did state that the 11V supply was up after the failure. It was 11V,
    just like it should be so that clears the zener, the cap and the resistor. No
    sweep, yet the 11V was ok. You're not the only person that mentioned that, but
    if the voltage is there. . . . . .

    ----------------------------------------------------------

    Thank you all for your time and interest. <u>When</u> we beat this $%&*^#@# I'l
    make sure you're the first to know, well realistically second.

    JURB
     
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