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Horizontal frequency

Discussion in 'Electronic Repair' started by Golf, Jun 4, 2007.

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  1. Golf

    Golf Guest

    I have a general question regarding horizontal frequency in a TV. I
    have this set that tries to power up but shuts back down. Anyhow, I am
    going to scope the H drive pulse and such, but if the frequency isn't
    what it should be (suppose to be around 15.7Khz?), would this likely
    be part of a protection circuit shut down in some models? Would the
    same be true for Vertical as well? The screen and heater voltage is
    swinging also. I am trying to repair this with as little help as
    possible. This is why I'm not going into details on model #, make, and
    such. Thankyou group.
     
  2. PeterD

    PeterD Guest

    OK, we'll be all too glad to give you as little help as possible.

    The answer to your question is maybe.
     
  3. Art

    Art Guest

    Without the make, model, and chassis number of the set this is the kind of
    answer you will receive. Post relevant data and symptoms so we can at least
    attempt an educated and relevant guess.
     
  4. Golf

    Golf Guest

    OK Art, here it goes - Sanyo model #HT32744, chassis #32744-00. Hit
    power button, Relays click, high voltage starts, then relay clicks
    again and set shuts down (approx 2 seconds time). Can repeat this once
    before having to unplug the set to repeat this sequence. I have
    checked the power supply including the SMPS which is operating
    normally. I have 140vdc at the collector of the HOT (steady). There is
    a power failure circuit that I am trying to understand by looking at
    the schematic. I disabled this (set stays on) to keep the set running
    long enough to check a few more voltages. Even though the set stays
    on, there is a faint raster in the middle of the screen about 2"
    square that pulses (seems to be in sync with the slight ticking noise
    of the flyback). The screen and heater voltage is swinging all over. I
    have never checked these voltages on a working set, so I'm not sure
    what's normal.
    I work on TV's as a hobby, and have about 2 years experience.
    I have a good basic understanding of electronics and electricity, and
    am self taught with the exception of help from this group. This is why
    I may ask some seemingly retarded questions. I hope this is enough
    info. Thanks for replying.
     
  5. Ken

    Ken Guest

    As others have said, information is key. However there are a couple of
    things you might try to zero in on the problem:

    From your comments, I assume that the HOT is not shorted. Jumper the
    base to the emitter of it to force it to stay turned off and see if the
    power will stay up. Of course you will not see a raster due to the HOT
    being shorted out, but if the power stays on your problem is located on
    one of the output terminals of the flyback transformer. If this is the
    case look for a shorted diode off of one of those secondary windings.

    It could be other things, but that is often the reason based upon the
    symptoms you described.
     
  6. Golf

    Golf Guest

    Thanks for the reply Ken. For the sake of me explaining everything
    I've done in the past 2 hours, I think I found a problem on a 6.5V
    supply. It goes to the P in P module, an RGB drive IC, and somewhere
    in the video deflection circuit I think. Long story short - removing
    power from this 6.5 regulator keeps all voltages stable off the SMPS.
    I was going to disconnect the output of this regulator first though to
    see if it was OK.
     
  7. Golf

    Golf Guest

    Art, I thought my questions (yes/no/possibly) were pretty straight
    forward, but obviously I was wrong. "if the frequency isn't what it
    should be (suppose to be around 15.7Khz?), would this likely
    be part of a protection circuit shut down in some models? Would the
    same be true for Vertical as well? "
    Peter gave a better answer (maybe) anyway. Thanks
     
  8. Franc Zabkar

    Franc Zabkar Guest

    I have seen a fault of this kind in an old IBM monitor. A ceramic cap
    drifted in value, causing the horizontal frequency to increase (or was
    it the duty cycle?), which in turn caused the pulse amplitude on the
    collector of the HOT to increase. This was reflected in a secondary
    winding of the FBT which then triggered the HV protection and shut
    down the unit.
    - Franc Zabkar
     
  9. Meat Plow

    Meat Plow Guest

    I gave this guy several pointers in multiple posts about HV protect over
    the last several days. He seems pretty able for a hobbyist but it's tough
    when you don't have shelves stocked with new and good pulls to sub.
     
  10. Guest

    I give it about a 70% chance it is the flyback.

    It ain't the frequency, it is the waveshape that tells you.

    Just to give it from the top once, you put the scope in 200Mv, 10uS
    auto triggered. you put the probe near the fly, and you should see a
    pulse, about 12-13 uS for a nonHDTV. If you do, you scope the vertical
    windings of the yoke, put yout finger on the scope probe, take it into
    freerun mode and use the vernier to sync it with the powerline pickup
    from your body. Then you go to the yellow and green wires to the yoke
    at 20V/div.

    You should see a similar pulse but with a ramp on the end. It will be
    moving, but you can still see it. If you do, you go to the main B+ and
    set it to 50V/div and watch it rise, if you see it go over three divs,
    suspect caps on the primary side of the SMPS or a bad opto or
    something that drives it.

    If the set is regulating properly, don't forget the audio IC, scope
    the Vcc to it and see if it drops like a rock, if so, remove the audio
    IC and try it again.

    And never forget, if you get a fucked up waveform out of the fly,
    check ALL diodes connected to the fly, especially the big ones.

    If all this fails, well it may be junk. Needing proprietary parts is
    not an option. Aftermarket flys are out there, ome of better quality
    than others, but when you get deep deep into the circuitry, it is not
    usually worth the time unless the unit was expensive and hs as good
    chance of having a good CRT. CRTs do not last like they used to, and
    the old ones are old.

    Off to bed.

    JURB
     
  11. Golf

    Golf Guest

    Sorry I haven't updated in a few days - Iv'e had my head stuck in the
    back of this bastard TV going on 4 days straight. Here is the latest -
    I managed to narrow down the problem to the 14V or 15V supply off the
    SMPS. If I disconnect the 14V from the main board, the power supply
    stays up. I disconnected each individual 14V source (feeds a couple
    regulators, PIP module ,etc) in hopes of finding the bad actor. Funny
    thing is, if anything is connected to the 14V source, the voltage
    swings wildly putting the set in shut down. Now it seems the problem
    is the source voltage. Steady 14V with no load. Goes ape shit with any
    load. I connected a flashlight to the 14V source (disconnected from
    main board). The voltage was steady, but dropped to about 9.5V. I did
    the same thing on the 15V source (steady with no load), and whamo, the
    flashlight started clicking on and off as was this voltage swinging
    big time (kinda looks like some AC in this voltage too). I checked
    every component off the SMPS secondary and found nothing bad yet. I am
    wondering if the switching transformer is gimped. The 35V supply is
    reading 45 - 50V, but is steady. The other voltages are steady - 140V,
    -15V. Sooo, I don't have experience with faulty SMPS transformers, but
    if it can't supply the load demand without dropping the voltage,
    doesn't it have to be this? Thanks again guys for all replies and
    suggestions.
     
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