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Zenith SR3587DT Sound OK, No Picture????

Discussion in 'Electronic Repair' started by Q.E.D., Aug 2, 2003.

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  1. Q.E.D.

    Q.E.D. Guest

    Looking for input into whether this June 1996 build TV is worth having
    repaired.

    TV has worked great since purchase in 1996. Never got a great deal of
    use...maybe 10 hours a week on weekends to watch mainly movies on DVD
    or HBO. I figure less than 4000 hours total use. Set had been on for
    about 4 hours one day and the picture just went blank...like someone
    turned it off. No warning of trouble before that...no picture fade or
    blooming, it just went dark. Sound still works OK.

    The TV is long out of warranty but due to it's light use I was
    thinking it might be worth repairing. Can anyone give me an idea how
    much would be a fair rate for a repairman to come out and diagnose (I
    live in Philadelphia, PA)? What's an average repair cost on a 35"
    set? I realize the repair costs would be on top of the initial
    estimate depending on what is wrong. On that note, are my TV's
    symptoms indicative of a common problem? Based on my novice research
    I think it could be a high voltage problem.

    The neighborhood repair guy went out of business about three years
    ago. Choosing a repair service, I'm finding, is almost impossible to
    decide on blindly.

    Any advice would be appreciated.

    Thanks
     
  2. Bill Jr

    Bill Jr Guest

    Could be as simple as the 110k ohm resistor that connects to the H.O.T. or a
    bad flyback.
    You really should contact Zenith and ask for a local authorized servicer.
    Try www.zenith.com

    Good Luck,
    Bill Jr
     
  3. Mike

    Mike Guest

    im sure you all know me by now,

    but I had the same problem with mine, NO HV, and i have narrowed it down to
    the scan gen circuits, it wasnt spitting out a waveform, not even a startup
    pulse.

    I found a scope mod that can ring check the flyback, and it checks good, so
    only thing left: Scan Gen IC. which the shutdown sequence is computer
    controlled.

    I NEED TO DISABLE THE SHUTDOWN TO TEST FURTHER. anyone got any insight on
    this?
     
  4. Mr. Lee

    Mr. Lee Guest

  5. Mike

    Mike Guest

    see, thats the problem. I live off of social security, and I dont have 100
    dollars to put in a board.

    I understand the flyback is 60, but its not bad. I ring tested it.

    I need to disable shutdown of the scan circuity, and someone here with a
    schematic can tell me.
     
  6. Mike

    Mike Guest

    BTW, i dont have a remote for it, to get into service menu, but of course it
    doesnt matter, because there is no HV, raster, or filiment. Just sound.
     
  7. john

    john Guest

    DO NOT disable the shutdown ,are you stupid ?
    Its there for the TV,s and your protection .
    kip
     
  8. Mike

    Mike Guest

    yea im stupid, if thats the only way to find out whats wrong, because other
    approatches just arnt working.

    like, The HOT has B+, flyback rings good, but

    there is NO drive signal from the scan IC for the horiz or vert. so im
    suspecting that the IC is either in shutdown, or is bad.

    there isnt even a startup pulse, and I thought that the startup pulse has to
    be there in order to get feedback from the flyback to shut it down. Is this
    true?

    if so, there isnt any.
     
  9. David

    David Guest

    You cannot do it. Disable the xray protect in that set.

    Just because the flyback ring tests ok does not mean it is good, means it
    does not have excessive shorted windings. Zenith flybacks are well known
    for having the triplers go bad, you will be able to read some resistance
    between the hv anode and the ground of the flyback, usually around 400K when
    they are bad.

    The 35" in that era is also well known for the shaping coil and pin cushion
    coils shorting out internally. There is a fuse in series with the voltage
    to the flyback for the main b+, without it present you also will not get any
    drive pulses.
     
  10. Mike

    Mike Guest

    ok, I finally got to it.


    All volteges are presant and none of the fuses are blown.

    now, there was a pin problem.

    Now, I set my scope up, and did some more testing on the tv,

    the scan gen IS working. there is a square wave pulse on the pin 8. there is
    a square wave pulse on the base of the horizontal driver transistor. BUT,
    there is NO DRIVE PULSE on the collector of the driver transistor. so, i
    guess its a bad transistor. I did some voltege measurements, and there is
    about 41VDC on the collector.

    There is 158VDC B+ on the HOT.

    I did a BtoC, and BtoE check in circuit on the transistor, and it checks OK,
    but i never pulled it out of circuit.

    Any Ideas?
     
  11. Mike

    Mike Guest

    P.S., the heatsink on the QX3201 stays cool.

    the other 3 in the chopper circuit get regularly warm, thats the only one
    that stays cold.

    could that be something? it junction tests good.
     
  12. Mike

    Mike Guest

    oh yea, one more thing, the square wave pulse is continuos, its not shutting
    down, probably because its not getting to the flyback, or the base of the
    HOT, it dies off at the driver xistor. its there on the base, gone on the
    collector
     
  13. Mike

    Mike Guest

    one more thing, the square wave pulse is continous, its not shutting
    down, probably because its not getting to the flyback, or the base of the
    HOT, it dies off at the driver xistor. its there on the base, gone on the
    collector
     
  14. Bill Jr

    Bill Jr Guest

    IS the frequency of that square wave accurate?
    IS the amplitude sufficient to turn on the horiz driver transistor?

    Bill Jr
     
  15. Mike

    Mike Guest

    i dunno.

    I just have an older scope.

    Its a square wave. typical old squarewave.

    I seen a picture of the CTC130 square wave in my tv repair handbook, it
    peaks out at the top of each wave, well, this one is jsut square wave. I
    have to put my sope on 5V to see the drops and rises.
     
  16. Mike

    Mike Guest

    and thats about right, the datasheet to the scan IC says 5vp-p
     
  17. Mike

    Mike Guest

    the only information i can tell you about the amplitude, is that the
    waveform at the base is a little weaker than it is comming out of the IC,
     
  18. john

    john Guest

    Why not just take the set to a TV shop and get a proffessional
    to take a look and advise you , its pretty obvious you dont have the skills.

    kip
     
  19. Mike

    Mike Guest

    excuse me?
    I work at a shop.
     
  20. Mike

    Mike Guest

    you may be right about not exciting the driver.

    Here is what I have come up with.

    There is 1.8VDC comming into the board through the cable comming from the
    scan gen IC, and there is only .1VDC at the driver. and 41vdc at the
    collector.

    any ideas?

    and there should be 5V p-p comming from the IC, but it might go through
    resistors, dropping it down.
     
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