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hot shot -- or not?

Discussion in 'Electronic Repair' started by Gil, Jun 15, 2004.

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  1. Gil

    Gil Guest

    Hi all,

    On a dead Daewoo 27" c.1999 I replaced the H-output Xsistor, D1880,
    Q402 with the equiv. NTE2353.

    Now the set just goes click, click, click, etc on startup while
    jumpering the fuse contacts with a lightbulb.

    I'm wondering if the new HOT is defective. I know there's supposed to
    be a damper diode in there, but Rbe = Reb = ~ 0.3 Ohms. Is that right?

    TIA

    gj
     
  2. Sofie

    Sofie Guest

    Gil:
    You should test the HOT out of circuit. The very low "in circuit" Reb
    resistance you are reading is completely normal even when the HOT is good.
     
  3. Joe Rongen

    Joe Rongen Guest

    A bad horizontal output transistor could have shorted the B+ regulator
    which in turn would feed an unregulated (higher) voltage into the horizontal
    circuit. This would repeatedly activate the x-ray protection circuit and
    shut down the TV.
    ( One should always check the B+ voltage after any serious short.)
    As such, you are measuring across the driver transformer. The damper
    diode is located between the collector and emitter of the output transistor.

    Regards Joe


    " ...not only are the drops of rain mere appearances, but even their
    round shape, and even the space in which they fall, are nothing in
    themselves, but merely modifications of fundamental forms of our
    sensible intuition, and the transcendental object remains unknown
    to us" ..... Kant, I. Critique of Pure Reason.
     
  4. Jason D.

    Jason D. Guest

    In Daewoo especially CN200 chassis I bet it is in case for yours, if
    horizontal transistor is shorted, I always assume the power supply is
    also blown. Three transistors, shorted find them by it's looks and
    meter for resistance, crispy resistor I think is 3.3 ohms 2W.

    I use the one of two transistors used for sony rebuild kit, correct
    resistor and other two small transistors to rebuild daewoo power
    supply since their oddball transistor is hard to get now.

    Main cause for horizontal fry was about six electrolytic capacitors in
    that chassis, Two 100uF use same volts as old near vertical heatsink
    and other one between bank of high power resistors, two 1uF 160V (can
    use 200V), another two (small ones) around the power supply.

    Honestly, that daewoo is disposeable because flyback doesn't get good
    focus even that set had good RCA tube in it.

    Get a older or newer RCA or JVC instead. They hold up very well and
    gives very little trouble.

    Cheers,

    Wizard
     
  5. Ross Herbert

    Ross Herbert Guest

    On 15 Jun 2004 10:11:50 -0700, (Gil) wrote:

    |Hi all,
    |
    |On a dead Daewoo 27" c.1999 I replaced the H-output Xsistor, D1880,
    |Q402 with the equiv. NTE2353.
    |
    |Now the set just goes click, click, click, etc on startup while
    |jumpering the fuse contacts with a lightbulb.
    |
    |I'm wondering if the new HOT is defective. I know there's supposed to
    |be a damper diode in there, but Rbe = Reb = ~ 0.3 Ohms. Is that right?
    |
    |TIA
    |
    |gj


    What is the wattage of the bulb across the fuse? The fact that the PSU
    is running in pulse by pulse current limiting mode could simply be due
    to the excess voltage drop across the bulb causing the PSU to limit
    because the input voltage is too low under normal load.

    That is in fact how I test the operation of SMPS control IC's. If they
    don't go into p x p limiting (under load) using a 60W series input
    bulb then I know the controller IC is kaput.
     
  6. Gil

    Gil Guest

    Thanks, I realized just after making the original post that the best
    test is out of circuit so I unsoldered the thing and still got Rbe =
    Reb = ~50 Ohms. Which still seems odd.
     
  7. Rbe of 50 ohms is normal for a HOT with a built in damper diode.

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  8. Asimov

    Asimov Guest

    "Sam Goldwasser" bravely wrote to "All" (17 Jun 04 21:13:23)
    --- on the heady topic of "Re: hot shot -- or not?"

    SG> From: Sam Goldwasser <>

    SG> Rbe of 50 ohms is normal for a HOT with a built in damper diode.

    Indeed, many HOTs have a built-in low value resistor across the
    base-emitter junction along with the reverse biased damper diode
    between collector and emitter. Was this supposed to be a secret?

    A*s*i*m*o*v

    .... The current limits placed are based on resistance
     
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