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CTC169AS Chassis shuts down after a few seconds

Discussion in 'Electronic Repair' started by BobD, Nov 13, 2003.

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  1. BobD

    BobD Guest

    Hi,
    This chassis is in a 31GT656 GE TV. Upon power up, the screen has a
    horizontal line made up of seperate RGB lines. After 3 seconds, the
    relay clicks and the set comes up with a full bright screen with white
    retrace lines. After about 3 more seconds, the set shuts down and
    requires a wait before restarting. The CRT voltages seem to check
    out(G1=5.0v, Cath=225v). If I power up with a Variac and keep the
    voltage around 80%, the set will stay on. I checked the resistance of
    the heater to the other pins on the yoke. All except G1 are infinite.
    I get about 1.2 ohms to G1 from the heater (with the CRT PCB
    disconnected). If I disconnect the heater, the power supply will stay
    on (with the CRT PCB connected). I can power the heater from a remote
    supply to verify it works, but with the set supply connected, the
    heater filaments appear dim. My heater voltage, measured with a DMM,
    is around 2.2 volts loaded, 8.5 volts unloaded (heater disconnected).
    Bad tube? Any ideas?
    Thanks.
     
  2. ---------------------------------------------------------------
    Text pasted from WinSTIPS 2.0
    http://www.ServiceSoftware.com
    ---------------------------------------------------------------

    RCA CTC169

    Bright Pix W/Retrace. Audio Ok. Shuts Off In 10 Sec

    Bob'S Tv

    Q2906 (Q906) Ecg 159 Ok

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



    --
    ==========================
    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
     
  3. David

    David Guest

    Verify all SMPS power supply voltages first, both in standby and run.
    Especially ESR the capacitors and check the 12v supply diode.

    A borderline 12v line will cause all kinds of problems.

    Seems like you are starting at the wrong end of the tv set for
    troubleshooting of a shutdown problem. You have to verify power supply
    first, then the deflection/high voltage circuits, then follow the remaining
    shutdown causes.

    David
     
  4. David

    David Guest

    Once the power supplies are all verified. Bright screen with retrace
    indicates a problem in the video output stage or sweep derived power supply
    voltages causing the shutdown condition.

    I would bet if you have bright screen with retrace, the crt voltages do not
    check out as you indicated. Normal G1 low voltage, 20v or so or zero;
    Cathodes, 200v with no video at all, lower with some video, never below 125
    volts dc even with very bright video unless a problem; G2 can be anywhere
    between 200 and 600 volts depending on tube and design.

    I would look for low video output power supply voltage, 200v. or a problem
    with the luma buffer transistor Q2906 being shorted. Q2906 often shorts due
    to intermittent arc-back shorts in the picture tube. Repeated failure of
    this transistor after replacing with an EXACT part generally indicates an
    intermittent tube problem.

    David
     
  5. BobD

    BobD Guest

    You guys are good! The Q906 transistor is shorted. I will replace and
    reply once the new one is in.

    Thanks again.
     
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