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Dead RCA CTC195 - Fixed!!

Discussion in 'Electronic Repair' started by Bob F., Feb 18, 2004.

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  1. Bob F.

    Bob F. Guest

    My big screen set, an RCA CTC195, purchased in 1997 died a few weeks ago. I
    purchased the schematics from TCE (Thompson Consumer Electronics) and
    carefully read the section on the input power supply, then checked all of
    the waveforms and voltages. I could see that the B+ voltage was too high
    (170vdc vs. an expected 150vdc) and pin 5 of U101 (a PWM IC for voltage
    regulation) was NOT oscillating.

    My first step was to take the main video board out and clip some capacitors
    across the large input capacitors and see if the power supply would start
    up, but no response was seen.

    Next, I started checking resistor values. All of the resistors on the top
    side of the board around U101 checked out okay, so I flipped the board over
    and started checking the surface mount components. I went to check R121, a
    1K-ohm, surface-mount resistor, but lo-and-behold, it was missing! In fact,
    it looked as if the component had never been installed because the glue dot
    to hold it in place before solder reflow was perfectly formed, not mashed
    down.

    Anyhow, I added a resistor to this point and fired it up. Amazingly, the
    damn power supply came up. I put the main video board back in the chassis,
    connected everything back up, and its been working for the last four days.

    My theory is that R121 was never added or fell off during production.
    During testing at the factory the set checked out okay, but with the passing
    of time, component values in the voltage regulation circuit have drifted
    causing the set to shift from a state of marginally operating, to shutting
    down.

    My lesson learned is that its sometimes necessary to ADD parts to fix a dead
    set.

    Thanks,

    Bob F.
     
  2. Jason D.

    Jason D. Guest

    Anyhow, I added a resistor to this point and fired it up. Amazingly, the
    AMAZING! I had a JVC horizontal shrieking at 22KHz 27" set (narrow
    raster). It was missing resistor for the bus controlling that digital
    jungle. :-O

    I think that small electrolytic cap in the primary side SMPS went old
    making the problem show up in yours, just in case change that small
    cap.

    BTW, R121 is partial location number, The RCA has number of different
    areas designations: In your case for SMPS is R14121. Horizontal
    circuits is numbered with x144yy where x is type of components, yy is
    location numbers. For example L14401 is inductor coil for base drive
    between HOT and horizontal transformer driver.

    Bravo! & Cheers,

    Wizard
     
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