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Thomson (ICC17 chassis) TV: fault related to focus voltage

Discussion in 'Electronic Repair' started by Robin, Mar 19, 2007.

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  1. Robin

    Robin Guest

    I have read the FAQ (and much else) without finding a clue to the
    precise problem I have.

    The set is a Thomson 28WT25US around 6 years old. It has a 28 inch 16:9
    tube and ICC17 chassis. From about its second year there was an
    intermittent fault: the set would go into protection mode (error 27: =
    deflection failure more than three times). But it always used to
    recover within a few minutes. Recently however it failed to recover. I
    changed the LOPT/flyback transformer. That has not solved the problem

    If I adjust the focus voltage to give a sharp picture the set fails
    quickly (within a few minutes and sometimes almost immediately) with
    error 27 as before.

    But if I leave the focus voltage low then the set continues to operate
    for hours.

    I have tried reducing the G2/A1's but that made no difference. So I
    don't think it can be arcing from G2 or a breakdown in the capacitor
    between G2 and ground (CB01 on the board at the back of the CRT).
    Right, please?

    I have read that these sets can have problems with the capacitors around
    the LOPT (especially CL21 and CL22). But I am puzzled as to how that
    would explain the way the set is stable with the focus voltage low -
    unless (as is very possible) I am misunderstanding the way the focus
    voltage is tapped from within the LOPT.

    I have looked for a capacitor between focus and elsewhere which might be
    breaking down but could not see one. So I am at a bit of a loss.

    In short, any ideas please?

    Please note I am a novice at TV repair and have only an ordinary meter -
    no access to scope or EHT meter these days.
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