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CTC203 "sacrificial" output transistors

Discussion in 'Electronic Repair' started by [email protected], May 22, 2005.

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  1. Guest

    I recently did a CTC203 that was squealing. I found the horizontal
    output transistor shorted and replaced it. I then dug out the infamous
    coil and cleaned/ resoldered it as well. I fired the thing up only to
    have the flyback arc and take out the new transistor. I then replaced
    the flyback and of course the new transistor again and the set worked
    fine. I don't have a ringer but I have been told that this type of
    insulation breakdown is somewhat common on these 32 inch and larger
    chassis and very often won't show up on a ring test anyway. I don't
    want to just replace a flyback as a prophylactic measure every time one
    of these squealers comes in but is there any way to predict this might
    happen before powering up a new repair. Thanks, Lenny Stein, Barlen
  2. Art

    Art Guest

    Sad to say, either a ringer to test the LOPT before replacing the output
    transistor or estimate to include the LOPT as a nominal repair. IMHO the
    cost of the output transistor is small price to pay to do a dynamic test on
    the LOPT. Seen a lot of these transformers fail not only in the larger sizes
    but also the 27" versions.
  3. Did I read that right? You think that blowing outputs is a reasonable way
    to diagnose a bad LOPT? Please finish this line of reasoning...

  4. RonKZ650

    RonKZ650 Guest

    I really don't know any other way to do it other than powering up the
    set. Ringing a flyback is not going to reveal a cracked case arcing
    problem. What I have done is make 2 test transistors, one with a
    damper, one without out of old obsolete TV outputs such as old 2SC1172
    metal case outputs, and old outputs from TVs like the GE PC chassis.
    Use the original metal mounting plate from an old set that had a socket
    like a mid 80s Zenith. Run a foot or two of wiring to the socket. Now
    when an output transistor is shorted, after doing all the pre power up
    checks, instead of replacing the transistor, I simply run the wiring
    from the test transistor to the board and solder. Turn the set on with
    a HV probe that has min/max memory in the anode then quickly unplug and
    see if HV normal. If arcing happens and the test transistor is shorted,
    takes 30 seconds to replace it with another junk TV output. All free.
    Works for me. Saves some work, and I've found these old transistors
    pretty much will work any TV for a test anyway.
  5. Art

    Art Guest

    Leonard: Unless you have an absolute failsafe way of testing the LOPT
    without sacrificing a transistor we will continue to do it our way. Ringing
    will tell us if indeed there is a fault in one of the transformers windings,
    or maybe a shorted diode in the rectifier portion. However, without dynamic
    testing we will not find the cracked casing unless there is the brown ooze
    already coming out of it. Even with these nice Sencore 325's that we are now
    using. I like Ron's idea for bench testing however in the field it may
    become a bit cumbersome.
  6. I never said that there was a failsafe test. I simply question the routine
    use of output transistors as test devices. We test LOPTs by ringing and
    with the Sencore drive test which is simply a H pulse at 30vpp applied to
    the primary. Get 500-700 vdc out and it is likely good. Rarely do we find
    a bad one that passes both tests. Sometimes a bad one will cause
    intermittent failures or overheating, but these are rare. I paid $200 for a
    used VA62 on Ebay and the flyback test and ringing tests alone have made it
    a worthwhile investment.

    It just seems wasteful to me to consider output transistors to be
    sacrificial test devices. It seems dangerous as well, since you never know
    what else you will take out when it fails. Well, in the case of an AA1
    Sony, you do actually...

  7. me

    me Guest

    its a lot cheaper than the hi-pot test equipment you would need...
  8. Mark

    Mark Guest

    maybe we can improve on ron's idea....

    get a really large test transistor and add some extra zener protection
    and current limiting so that if there is a arc, the test transistor
    will survive.

  9. What about the power supply?

  10. Jason D.

    Jason D. Guest

    Knowing the history and specific brands tendency to kill HOT
    instantly, had no time to react. That how instant: Push (power
    button)CLICK-POW. Either a fuse blows instantly or switching power
    supply goes Eeeeeeee!

    In generic monitors if I see shorted HOT and checked the driver
    circuit if it has electrolytic cap, otherwise JUNK. Cost of flyback
    is too costly in repairing out of warranty monitors.

    In RCA, Citizen and some other models, when I see a blown HOT
    depending on chassis, I usually replace both flyback and HOT and they
    work. A failed cap (plastic ones) is extemely rare in many TV except
    those few that had small blue disc or had to replace many caps in like
    in CTC169. It is all based on experience to make the judgement call.

    Cheers, Wizard
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