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Magavox PTV705 blows HOT instantly

Discussion in 'Electronic Repair' started by David, Dec 5, 2003.

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  1. David

    David Guest

    Bad flyback, cracked blue crt (very common), bad hv divider, etc.
    Just use standard tv set high voltage circuit troubleshooting techniques.
    David
     
  2. Guest

    Guest Guest

    Hi. Magnavox PTV705, blows Q802 HOT instantly. Have checked, good H drive to
    base from proc chip, with HOT removed and HV cable detached to avoid crt
    problems. Anyway, done up with HOT removed and HV isolated, I can fire up
    for 1 second. During this time I have good base signal to HOT base
    connector. If I put in new transistor, makes a not pretty noise, then is
    instantly shorted. Have found no obvious bad parts near the area. Any ideas?

    Thanks in advance,

    Danny
     
  3. Bill Jr

    Bill Jr Guest

    That Q802 is the Horizontal Output Transistor.
    The High voltage is done with Q906, the HVOT.
    These sets suffered from lack of sealant for the anode connectors to the
    CRT's causing your exact symptoms.

    Good Luck,
    Bill Jr
     
  4. Guest

    Guest Guest

    Hi. Thanks for the ideas. It does blow the Q802 with HV cable removed from
    splitter and isolated also. Reckon this set doesn't like having no load on
    HV output? First time I've run into this, so a bit of a learning curve. I
    gotta change the tube oil anyway, it is grungy, so I'll redo the RTV at HV
    connectors while I'm at it. Just trying to sort out the running problem
    first before the cosmetics.

    Cheers, Danny
     
  5. David Heller

    David Heller Guest

    Danny

    I would reflow the horiz drive transformer. You need to do the four pins
    where the tiny leads from the windings are connected to. and resolder
    the connections where the transformer is soldered to the board. Other
    possibilites are a bad yoke or horiz drive problems Also the damper
    diode could be bad if not internal to the output transistor.

    Dave

    Good Luck
     
  6. Also check for open safety (tuning) caps off the horizontal output. I recall a
    Philips PTV620 few years ago the would blow the HOT instantly. In that case, the
    cause was C817 (0.0091uf/1600v)

    Alan Harriman
     
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