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Pioneer TV symptom - all three tubes barely glowing

Discussion in 'Electronic Repair' started by Golf, Jul 20, 2007.

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  1. Golf

    Golf Guest

    I have searched, but can't seem to locate anyone who has a Pioneer
    model #PRO97 schematic. This is the same set that has 119V reg B+ at
    HOT with voltage adjustment at max. Do get 160V when I remove a
    darlington driver transistor out of the power supply (was also
    replaced with new part). All three Pic tubes are on, but glowing very
    faint. I suspect this low voltage is the cause. Anyone seen a similiar
    problem or know if this low HOT voltage would cause this problem? I
    really need a schematic since I'm not a seasoned pro. Any idea where I
    could get this also? Thanks a ton as always.
     
  2. hr(bob)

    hr(bob) Guest

    Do you mean the crt filaments are glowing or the phosphor end/display?
     
  3. Guest

    Always hated working on those. Never had a print, they are like $100.
    One shop where I worked had a print for a much older Pioneer but I
    don't even have that now.

    I think you better be more careful with that power supply or you're
    going to burn it down. I am a bit familiar with them, and I do fix
    them, but it is time consuming. Difficult to reverse engiuneer and
    access is poor.

    First of all, some Pioneers use a deflection regulator and the B+ you
    read at the HOT is not always the main B+, same with the HVOT. I am
    not sure about the PRO97, it may or may not have discrete HVOT, which
    is a seperate transistor driving the flyback. Years ago this almost
    became universal in the better PTVs but many have dropped it in favor
    of a complex algorithm to keep geometry using the pin modulator, even
    modifying vertical sweep dynamically to deal with the varying high
    voltage load.

    I don't recall what normal B+ is in these sets, but if there is a
    regulator after the PS going to the HOT, 119V seems about right.

    You have to examine and reverse engineer the thing. Whatever
    transformer the collector goes to, look for another power transistor
    going to another pin. If there is, DO NOT remove that darlington
    again. If the darlington was bad, it must have been leaky, because the
    last time I had one bad the chopper output went into self destruct
    immediately.

    In this one, it wound up shorted B to E, which for all practical
    purposes made it an open circuit. How it managed to survive in your
    case is beyond me, so don't push your luck.

    Without knowing what the B+ is supposed to be there are other
    indicators. (now I am giving out secrets here). For example, somewhere
    in the set find a 7812 regulator. Look for like 16V at the input pin.
    A 7812 is a good choice because they are usually fed direct from the
    main PS. A 7805 or 7809 on a run supply might be fed by a post
    regulated 12 or 9 volt source.

    Another main clue is the raster if you can see one. Is it too small ?
    Is it too big ? If too small you may indeed have low B+, but don't
    just jump to that conclusion. If the raster is too big you probably
    have low HV.The B+ really can't go all that high without triggering
    shutdown, but a lower HV, if the H scan is seperate, results in a
    bigger raster because lower HV = higher deflection sensitivity. If
    you've been in the business long enough to have seen a weak HV
    recifier tube in a TV, that is abundantly clear.

    So you say light dimly, can you turn up the G2 and hit a menu button
    or something that will invoke an on screen display ? That can give you
    at least an idea of the geometry even if you can't see normal video.

    So report back and we'll see what's up. If you only have like 13V
    going into a 7812 regulator, I'd say the main B+ is probably low. If
    the input voltage seems plausible, like at least 14.5V or so, turn
    your attention to the HV or deflection post regulators.

    And see if you can get any idea if the raster is bent out of shape or
    out of size.

    If HOT and HVOT are seperate, a loss of voltage to either will dim it.
    A deficiency in the HOT will cause low filament voltage, and the
    raster should be too narrow. A deficiency in the HVOT circuit will
    also dim it down, but the raster will be too big, and it should be
    equally horizontally and vertically, usually equally. But don't take
    that to the bank because of dynamic geometry correction. They may
    still use it even with discrete HOT and HVOT.

    And, don't take that darlington out again, I think you are damn lucky
    not to have a shorted chopper and a blown fuse. I might not know what
    the B+ should be, but it certainly shouldn't be 160V. No way it is
    over 140V.

    Here's another little thing might work, look at the pots on the power
    board. They might be sealed, glued up or melted to prevent tampering,
    but they might be labelled. If one of them says "140V adj" or "135V
    adj" that'll tell you. In fact it occurred to me that it should be
    135V. I am almost sure, but this is strictly from memory.But that is
    not necessarily fed directly to the HOT or HVOT. If you had 160V at
    either you are one fucky lucker to have it still fire up.

    Good luck, you'll need it. Last time I had to work on one of those
    power supplies it took time. Damn nightmare. In this case the
    darlington was shorted E to B, and I disregarded the in circuit
    reading because I saw a low value resistor across it. Only by lifting
    the pin did I find out.

    And there is one more little nail in your coffin on these. If you
    bring it up slow on a variac, it won't start. And it won't start with
    a lightbulb ballast either. (another secret for another time)

    So, I don't think you have a problem in the main PS, but if you did
    and fixed it without wasting $100 worth of transistors, I would
    personally write you a diploma. What a bitch, especially without a
    print.

    JURB
     
  4. Golf

    Golf Guest

    I will try to see what the raster looks like and post a follow up when
    I get back to the set. I recall checking voltage on a diode off one of
    the flyback windings. I could get the raster on all three tubes to
    flicker brightly every time I touched one particular side of this
    diode with the probe. I resolder everything.
     
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