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Marantz 2275

Discussion in 'Electronic Repair' started by GregS, Jul 21, 2009.

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  1. GregS

    GregS Guest

    Wondering if anyone is familiar with this series. I was
    testing it out and the bias circuitry is not
    functioning. There is shown on the schematic a bias transistor
    along with one diode and pot.
    There is no bias transistor on the two stages, just looks
    like one diode. I intend on adding a diode or two to get it going.

    There is a place on the boards for the transistor, but it was never there.

  2. GregS

    GregS Guest

    Just to clarify. You can see the holes in the board for the transistor, but the copper
    is solid on the back plane. I don't figure how it could have worked right
    from the factory. The diode under the driver heatsink seems normal, about
    6 volt drop. I might need at least 4 diodes in addition to the one.
    H709 is MISSING.

  3. David Farber

    David Farber Guest

    I found a service bulletin from 10/29/1975 that may or may not have an
    effect on your problem. If your serial number is below US 7701 or Europe
    51001, replace diode H720 and diode H721 with a 1S1555. When you say the
    bias circuitry is not functioning, do you mean there is zero bias current?
    It is supposed to be 10mv from TP701-TP702 and TP703-TP704.

    Good luck.
  4. David Farber

    David Farber Guest

    If both channels are missing that transistor, and one of them works
    properly, then it probably was meant to be like that. You can see in the
    schematic that there is still a path from the base of H713 to the base of
    H714. If anything, H709 would lessen the voltage between the two points and
    decrease the bias even more. What is the reference number of the diode which
    you say has a 6 volt drop?
  5. GregS

    GregS Guest

    Neither channel has any current. point 6 .6 volt.
    I'll check on the diode specs.

  6. GregS

    GregS Guest

    H720 and H721 are for the bias of a couple stages back. I'll
    check that out too.

  7. GregS

    GregS Guest

    One mystery solved. There is a bias transistor. Its hidden under one driver
    heatsink. There are two positions for it on the board. One
    away from the heatsink. That position I first saw is vacant.

    So far a bad pot on one channel is the only thing I have found wrong.
    I still have to figure why the bias on the other channel is
    not working.

  8. One mystery solved. There is a bias transistor. It's hidden
    Doesn't each output stage have its own bias transistor?
  9. GregS

    GregS Guest

    Right. There are seaparate assemblies.

    I was fixing some Sansui integrateds some years ago. A particular series
    had a resistor that would change value and screw up the bias. It
    was an esy fix, except for the conductive glue on the power supply

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