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Thread: Fender Jazzmaster Ultralight - Power amp board

  1. #1
    Member lalunatech's Avatar
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    Fender Jazzmaster Ultralight - Power amp board

    Hiya all
    I have a Fender Jazzmaster Ultralight with a faulty power amp board. It looks like someone has had a go at it already. The eight power mosfets mounted (in a horrible way) under the PCB have no insulation from the heatsink, all their tabs are connected together. They are all the same N-Channel IRFZ24N which do not have isolated tabs. This is a mystery because I cannot see how it would work with high side and low side devices with their drain terminals shorted. I will try Fender for a schematic. Has anyone worked on one of these before? Has someone substituted an incorrect mosfet then given up on the repair before I got it?

    Thanks.
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  2. #2
    Senior Member Enzo's Avatar
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    Do a search on this forum for that schematic, I think we posted it somewhere.
    Education is what you're left with after you have forgotten what you have learned.

  3. #3
    Member lalunatech's Avatar
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    Enzo
    It is OK, I emailed Fender last night and had them in my inbox this morning. I wish all manufacturers were so accommodating.
    BTW the power amp board (probably all the boards) are a discontinued Fender part now and they told me they don't have old stock in any of their warehouses.
    I normally don't attempt repairs on these horrible surface mount D-Class amp boards because they usually end in failure but I had a slack day and thought what the hell!
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  4. #4
    Member lalunatech's Avatar
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    Turns out the D-Class amp driver chip is faulty. It is just a piece of Jazz scrap now.
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  5. #5
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    Would you care to share the schematic please?

    Something does not add up to me, mosfet type with mounting type you mentioned and power figure to be specific.

    What's the driver? IR2011, IR2110 or similar?

  6. #6
    Senior Member Enzo's Avatar
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    Education is what you're left with after you have forgotten what you have learned.

  7. #7
    Senior Member Enzo's Avatar
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    Here, try this.

    The driver IC directly controls both channel MOSFETs.
    Attached Files Attached Files
    Education is what you're left with after you have forgotten what you have learned.

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    Thanks a lot Enzo.

    And bloody hell, me so dislikes what me saw on the schem...

    And 250W, yeah, right on 2ohms load

  9. #9
    Supporting Member Jazz P Bass's Avatar
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    "The eight power mosfets mounted (in a horrible way) under the PCB have no insulation from the heatsink, all their tabs are connected together. They are all the same N-Channel IRFZ24N which do not have isolated tabs."

    That is not good.

    (4) of them should have the Source grounded.
    J M Fahey likes this.

  10. #10
    Old Timer J M Fahey's Avatar
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    Agree and add:
    It looks like someone has had a go at it already.
    You bet.
    It did not leave Factory that way.
    The eight power mosfets mounted (in a horrible way) under the PCB have no insulation from the heatsink, all their tabs are connected together.
    See above
    They are all the same N-Channel IRFZ24N which do not have isolated tabs. This is a mystery because I cannot see how it would work with high side and low side devices with their drain terminals shorted.
    It doesnŽt.

    Just peeked at bthe schematic , and that confirms my decision: I will never ever even try to repair such a "thing", period.

    Quite complicated, and it would be worth trying ... if the f*ck*ng thing cost, say, $3000 or $5000 so a used one would hover around, say, U$2000 so a $500 to $800 service charge would seem acceptable to owner.

    At U$800 over the counter new price, forget it.

    "hey .... but with that mentality eventually youŽll give up Servicing"
    ..... so what?
    What IŽd drop is unrewarding servicing ..... wonŽt miss it.

    IF I wanted to keep on, IŽd learn instead to service Avionis, Communications, Medical or Industrial stuff, even "smart house" Electronics , even "humble" modern car Electronics .... instead of dead end Musical Instrument stuff.

    Dead end because of automated manufacturing at virtual zero cost, which to boot lets designers complicate stuff as much as they want, in any case "it does not carry a cost penalty" so why not?
    Juan Manuel Fahey

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