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Thread: Trace elliot gp7sm loud hum

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    Question Trace elliot gp7sm loud hum

    Hello, my first post here. I recently purchased a Trace Elliot bass amp head from kijiji. It was shipped to me and was listed as "working great". When it arrived it was definitely not working great. Most of the screws were missing. Someone had opened it up, tried to fix it and just barely put it back together. So I'm stuck with it and would like to try and fix it. I'm a total amateur but I have modded some guitar pedals and repaired a few amps in the past. Replaced capacitors, pots etc.
    So what the amp is doing is making a loud hum. Turning the amp up or down makes no difference, there is no sound from the guitar. Just a steady loud hum. It may get a little louder or quieter if left on for a minute. I also notice that one of the two mosfet gets hot the other doesn't. The direct out works fine. So the premap is good. Looking at the board I see one burnt 220 ohm resistor. . I also see one capacitor that looks a little swollen. . I've ordered some resistors, and thinking about ordering the capacitor. Could these problems cause the issue with the amp, or are they likely just a result of a larger problem?
    Just hoping someone could lead me in the right direction while I wait for parts. Thanks.

    The power board - https://imgur.com/a/U2oTs05

    Schematic... https://music-electronics-forum.com/...9&d=1434038910 (I think this is the correct one)

    Swollen cap - https://imgur.com/a/wZI9PYj

    Burnt resistor - https://imgur.com/a/5R3qac7?

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    Lifetime Member Enzo's Avatar
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    Does teh speaker cone move one direction and stay there? In other words is there DC on teh amp output? If yes to either, TURN IT OFF, you will melt your speaker - if not already done.

    Work with NO speaker load until we know the amp is stable and not making DC.

    The swollen C13 ought to be replaced, but it appears to just be the fan supply, so no audible difference.

    Resistors never burn up on their own, something else takes them down. Usually a transistor. Especially if some of your power transistors are getting hot. They aer very suspect.

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    Thanks for the reply. What's getting warm is the buzz905. I figured it should be hot and the cold one was maybe not working. I'll check for DC on the speaker output.

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    I'm reading 61 volts DC on the speaker out.

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    Supporting Member Jazz P Bass's Avatar
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    Quote Originally Posted by Rotti123 View Post
    I'm reading 61 volts DC on the speaker out.
    Ouch!

    + or - Vdc?

    Mosfets usually fail shorted.

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    Quote Originally Posted by Jazz P Bass View Post
    Ouch!

    + or - Vdc?

    Mosfets usually fail shorted.

    Just checked again. 59 Vdc positive and it slowly creeps up the longer its left on.

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    Supporting Member Jazz P Bass's Avatar
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    Pull out the mosfets.
    Possibly, at least one is shorted.

    The fact that R13 is burnt would indicate a lot of current passed through that section.
    So check Tr3, 4 & 5.

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    Thanks I will check them. These mosfets only have two pins. Im assuming the casing is another? Just trying to figure out how to test them. I've found some simple instructions but those mosfets all have three pins.

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  9. #9
    Supporting Member Jazz P Bass's Avatar
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    The BUZ mosfets indeed have three connections.
    The pins are the Gate & the Drain.
    The case itself is the third one. (Source)
    BUZ900.pdf

    As stated, mosfets typically fail 'shorted' from D to S. (or all three)

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    Perfect,. Thanks for the info

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    It looks like there are issues with both mosfets. One of them has continuity between all three pins. The other seems to be shorted between the source and either the gate or the drain.

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