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Thread: Earth Module 440 Bass "Producer" Head (i.e Peavey Musician 400) Problem

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    Earth Module 440 Bass "Producer" Head (i.e Peavey Musician 400) Problem

    Hi all, I'm working on a Earth Module 440 Bass Head, direct rip-off of the Peavey Musician 400. When I first powered up, it hummed very loudly, then blew the fuse. I replaced the fuse, powered up, and now it's quiet. No sound at all. I measured no DC at the speaker jack, I've been using a schematic for the Peavey Musician 400 to get into the ballpark.

    What would cause the hum to stop and no longer blow fuses? I suspect a power transistor, but I'd like to hear from more experienced folk.

    I appreciate the help, thanks!

  2. #2
    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by JerkyPudding View Post
    What would cause the hum to stop and no longer blow fuses? I suspect a power transistor, but I'd like to hear from more experienced folk.
    Right, a shorted output transistor would be a top suspect. What usually happens is that transistor serves rail voltage (plus or minus what, 60V or so?) straight to the speaker, with lots of hum riding on it. That big DC jolt isn't good for a speaker, perhaps the one you had plugged into the amp is now toast. I hope not, but you could test the speaker real fast with an ohm meter, do the battery "pop" test, or plug it into a known working amp.

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    Quote Originally Posted by Leo_Gnardo View Post
    Right, a shorted output transistor would be a top suspect. What usually happens is that transistor serves rail voltage (plus or minus what, 60V or so?) straight to the speaker, with lots of hum riding on it. That big DC jolt isn't good for a speaker, perhaps the one you had plugged into the amp is now toast. I hope not, but you could test the speaker real fast with an ohm meter, do the battery "pop" test, or plug it into a known working amp.
    Yeah, it's running in the upper 40v range. That thought crossed my mind, so I tested the speaker earlier, it's OK, it also has a fuse in the cabinet. Lesson learned, regardless. I'm still rather green with the solid state amps. Would an open resistor on the likely culprit transistor have caused the hum to stop?

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    It could also be that you have a shorted output transistor and it burned open the emitter resistor that connects it to the circuit. Now that the resistor has acted like a fuse, the amp will power up to some extent without blowing the line fuse.

    Additionally, are there any internal power supply fuses in there?

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    Quote Originally Posted by 52 Bill View Post
    It could also be that you have a shorted output transistor and it burned open the emitter resistor that connects it to the circuit. Now that the resistor has acted like a fuse, the amp will power up to some extent without blowing the line fuse.

    Additionally, are there any internal power supply fuses in there?
    No, there is only the main fuse.

  6. #6
    Senior Member Enzo's Avatar
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    Try not to focus on what made teh one problem shift into anothwer problem. Just fix what is wrong.

    Work with no speaker on it until we know it is stable and not producing DC. Check power supplies, no circuit works right without proper power supply.

    Any shorted transistor you find, always check any associated resistors for opens.
    Education is what you're left with after you have forgotten what you have learned.

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