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Thread: Problems with Laney DB300C bass amp

  1. #1
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    Problems with Laney DB300C bass amp

    Hi, I'm the new rookie around here.
    Although my tech skills refers to tube amps, sometimes I get into the shop some SS amps for a fix. Most of the time it's a two hours job, but sometimes are a-hard-nut-to-crack job.
    This time is a Laney bass amp, model DB300C (schematics HERE)
    The amp came to me with a broken transistor (TS10), wich blew out the 5A fuse from the negative path. I have replaced it, and now It works fine.... for half an hour. Then it suddenly starts drawing current and one of the two 5A fuses blows up (sometimes the one from the positive path, sometimes the one from the negative path) I have measured voltages, everything's ok. Bias and offset are OK... resistors and transistors were all checked.... The fail happens with or without audio signal at the input. speakers are OK too.

    What kind of component will fail after, more or less, half an hour? electrolytic capacitors?

    Nice to meet you all.
    (...and sorry for my bad English language skills)

  2. #2
    Senior Member Enzo's Avatar
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    What do you mean the bias is OK?

    I would monitor the mains current draw. At idle it should not draw much current. Just a guess, but less than 30 watts I'd think. Watch it as it sits there. Does the mains current slowly rise until the fuses blow? Or does it suddenly take off.

    Try setting the bias colder too.
    Nickie likes this.
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  3. #3
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    Hi Enzo.
    In fact, mains idle current is something about 200ma, can't remember now, but it maintains stable. I've tried to "catch" the sudden current rise with an ammeter, but it happens really fast, in a matter of 2 seconds i would say.
    I'll try to set the bias colder, but it's actually set really low, 20ma.

    Thanks.

  4. #4
    Noodle of Reality Steve Conner's Avatar
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    A blown TS10 shouldn't cause the fuse to blow. There's probably some other fault that you missed.
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  5. #5
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    Hi Steve, you're right about that, the blown fuse was caused by a short-circuited capacitor (C3 10uF 100V), which was replaced.

  6. #6
    don't forget the joker g-one's Avatar
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    Here's the schematic (in case that link gets removed)
    laney_db300_sch.pdf
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    "Thank you. Now on this next one , ladies and gentlemen, I'd like you to pay attention to my tone - not so much my singing or the band... " - JP Lepage

  7. #7
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    Ok, this one was a tough fight, but the amp is working pretty fine now.
    The clue that led to the final solution was found in this same forum, here.
    The problem was TS4, a smd version of the MPSA92, but it can handle only 250mW and it works over the edge all the time (bad design there!). It was replaced by the TO-92 MPSA92, requiring a bit of craftsmanship, bending the legs and sticking with glue.

    Thank you all for helping me and being so cool.
    g-one likes this.

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