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Acoustic B20 with blown fuse...

Discussion in 'Amps [BG]' started by handjyve, Mar 9, 2012.

  1. handjyve

    handjyve

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    I recently acquired an Acoustic B20 that was not working. I opened the cabinet to reveal a blown fuse. I thought great this should be an easy fix. I initially replaced the blown T315mA with a .315A from Radio Shack only to find that these fuses were blowing instantly. After more research I found that the "T" meant slow blow or time delay and that the fuses I had gotten were fast acting. So back to Radio Shack. This I would later realize was where I really goofed. I mistakenly got T3.15A fuses. Put one in and vuala! The amp fired up..... For about a minute until one of the caps started to smoke and ooze. So it appears by putting the larger fuse in I defeated the purpose of the fuse and allowed the problem to damage the next weakest link. The strange thing is that the amp still works. For how long I don't know. Can anyone advise me on what the problem might be and if and how it could be fixed??
  2. BassmanPaul

    BassmanPaul Gold Supporting Member

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    that cap will need to be replaced for a start. You should also look at the rectifiers feeding it to check if one is shorted.
  3. RickenBoogie

    RickenBoogie

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    Usually, a blown fuse is an indicator that something else is amiss, and that's WHY the fuse blew. Not the other way 'round. That amp isn't worth too much repair expense.
  4. handjyve

    handjyve

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    Thank you guys.
  5. T-Bird

    T-Bird

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    Hi.

    ^What Paul said.

    If Your net question is going to be: "what's a rectifier, and how do I test it?", please do yourself a favour and either donate the amp to someone who has some spare time for non-profitable tinkering, or read one of the hundreds of on-line el. ed. courses.

    And as always when poking around mains operated equipment: BE SAFE.

    Regards
    Sam
  6. KWthewanderer

    KWthewanderer

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    hello handjive!
    Did you manage to find the problem?

    Hope you didn't let that previous clown discourage
    you with it's sad, pseudo-superior backdoor
    be-littlement & public forum abuse of you AND
    your legitimate plea for help on this electrical issue.

    I just bought one of these Acoustic B20s used,
    and it sounds just great!
    So I wanted to know if you saw something in there
    that I can avoid, like maybe it became to hot,
    or it was on for to long at once?
    I really don't want mine to meltdown...
    :)
  7. Thor

    Thor Back. And grumpier than ever. Supporting Member

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    Did someone call me. Clown? Really? Did you want to reconsider what you recently posted on a 6 month old thread or would you like to own that one full time? This is your chance to redact that post a smidgeon.
  8. mech

    mech

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    Totally agree with Thor. Anything that plugs into a wall socket can and will kill you dead if you don't know what you are doing and observe safety precautions. Subing the 3.15A fuse for an .315A is and example. Although electrolytic capacitors are supposed to fail in a safe, non-violent manner, they have been known to explode and the guy could have easily been blinded. The "clown" is the person who tells someone to go ahead and do something hazardous when it's apparent the OP lacks basic electronics knowledge. Electrical problems with 120VAC powered units are not for the amateur. Tear into the guts of your bass all you want. It won't kill you.

    mech
  9. xxx666

    xxx666

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    Disclosures:
    Endorsing Artist G&L Guitars
    Mega troll post right there - looks like an account was opened especially...
  10. T-Bird

    T-Bird

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    Hi.

    The "clown" here ;).

    It's just that I for one cringe every single time when a person asks a question about something that's obviously way over their heads to fix safely, and some do-gooder urges them to try to fix it it anyway. Be that problem electrical or mechanical in nature, mind You.


    Wall voltage can be lethal, and should not be toyed with.

    That said, I know no-one who's died while working on an amp but several persons, Yours truly included, have burnt a nerve or two while doing some stupid mistake for a reason or another.
    Stupid mistakes that we do have known how to avoid, but perhaps we were in a hurry, tired, exhausted or our minds just weren't in the work ahead of us like it should've been.

    An uneducated person usually doesn't even have a clue how to avoid getting zapped, so they're more likely to get zapped than not when poking in a live circuit.



    The burnt nerve will hinder Your playing ability for a while, or forever.
    It may also stop You playing for good.

    To me that'd probably be worse than death. And I'm pretty old, with no delusions about a career in music anymore.

    For an aspiring young musician, a damage like that could mean the end of his/her dreams and probably a severe hit to their livelihood as well.


    The same goes for us wrenches fixing our rides or other machinery, crack a knuckle with a slipping cheapo open end wrench, and Your playing ain't the same anymore. EVER.


    The above rant isn't in no way to discourage youngsters from pursuing their dreams in being a tech or a wrench -be that professionally or as a hobby- far from that, but PLEASE, do that SAFELY.
    Preserve Your hands, after all those are all that You got.

    Regards
    Sam
  11. B-string

    B-string Supporting Member

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    Music equipment is called equipment, not toys. Toys are relatively safe things to "toy" with. If you want to get in there and play with them then get the education first. At the very least the basics. I encourage anyone to as techs are a disappearing breed.
    I know this is an old thread. just adding to the above.
    I would imagine the amp suffered more serious damage from the improper fuse.
  12. ptdorris

    ptdorris

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    I had the same problem with my Acoustic B20 - blown fuse. The problem was caused by a bad capacitor (C11 - 2200uF @ 25 V) in the power supply (PS) circuit.

    The capacitors are used to smooth out the DC voltage rectified by four diodes (D8, D9, D10 and D11 - 1N4003). If the capacitor shorts out, too much current is passed to ground.

    As long as the rated fuse (T315 315mA 250V) is installed the fuse will blow. If a larger fuse was installed, then the fuse may no longer be the weakest link and something else is going to blow. As others have stated it was the defective capacitor.

    All four diodes in the PS circuit appear to be fine so I will be ordering new capacitors.

    Yes, replace both capacitors. The problem appears to be caused by either a batch of bad capacitors that made their way into this amp or the ripple rating of the capacitors is insufficient for the design.

    For those interested in the effects of ripple current on capacitors check the following link and go to page 10.

    http://www.cde.com/catalogs/AEappGUIDE.pdf

    You can see what "may" have caused your B20 to fail.

    For comparative purposes, here is the info from my circuit board:
    Date: 22 Nov 2007
    Rev: E
    Eng: J. Albert-LWJ

    Here is a picture showing the defective capacitor and the location on the circuit board.

    [​IMG]

    Regards,

    Paul T. Dorris

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