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Acoustic B20 with blown fuse...

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

  1. handjyve


    Nov 30, 2007
    Long Island, NY
    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

    Aug 25, 2007
    Toronto Ontario Canada
    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


    Jul 22, 2007
    Dallas, TX
    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


    Nov 30, 2007
    Long Island, NY
    Thank you guys.
  5. 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.

  6. KWthewanderer


    Aug 10, 2008
    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 Moderator Staff Member Gold Supporting Member

    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 Supporting Member

    Jun 20, 2008
    Meridian, MS, USA
    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.

  9. xxx666


    Jan 12, 2008
    Endorsing Artist G&L Guitars
    Mega troll post right there - looks like an account was opened especially...
  10. 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.

  11. 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


    Mar 4, 2008
    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.


    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.



    Paul T. Dorris
  13. Wilbern "Butch" Lewis

    Wilbern "Butch" Lewis

    May 16, 2014
    I just bought a B20 today and found it not working. I ran across this site looking for information on how to access the "brain" as you call it. Several members complained about not being long enough speaker leads to allow removal of the brain. The secret to easily removing the brain is quite simple. After removing the four screws on the top of the cabinet remove the speaker grill (it's held with velcro) and then the six screws holding the speaker which will then come out giving more than enough room to reach in and disconnect the speaker leads from the speaker. Then feed the leads up through the hole six or eight inches (once removed from both ends, speaker and PCB) just leave them hanging in the cabinet and the removal of the head becomes a non-issue. No modifications, no soldering, easy. By the way, all the diodes checked good but the two large caps were bulging and leaking and the fuse was blown. Should be an easy fix.
    Hope this info helps.
    Butch Lewis
  14. JGbassman


    May 31, 2011
    Sorry to open up an and old post, but I too have the same issue with the blown fuse.

    I will take a look at the caps to see if there are any issues before I replace the fuse, as this has been a good little amp I've had for years and I'd like to keep it if it's not too expensive to fix. I've not done a lot of repair work but replacing a cap or two isn't out of my abilities. Anything further would be cause for a trip to the shop.

    Also regarding removing the electronics, I had attempted this already, but my speaker leads running to the sealed box were glued for an airtight fit and I'm going to have to remove that if I take the amp out of the cabinet. Just throwing that out there for others.
  15. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    Just make sure you do not damage the PCB traces. One slip and your PCB is ruined.
  16. JGbassman


    May 31, 2011
    I'm probably going to take it in, not that I couldn't do the work, I just don't know enough about amps to see if there were other issues going on as well. Trouble shooting amps, at least for me, is better left to the seasoned pros. I've done plenty of soldering regarding guitars, but pcb's are a little more delicate and not as resistive to heat. Plus I don't know which components to heat sink and all that jazz. I'd love to learn as I'm sure it's not out of my scope, but I'd want to learn by watching a pro.

    So thanks for the tips. I do have a question. If I replace the fuse, will that possibly cause other issues, or just try and see if it was a freak situation that blew the fuse. (wishful thinking here)

    I just want to make sure that it won't cause further or more extensive damage by replacing the fuse and applying current to the amp
  17. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    Fuses blow for a reason, that’s what they are there for. If one blows and you replace it and it blows again you can cause more damage. Taking it in is a good idea. :)
    JGbassman likes this.
  18. JGbassman


    May 31, 2011
    Thanks. 20 years ago I would have had a go at it myself, disregarded every shred of reasonable advice, and ended up tossing it in the bin after I would have destroyed it trying to fix it. (Thank goodness guitars are more resilient).

    Once again, thank you for your time and expertise. I will not disregard your wisdom. :)
    BassmanPaul likes this.
  19. Marty Hewes

    Marty Hewes

    Nov 20, 2014
    Woodstock Il
    I just got a dead B20 also. I found the fuse blown. I replaced the fuse with a slightly larger one (750mA, all I had handy), and before I could test it, one of the supply capacitors blew wide open. I've been a tech since 1973, I've seen a lot of caps fail and draw high current. The caps used in these B20's appear to have problems. I suspect the caps go leaky, start drawing more current, and the fuse blows. Put in a bigger fuse, and the caps might get hot enough to pop, like mine. Not too dangerous, that is why the top of the cap is scored, so it will split before the internal pressure gets dangerously high. Some of the bigger ones actually have a rubber plug that will blow out first.

    It's good policy to replace the power supply electrolytic caps whenever you service an amp that's got a few years on it, especially if they are not name brand caps. They tend to be a relatively high failure rate item. Use good, name brand parts. This is not a place to use cheap no-name parts if you want reliability. There's a reason 1960's Fenders are still going, and these B20s are all blowing fuses. Leo was a tech, he knew where money needed to be spent. He cared more about reliability than making an extra $0.50.

    Most likely, replacing the caps and the fuse will bring it back to life. BassmanPaul is right though, if you don't have PC board repair experience, practice up on a junk PC board first, it's fairly easy to lift a trace with too much heat. And remember that electrolytic caps have a polarity. Put them in backwards and they won't last long, and glue them down so the vibration won't break the solder joints.

  20. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    750mA is "slightly" larger? If you do the math, it's more than double. Falls into the not safe category in my book.

    I do not ever recommend replacing caps after a few years. In a properly designed amp they should last about 30 or more years. In an improperly designed amp, a failure is not a cap problem but a design problem.
    BassmanPaul likes this.