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Ampeg PF-500 fuse problem

Discussion in 'Amps and Cabs [BG]' started by LumberjackTBird, Oct 22, 2012.

  1. LumberjackTBird


    Aug 5, 2012
    I had problems finding info on this, so I'm sharing it with anyone who needs it...

    After a couple months playing my pf-500 through my 4ohm 210 with no problems, I took it to my band's practice and plugged it into an 810, and it peed it's pants.

    Turned it on, and the fault light came on, line normal, but didn't turn off. I didn't notice for a couple minutes or so, we were all chatting and hadn't started playing. When I noticed, I shut it off, and switched it back on. Lights came on for a couple seconds, then a small pop sound, and all the lights, except the power switch, went off. No pop through the speakers, it came from the head. No burnt smell either. Sounded like a fuse popping.

    It was plugged into the same power conditioner as 2 guitar tube heads. Only problem I saw was the 1/4 in speaker cable wasn't fully inserted...

    So I took it home, popped the top, and pulled the fuse. I don't have a multimeter to test the fuse, so I went on the hunt for a new one. That wasn't easy....

    E-mailed ampeg, they said send it in. Its not warrantied, so I wanted to try to fix it before shelling out money to send it in. A few e-mails later, they gave me the part number, but I couldn't just order one from them... I e-mailed the local service department, which is 2 hours away, and they sent me a radio shack page, and the item was sold out. But I got the info I needed.

    Its a ceramic fuse, 5mm X 20mm 10A 250V - or the part number - FUSE SB 10A/250V 5X20MM UL CSA

    Service guy said any slow break fuse that size would work, but I thought it would be best to find a ceramic.

    Well, 2 radio shacks, 3 music stores, 3 electronic stores and I couldn't find one. Found one on eBay, where I should have looked to begin with.

    So the fuse is headed my way. I wanted to post this in case anyone else has this same problem, because I couldn't find any info on the fuse anywhere. Hopefully this will help someone out there down the line. I'll update when the new fuses get here, which can't be soon enough!
  2. hodgy


    May 5, 2004
    Bothell, WA
    Tech Support- Ampeg/ Mackie
    Generally if the fuse goes, then you've got bigger problems.
  3. LumberjackTBird


    Aug 5, 2012
    thats what everyone keeps telling me... I'm hoping not... but we will see...
  4. seamonkey


    Aug 6, 2004
    And you never tested the fuse to see if it is blown?
  5. oerk


    Oct 16, 2009
    This could be the reason for the fuse popping. A 1/4 inch plug can short circuit if not plugged in fully. If you're lucky the amp didn't suffer anymore damage.
  6. LumberjackTBird


    Aug 5, 2012
    I don't have a multimeter to test the fuse, again fuse is cheaper than multimeter haha
  7. LumberjackTBird


    Aug 5, 2012
    if nothing is damaged, I will never leave home without my speakon cable again haha
  8. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    That's odd...10 amp slo blo fuses are pretty common. I bought a bunch for my SVT at Radio Shack. Am I missing something here about this fuse that sets it apart from other 10 amp fuses or something?
  9. LumberjackTBird


    Aug 5, 2012
    dunno, they were just sold out in my area I guess...

    The fuses I ordered came in today, and sadly but not surprisingly as soon as I turned it on it blew another one. Calling a local amp tech tomorrow, hope this doesn't take too much time or money...
  10. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    That sucks, bro. Hope you can get it worked out for not too much cash.
  11. James Judson

    James Judson

    Jul 16, 2009
    Sometime fuses get tired and weak and blow for no apparent reason. Seen it happen many times. I recommend carrying a few spares cuz its a cheap test. If a second fuse blows you have a more serious problem and its time to take it to a tech.

    You are on the right track here. Let us know how much it cost and if the tech thinks the speaker cable had anything to do with it.
  12. LumberjackTBird


    Aug 5, 2012
    well, anyone want to buy a pf-500 shell?

    Didn't get to talk to the amp tech, but here are his notes...
    Found main switching transistors shorted, drive transistors shorted, PWR controller and many other surface mount components blown.

    Needs new main circuit board.

    Parts - 312
    labor - 40
    s/h - 12
    total - way more than the $260 I paid for it...
  13. Woodstockz


    Sep 23, 2011
    San Diego, Ca
    Check for PM.
  14. Woodstockz


    Sep 23, 2011
    San Diego, Ca
    Success!!! PF-500 brought back from the dead

    It's Alive!!!

    - The short circuit protection is too slow to protect the power transistors.
    - And the short circuit blew the power amp transistors.
    - And the shorted power amp transistors blew the power supply transistors.
    - And the shorted power supply transistors blew the fuse.
    - And the knee bone connected to the ankle bone.

    [For some reason, I can't upload pictures]
  15. arai

    arai Banned

    Jul 16, 2007
    Sounds like a design flaw.
    And lol at the knee bone joke
  16. Woodstockz


    Sep 23, 2011
    San Diego, Ca
    I'm glad that someone got the joke.

    The flaw is with the chips. Others that have used other chips from this family have noted that the over current protection is too slow to protect the power transistors from a direct short circuit.
  17. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    Your SVT uses standard 1.25 X O.25 fuses Jimmy. This amp uses the smaller type more popular in Europe. MCM or parts Express, among others, will have the required fuse in stock.
  18. Woodstockz


    Sep 23, 2011
    San Diego, Ca

    Attached Files:

  19. Good job fixing the disabled PF-500! After all the moaning about PF-500 fails, someone actually had the moxie to fix one themself. Kudos. :hyper:

    Can you tell us what the transistors are?
    What is the class-d chip(s) that are used?
    What do you think may have caused the short circuit?
    Do you have any affiliation with Ampeg/Loud?
  20. KrisHayes


    Sep 30, 2012
    Yup. Main board went in mine too. Thankfully still under warranty!