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Help Fixing my Amp? Only Works With Headphones.

Discussion in 'Amps and Cabs [BG]' started by SoupCann, Aug 22, 2017.


  1. SoupCann

    SoupCann

    Feb 3, 2017
    Hey Talkbass. Had this account for a while but never really had much to post. Until now!

    My Marshall B25 MII (It's a combo amp) has recently stopped working unless I plug headphones into the headphone jack. If I don't do that, no sound comes from the amp at all, even when it's on with the volume at max and the headphones unplugged.

    I've tested my bass, cables and pedals and all of it works fine, so I know it's the amp itself.
    Searching online didn't really give me a concrete answer of what was wrong or how to fix it, I got everything from spray air into the jack to take the amp apart and resolder everything.

    Has anyone experienced this before and do they know how to fix it? This amp has been with me throughout my bass playing and it'd be a shame if I couldn't keep it up and running. Thanks in advance everyone.
     
  2. If the headphone jack is fed from the power amp’s output, it could be that the contacts that close when the headphone plug is withdrawn are corroded and in need of cleaning.
     
    Bob Lee (QSC) likes this.
  3. SoupCann

    SoupCann

    Feb 3, 2017
    Thank you. Any idea how I can remove the corrosion?
     
  4. If you can get to the switch without taking too much apart a tiny drop of Deoxit should clean it up.
     
    Haans likes this.
  5. SoupCann

    SoupCann

    Feb 3, 2017
    Not 100% sure where the switch is. Is it just behind the jack?
     
  6. Haans

    Haans Altruistic nihilist Supporting Member

    Oct 31, 2005
    Bergen, Norway
    Probably a switching jack, so you could spray some electronics cleaner in there and just insert and remove the jack plug a few times. Repeat if necessary.
     
    Aqualung60 likes this.
  7. SoupCann

    SoupCann

    Feb 3, 2017
    I might try that. I'll see if I can find a good electronics cleaner in my house somewhere.
     
  8. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    You might search on my warnings and suggestions about using "contact cleaners" before attempting your "repair". Fortunately it's not an expensive amp, but learning before is better than after the fact.
     
    Haans, Spidey2112, ezra1 and 4 others like this.
  9. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    It isn't an electronics cleaner which is typically a solvent, deoxit is something else, an oxidization cleaner.

    This is a fairly common problem seen in amps from many different companies.either it is oxidized contacts or a bad solder joint.

    The jack has a switch (called a shunt) built into it. When the headphones are plugged in, the switch opens turning off the speaker. Remove the plug and the switch closes connecting the speaker. If the switch contacts have oxidization on them, the contact closing the speaker circuit is not made. The oxidation acts like an insulator.

    It needs to be cleaned fully for the contact to be 100% conductive.

    Sometimes you can get a partial cleaning by cycling the headphone plug in and out of the jack in rapid succession many times (say 10-15 cycles). The bouncing can knock some of the oxidization loose. Worth a shot but chances are it will not work. Even if this works, the jack should still be inspected and cleaned properly. Worst case, the jack might need to be replaced.

    When you use a spray, you risk getting it into the electronics.

    Take it to a tech, it is a short job and cost should be minimal. They will know what to use and exactly how to apply it.
     
  10. SoupCann

    SoupCann

    Feb 3, 2017
    That seems like a much better idea than doing it myself. Thanks man.
     
    beans-on-toast likes this.
  11. Hand slap

    Hand slap

    Feb 14, 2016
    Contacts on the jack, or Bad connections, where it is soldered to the board, easy fix, either way.
     
  12. Spidey2112

    Spidey2112

    Aug 3, 2016
    When did the problem start? Gradual, or boom, there it is? Anything unusual happen prior to symptom rearing it's ugly head?
     
  13. Tim Skaggs

    Tim Skaggs

    Sep 28, 2002
    Fried speaker?
     
  14. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    A possibility certainly. Fried, or damaged in another way.
     
  15. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Yes, the speaker wires should be disconnected and the speaker tested.
     
  16. Spidey2112

    Spidey2112

    Aug 3, 2016
    I'm calling the headphone jack... when I troubleshoot, I go for simple first, and in my mind (believe me, none of you want in there) I can picture the prongs of the headphone jack (nothing inserted) being stuck in such a position, so as to "simulate" a normal condition of no output from the speaker and it's corresponding circuitry (when the plug is actually inserted)... I'd take a peek.
     
  17. Are the wires to the speaker secure? I once had an amp that wouldn't produce sound, turned out one of the speaker wires became disconnected at the speaker end.
     

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