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Please help with Repairing my Ampeg BA115!

Discussion in 'Amps and Cabs [BG]' started by psrdesigns, Dec 8, 2012.

  1. psrdesigns


    May 29, 2010
    Hello all, ive bought a Ampeg BA115 three years ago and i was jamming at 60% volume for 30-45 minutes last week and all of a sudden my audio cuts out. Ive driven this amp at max volume for over 3 hours on a hot summer day so i don't think heat was the issue. It is the BA115a i think, the 100-watt solid state combo, so its not the newer ones with the preamp tube for those who may not be familiar with the BA series.

    My brother has extensive electronics training, but nothing in the way of amplifiers and audio components.
    I will list some of the things we have done and tried already.

    1.We replaced the two MOSFETS in the "power stage" i believe he said, an OP-AMP integrated circuit chip that we saw some smoke come from, and also a resistor that started smoking. I can get the exact part numbers and values for the parts if need be, but there all generally in the same area.

    2.We used an oscilloscope to see if the signal (a guitar cable with a 1/4-1/8 adapter hooked up to an ipod) we were feeding it was going through and it seemed to be modulating right. But we still get no sound to come out of the speaker.

    3.Tested some other parts, the Transformer is putting out all the right voltages, and the speaker is fine, and the filter caps are good. I believe the DC Rectifier was also good. Oh and yes, the fuse is good as we have other 2.5 amp fuses laying around.

    This thing is like my baby and i don't have the money to get a whole new amp. Any suggestions would be greatly appreciated, we're running out of ideas on what to test. Besides seeing smoke come out of that OP-AMP and the discolored resistor, everything else looks all new and shiny.
    Please if there's any other information you need that might help you with helping me, please let me know, i'm pretty much begging here!
  2. If he used an o-scope on the amp and had signal in AND out (only louder) then the speaker or speaker wiring must be bad/defective.
  3. anderbass


    Dec 20, 2005
    Phoenix. Az.
    Check the headphone jack, its got switching contacts that mute the speaker, those contacts might just be dirty...
  4. With the scope you should be able to test for good signal at several points along the way from the input jack to the speaker wires. If you are testing by looking for the signal you put in with your ipod then did the signal cleanly appear at; the input of the first op-amp stage, the pre-amp out jack, the driver transistors, the output transistors, the Zobel network (if used), the speaker wires?

    Without some more info about where you loose the signal it is very hard to give good advice. How far your ipod's signal makes it through your amp before you can't find it on the signal path is how a tech might use the tools you have to narrow down the source of the trouble. Perseverance will lead to success. I own a BA-115 and like it a lot. The schematic is available online and with that try to locate your ipods signal (start by hooking the ipod to the scope to get an idea of what you are looking for while probing with the scope probe). Then look for that same signal at the points I suggested by referring to the schematic them find those points on your amp and look for the ipod signal. A signal generator with a signal tone at a constant level would be preferred over an ipods music that is constantly changing and therefore not very consistant for test purposes.
  5. I guess you could download some test tones from the web and then load them on the ipod, that might work better for this purpose.

    Here is a 50hz tone I found on you tube:

  6. psrdesigns


    May 29, 2010
    Yes i love my BA-115 alot also, its odd because when we put those new parts in we got what seems like line level out to this 5 inch speaker we are using to test with, but a few seconds later the audio cut and things started smoking again and this audio stopped.

    He told me he thinks it could be one of the transistors and two Zener Diodes. Ive linked the pdf to the schematic and we believe the problem is somewhere in the top right corner (the F 1-3 block of page 2 of the schematic). I guess that the 40 volts it should be getting isn't getting there and it might be those transistors not turning on and off or something. He also thinks that could be the cause of the MOSFETS getting so hot. After like 2 minutes of being on now they get really hot. So he thinks they might be taking the whole load since its not stepping down or up or what ever he said.

    Does any of this make sense?

    BA-115 Schematic