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Need Help w/ Ampeg BA210SP - blows the fuse, the output is bad

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  • Need Help w/ Ampeg BA210SP - blows the fuse, the output is bad

    hi everyone,

    I need help w/ an Ampeg BA210SP:
    While adjusting the BIAS something went wrong and now it blows the fuse each time it powers on. The amp was working before my 'intervention'. ;]
    Power.PDF = this is the schematic for the Power module.

    So far I have located 4 bad transistors: Q2, Q5, Q7 and Q8. (part no: IRFP9140N [Q2 & Q7] and IRFP140N [Q5 & Q8])

    First question:
    I have found that there are newer versions of the IRFP140N.
    Here is the link and the datasheet of the original part: IRFP140N International Rectifier
    This is a newer version (there are several): IRFP4410Z International Rectifier

    Should I go for the new version, or just get the original part?

    Next question:
    What other components should I check? Is there a real danger of damaging the new parts once installed? (The BIAS was turned CCW just a touch too much, the damaged occurred in about 1-2 sec after that.)

    *I have very basic knowledge/experience... just enough to get me in trouble... ;]

    Thank you in advance for your time and patience.

  • #2
    Welcome to the place. Is there a new version of the 9140 as well? If not, I'd stick with the original version.

    How were you monitoring current draw while you were adjusting the bias pot?


    • #3
      thank you. the 9140 does not have a newer version.

      I did not monitor the current in any way. My multimeter is in no way calibrated so I just went ahead and gave it a try given the circumstances -it had a 'distortion' when cold, and also the 330ohms resistors would get VERY hot very quickly. Anyways, I turned the trim pot CW very little (I would say 0.5mm) and the sound cleared... I turned it back where it was, and the 'distortion' came back. It was at this point that I got curious and went CCW a little more, just so I could hear the change in sound for future references ;]

      This is my first bass amp, got it on craigslist in bad shape. Lots of cold solder, a few bad connectors/wiring, dirty pots, etc. All the problems went away, except it would add like a distortion/overdrive sound when cold and at very low volume. If played loud, or just left the power on for a while, it would clear out -almost completely... I knew it wasn't the preamp and I was somewhat familiar w/ this issue on other amps in the past, or so I thought... I was planning to replace the BIAS trim pot w/ a sealed multi-turn trim pot knowing that dirt and corrosion can mess up the adjustment in time... So that's the whole story of the dead amp.
      Click image for larger version

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      • #4
        1) Why oh WHY did you mess with the Bias?
        Doubly so "by ear" , not measuring.
        Triple so because bias adjustment procedure is clearly spelled out in the schematic, and any multimeter is enough, following their instructions.
        Oh well

        2) Ok, damage is done, let's go ahead with repair.

        a) If Mouser (or ON Semi) suggest a replacement, I'd trust them .
        b) I'm quite certain that the actual transistor die is exactly the same , but they modified package or leads or tinning bath (and probably internal soldering) so as to meet ROHS `"no lead" rules.
        It will certainly work and sound like the original one, so use the one you get.
        3) build and use a lamp bulb limiter, until you are 110% certain bias is right.
        Light Bulb Limiter

        4) agree with replacing bias pot with a new safer one.

        5) those 330 ohms resistors, not shown on the schematic, are probably the dropping resistors for the preamp supply and it's normal they work hot.
        Juan Manuel Fahey


        • #5
          @J M Fahey
          thank you!

          is it the same thing if I use the 'light bulb limiter' instead of the fuse?

          *given the small amount of turn I planned for the Bias, I did not expect such a damage in such a short time... ;[ but who knows what's the condition of the taper on that trim pot...


          • #6
            Your photo of the board reminded me of the goop factor on these amps. You adjust the bias by setting a voltage across one of the resistors that usually has goop all over the leads, making it hard to do unless you remove the goop.

            And I think that you are supposed to set the bias with no speaker load.


            • #7
              Correct on both counts.
              Have a look at the Ashdown evolution, from new guy in the block (the way everybody starts) to presumed Magic Stars Mojo.
              Old schematics with all values printed > no values schematics > no schematic, just "replace R38 with 1K5" > goop everywhere .
              Oh well.

              A couple years ago, an old Tech mentioned "I used to repair Prince´s amps and guitars ... way back when he still had a name .... "
              Juan Manuel Fahey


              • #8
                So connecting a light bulb limiter where the main fuse would be, serves the purpose. The light bulb serves the purpose of a fuse too, auto-reset fuse that is.... I understand there is no chance of frying anything if the limiter is used, with or without a fuse in place.

                @52 Bill:
                based on your experience, is there a way to clear that goop mess other than mechanical 'force'?

                that is correct, the schematic states how the BIAS is supposed to be adjusted.
                But in my naivety, I thought making a little change to it just to figure if the sound would clear out , wouldn't hurt anything. And it worked, until the curiosity killed the cat...


                • #9
                  Juan, yes the 330 ohm are zener dropping resistors.

                  The goop is there for mechanical reliability, those big resistors will shake themselves loose otherwise. I recommend only removing enough goop to access a voltage reading. I can't tell from the photo if that is silicone rubber or hot-melt. Silicone, you can slice away some of it with a razor blade or an Xacto. Hot melt, if you spray it with freeze spray, it gets very brittle while cold. and you can shatter it away.
                  Education is what you're left with after you have forgotten what you have learned.


                  • #10
                    thank you.
                    It is silicone.
                    I do not want to clean it just for the sake of it, I understand what's the purpose of the goop. I have used silicone myself on some of the caps...

                    As of right now, I am waiting for the parts to arrive. 4 of each, just in case... ;] The repair shops around wouldn't sell me any parts, so 'online' was the only option.


                    • #11
                      Here is an upload of the Ampeg BA-210SP w/ the added distortion/overdrive noise for the interested:

                      It can be heard at min 1 that above a certain volume the sound is almost completely clear and it gets distorted when it falls under a certain level.
                      The details are in the description of the video.


                      • #12
                        It seems like R72 and R75 are nicely masked by the silicone... Instead of digging thru the goop trying to locate them, I would prefer measuring the BIAS voltage on some other more accessible/visible points.
                        I want to be sure that it would be alright to measure the voltage on the 'S' pins of Q5 and Q7.
                        See the attached picture for a quick look.
                        thank you.

                        UPDATE: The Q1 & Q4 are also bad. I am going to replace Q3 too...

                        Click image for larger version

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                        • #13
                          To be clear, we are not looking for the voltage on the S pin, we are looking for the voltage across the resistors. it is not necessary to measure it right on the resistor legs, the S pin is fine for ONE end of the resistor, but then the other end connects to one of the 50v rails. For example the S pin end of R75 is fine, the other end is +50, which can be found in the power supply or on the ends of R34 or D6.
                          Education is what you're left with after you have forgotten what you have learned.


                          • #14
                            thank you Enzo!


                            • #15
                              Ouch!! Just watched YT
                              Horrible sound, sounds worse than underbiased, if I'd had it on my bench I'd scope it right away, to rule out low level instability/oscillation, go figure.
                              That said, underbiased switching Mosfets are way worse than any other underbiased active device, because all others have less gain or show nonlinearity at low values, but it can be somewhat compensated by NFB.
                              While switchers, below a couple volts are *off* , plain and simple , nothing NFB can correct.

                              I use switchers because of availability and price but my big problem was always biasing.
                              Besides that, transconductance is so monstrous that a few mV extra can make them pass *amperes* while idle.
                              Tricky to bias, sorry.

                              Use a lamp bulb limiter until you are 100% happy with results, it's very easy to make expensive errors.
                              Juan Manuel Fahey


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