No announcement yet.

Bass Master XM100 Type:100B

  • Filter
  • Time
  • Show
Clear All
new posts

  • Bass Master XM100 Type:100B

    I’ve been trying to work on and repair this amp for nearly a month, I had some help stating it’s usually the output stage and told to check the transistors.

    I’m still new to components and components level repair, so I’m getting in pretty deep here, but don’t worry I know my way around high voltage and most electronics.

    working my way back from checking TP1/TP2 I get the correct voltage with the unit off that is mentioned in the service manual, I replaced 3 burned out resistors and soldered in jumper wire to its neighbor where two pads were burned away.

    R55 gets extremely hot shortly after being powered on, so I know there is still a problem in the output stage.

    The way I received the amp was no output from speaker, headphones low volume and distorted.

    attaching the service manual link and the results I gathered from testing the transistors, a few I tested in place and got good readings with my multimeter, the others base lead removed and two I tried with the collector removed because I didn’t realize at first it was different, though the reading was the same.

    I think several are bad, just need to confirm it so I can order the parts and see what happens, also of note to mention the fuse seems to have been replaced with a higher amperage.
    should be 1.6A, but is 5A (if I recall properly).

    Service manual:

    copying my unanswered post from another forum.

    Finally got around to trying to test the transistors Q1 seems good, Q2 passed exactly right, I think these three could be bad Q3,Q4,Q10,Q9.
    Will update after placing the collector leg for Q9,Q10 and pulling the base leg, unless it doesn't matter and someone comments.

    Followed this video for testing.

    Collector leg removed from circuit.
    CB +/- 2.343v
    EB +/- 2.343v

    BC +/- .565v
    BE +/- .565v

    CE +/- .002v
    EC +/- .002v

    CB +/- .600v
    EB +/- .598v

    BC +/- 2.735v
    BE +/- 2.735v

    CE +/- .002v
    EC +/- .002v

    Base leg removed from circuit.
    CB +/- 2.352v
    EB +/- 2.351v

    BC +/- .562v
    BE +/- .564v

    CE +/- .002v
    EC +/- .002v

    CB +/- .598v
    EB +/- .596v

    BC +/- 2.739v
    BE +/- 2.739v

    CE +/- .002v
    EC +/- .002v

    Q3 (Q5 passed while in place, same transistor) Definitely bad?
    CB +/- .695v
    EB +/- .701v

    BC +/- 2.535v
    BE +/- 2.951v

    CB +/- .565v
    EB +/- .240v

    BC +/- .565v
    BE +/- .240v

    Q1 (Seems good)
    SG +/- .717v
    SD +/- .018v

    GS +/- OL
    DS +/- 0.18v


    Stripe + 2.853 continues to build
    Stripe - .570

    Stripe + 2.853 continues to build
    Stripe - .570

    D9,D10 start at a low voltage and build up following the same testing.

  • #2
    Please say which resistors you replaced (that were burnt) and which trace connections were burnt. This will give a better idea of where there can be more damage.
    Your measurements show Q9 and Q10 both shorted from C to E (as you said C was disconnected from circuit).
    For reliability you should also replace Q2 and Q4.
    Do not connect any speaker or load until you are sure the amp is fixed and there is no DC voltage at the output.


    • #3
      Thanks for answering.

      R55,R57,R59 were all burned.
      R55 and R59 had failed pads, luckily near a neighbor that shared a trace.
      R55 jumped to R51.
      R59 jumped to C33.

      See attachment.


      • #4
        "R55 gets extremely hot shortly after being powered on" = Q4 faulty.
        Support for Fender, Marshall, Mesa, VOX and many more.


        • #5
          Anything else I should test?

          So far replace Q9, Q10, Q2 and Q4, after power on and test?


          • #6
            Those are what are most likely to be damaged.
            Change them and using a variac, monitor the quiexcent current as you slowly wind the mains up with no load/speaker connected.
            If the DC voltage is around zero and the quiescent current is between 18 and 20mv, you have fixed it.
            I presume the track was damaged removing the components.
            Support for Fender, Marshall, Mesa, VOX and many more.


            • #7
              I do not have or have access to a variac, at most I could make something similar with maybe a rheostat.
              Only other option available to me is to touch R55 and as @g1 said check that there is no dc voltage on the output and I am sure that I should replace the fuse that's preset with a stock rated fuse.


              • #8
                No you can't rig up a rheostat for the AC line. You can however build a 'dim bulb tester' or 'lamp limiter' fairly easily.
                And yes, make sure to install the correct fuse. The 5A in there probably caused the damage to be worse than it would have been with correct fuse value.


                • #9
                  I thought a variac was something entirely different, this is cool and simple to build.
                  I'll put one of those dim bulb testers together and test with it in place.
                  Jon Snell the pad was damaged from the faulty parts I would imagine, I didn't spend too much time with my iron on them and they were sucked away cleanly as if the resistor leg was all there was left holding the joint together.


                  • #10
                    4X the proper fuse size turns a $60 repair into a $200 one.
                    Juan Manuel Fahey


                    • #11
                      I'm aware of that problem, it's how it came to me and I suspect it was the usual 'it keeps blowing 1.6a fuses, this will fix that problem' solution, I would have expected damage elsewhere, but the output stage seems to be the only fully affected.
                      Anyone know of a website with 1.6a slow blow 6mmx32mm in stock? Nothing available locally, closest I can find is 1.5a and I'm not silly enough to pay $8 for one fuse on amazon.

                      I don't doubt .1a could be a huge difference, but would it be that much? Only other I can find locally is 2a.


                      • #12
                        mouser, digikey, etc. will have those fuses, but you will have to pay shipping costs. That is not considered a metric fuse, you will find it as .25 x 1.25 inch.
                        But I think a 1.5A slow there should be fine.


                        • #13
                          Thanks, I can pick one of the 1.5a easily for a couple dollars in a local shop.
                          Hopefully the other parts will fix it, I have a 4x12 waiting to be used.


                          • #14
                            Most if not all of the original parts are discontinued and I've no idea where else to look,, and do not have some of these, they do have a G series of some and out of others.
                            Where or how can I find proper substitutions?

                            Part: QTY:
                            MJH11017 x1
                            MJH11018 x1
                            BC550C x1
                            BC560C x2
                            2N5638 x1
                            BD139 x1


                            • #15
                              Try this source:
                              It queries all of the big parts houses.