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Fender M80 bass amp distortion on output

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  • Fender M80 bass amp distortion on output

    Hi everyone.
    I recently got an old dead Fender M80 bass amp. After examination, I concluded the output transistors were blown. I couldn't find that exact models, so I replaced them with equivalents. Now there is a sound coming from the amp, but it is extremely unpleasant and distorted. Can anyone please provide some assistance?
    Thank you in advance.

    P.S. If schematics are required: http://www.pmfst.hr/~zvodom/m80.pdf
    I can also post oscilloscope traces if needed.

  • #2
    First things first.
    Disconnect the speaker.
    You do not need a load until the amp is functioning.
    Is the output waveform clean without a load hooked up?
    Is there any measurable Volts dc on the output?

    Monitor the signal at the FX Send jack.
    Is it o/k?

    If the output section was blown you will have to check everything in that section.
    Especially the resistors & signal diodes in the failure path.

    There are plenty of voltages marked on the schematic. Compare them to observed voltages.
    Is the power supply checking good? Full Vdc , no Vac ripple? Including the + - 16 volts.

    What replacement transistors did you use?
    Did you replace the driver transistors?

    Comment


    • #3
      Now I have nothing on the output. Quick tracing of the signal path reveals that the signal is now lost on U1. Also, I have a high frequency noise on pin 2 of U1. Which means something is wrong with the preamp, right?
      I didn't bother to check FX Send, since there is nothing on the output of U1.
      Power supply:
      50V rails are at +-48VDC, with about 0.2VAC ripple. that's good, right?
      16V rails are at +-16.1VDC, no VAC ripple.
      DC test points 10, 16 and 17 are a bit off, about 0.5V. Points 8 and 9 are way off.

      I replaced 2SA1306/2SC3298 pair with 2SA1930/2SC5171 and 2SA1302/2SC3281 pairs with 2SA1943/2SC5200. I also changed all of the 2N4401 and 2N4403 transistors.
      I found a place online that sells exactly those devices I need, so I'll order them as soon as I get some money...

      Comment


      • #4
        Okay, I managed to get the signal past the U1. Something got desoldered. But now I see another problem. Mid part of the EQ section is dead. Specifically, the part around U3B and U2A. What intrigues me the most is the shape of the signal at TestPoint4. Here's a shot of an oscilloscope screen.
        Click image for larger version

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        Any input on this?
        Thanks.

        Comment


        • #5
          That is an amp oscillating hard at high frequencies - way above audio. RF.

          Look on the schematic over by the output to speakers. See R118,R119 in parallel? 47 ohm 2w each. Are they open?

          Scope your power supply rails both the 15v ones and the main powr amp ones. Is any of that RF on any of the power supplies?
          Education is what you're left with after you have forgotten what you have learned.

          Comment


          • #6
            R118 and R119 are inside the tolerance limits. Could those really cause RF in the EQ section of the preamp? I mean, the only connection between them and the ICs is ground... Also, I don't see RF anywhere else in the circuit, which eliminates the power supply, right? I scoped it anyway. Main rails do have some ripple on them, some 0.5V, but that's normal, right? 16V rails are dead silent. No RF on either of these.

            Comment


            • #7
              I was just starting at the start. Not being there, I can;t assume anything. And it was possible you had discovered the RF oscillation in the preamp, not realizing it had come in through the power rails. If those two resistors had been open, the amp would be sitting there trying to transmit radio.

              If the oscillation is localized to one stage, make sure none of the paths to ground are open - for example a gain pot or something similar. And try replacing the op amp in that stage.
              Education is what you're left with after you have forgotten what you have learned.

              Comment


              • #8
                Ok, checked all paths to ground with an ohmmeter, all seem fine. I have a few spare TL072s, all of them do the same thing when connected in this stage. I also tried removing R13 to break the signal path before the problematic op-amp just in case RF was coming from there, no use. Disconnection of C11 also yields no results. But, when I disconnected R19 and effectively broke the feedback loop, the oscillations stopped. However, the signal is then severely clipped. What would happen if the value of R19 changed?

                Comment


                • #9
                  Bump.
                  Anything? Enzo?
                  I tried to experiment a bit on my own. I took out the R19 and put a 100Kohm pot instead, just to see what happens when I change the value of R19. I found that all oscillations cease when R19 is above 24Kohms. Which I find strange, since the original R19 is 22Kohms. I also think that the frequency is changing, but I can't be sure, my equipment isn't very precise.
                  Could those oscillations be a product of some parasitic capacitances that, together with R19, form an RC circuit which is then amplified through the op-amp?

                  Comment


                  • #10
                    beats me, I'd be more inclined to think some little stability cap is missing, or some wire dress is wrong.

                    As a wise man once said, "When you see hoofprints in the dirt, think horses, not zebras."
                    Education is what you're left with after you have forgotten what you have learned.

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                    • #11
                      Any change to the frequency or amplitude of the oscillation by adjusting R14? I assume the volume pot R12 has not affect if you have isolated the problem to U2-3. If the R14 pot gangs do not have an affect, I would look for one either the low side of the the pot being open. If the volume control, R12, does control the amplitude of the oscillation your conclusion of the problem being in the mid-boost and cut filter is in error. and the problem is further back towards the front end.

                      Comment


                      • #12
                        Okay, this is odd. I did a bit more experimenting and found:
                        1. R12 has no effect on the amplitude of oscillations
                        2. R14(mid EQ pot) and R25(high EQ pot) DO have an effect
                        3. disconnecting or grounding of pin 5 of U3 stops the oscillations

                        Third point leads me to conclusion that I was wrong about the localization asumption made before, right? But what I cannot understand is: if grounding of pin5 stops the oscillations, RF is coming from somewhere before the mid EQ stage. But why can't I see it anywhere else?

                        Comment


                        • #13
                          Originally posted by zvir View Post
                          But what I cannot understand is: if grounding of pin5 stops the oscillations, RF is coming from somewhere before the mid EQ stage. But why can't I see it anywhere else?
                          Or it is being generated within the EQ stage.
                          Have you replaced U2 & U3?
                          I would snoop around that section searching for any caps that are maybe leaking Vdc.

                          Comment


                          • #14
                            Yes, I have. I also replaced every single component in the mid EQ section. No effect. Also, no Vdc anywhere in the circuit, except, of course, on pins 4 and 8 of U2 and U3.

                            Comment


                            • #15
                              Hi there
                              I have exactly the same issue with my M80 Bass : distorted output. I've checked all the test points with the speaker disconnected as described in Fender's schematics, everything is quite right (0.3VDC max difference). I followed sound path thru the preamp, everything is OK. I only get distorted waveform at TP 18, so it's not the preamp.
                              I recorded a little sample via line output, it sounds the same with the speaker : http://notsocomplicated.org/share/ampli_moisi.wav


                              We can see all types of distorsion here...

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