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Forward: a strange radio problem

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  • Dave C
    A friend owns a Mercedes and being pretty good at many things he did a fair diagnosis of the audio system s problem, but his solution didn t work. Any ideas I
    Message 1 of 5 , Oct 5, 2013
      A friend owns a Mercedes and being pretty good at many things he did a fair diagnosis of the audio system's problem, but his solution didn't work.

      Any ideas I can pass along to him?

      Thanks,
      Dave

      Sent from my phone.
      Please forgive auto-fill errors and typos.

      Begin forwarded message:

      The Becker radio-head in my '96 E320 (W210) has performed well through the turn of the century, but it developed a tic.  When I tried to turn the volume up or down, it lost the tendency to change in small, incremental steps.  Instead, the volume would make a series of random jumps, trying momentairily to blow out speakers or whisper with no predictable trajectory.

      It had to be a malfunciton the rotory switch that controls the volume.  It
      taps different switch contacts when you turn it left or right, causing
      digital pulses which a counted up or down as a binary number.  The resulting number is then passed to a D-to-A converter, and it becomes the digital approximation of an old-fashioned potentiometer.

      I carefully removed the offending component and soldered in a new one. Resassembled radio-head, installed it, and entered the code.

      No difference!  The thing still doesn't turn the volume up and down in a
      logical way. Puzzled, if fooled with the control, trying to at least shame
      it into operating properly, and I discovered something:  It works perfectly if I change the volume very slowly.  Click......Click.....Click.

      It acts as if the main processor is desperately trying to service an urgent interrupt request that is higher up on the priority chain.  Volume control is way down there, so it can only see to it occasionally as it struggles with something up high, and the real-time quality of the knob turning is out the window.

      A schematic will not help me figure this out.  Has anyone else seen this
      problem in an M-B radio?

      Jim
    • Dave C
      Another Mercedes owner has reported a similar -- but different -- symptom: he needs to turn the volume control many many times to get a change in volume. FYI,
      Message 2 of 5 , Oct 5, 2013
        Another Mercedes owner has reported a similar -- but different -- symptom: he needs to turn the volume control many many times to get a change in volume.

        FYI,
        Dave

        Sent from my phone.
        Please forgive auto-fill errors and typos.

        Begin forwarded message:

        A friend owns a Mercedes and being pretty good at many things he did a fair diagnosis of the audio system's problem, but his solution didn't work.

        Any ideas I can pass along to him?

        Thanks,
        Dave
      • Jan Kok
        Quite a puzzle! Is there a remote control cable, to allow changing station and volume from the steering wheel? Try disconnecting that and see if anything
        Message 3 of 5 , Oct 5, 2013
          Quite a puzzle!

          Is there a remote control cable, to allow changing station and volume from the steering wheel? Try disconnecting that and see if anything changes.

          The rotary volume control must be a quadrature encoder, in order for the CPU to be able to tell which direction the knob is turned. In other words, there are TWO switch contacts which produce this sequence as the knob is turned in one direction:

          contact A: 0 0 1 1 0 0 1 1 0 0 1 1 ...
          contact B: 0 1 1 0 0 1 1 0 0 1 1 0 ...

          Suppose the common connection got loose (bad connector, say). The code on the contacts could go from 0/0 to 1/1 or vice versa, and the microprocessor would think that the knob had turned, but wouldn't know which direction. Perhaps when the knob is turned slowly, there is not enough vibration or bending of boards, connectors, etc. to cause the loose connection, but faster turning does provoke the loose connection. Try tapping and wiggling the volume control in each of the four contact states, because only one of them, either 0/0 or 1/1 is subject to behaving erratically if the common connection gets loose.

          See what happens if the volume control is turned when the radio is switched off. Does the radio notice and adjust the volume when the radio is turned on?

          Check the voltages on the two contacts (A and B) and make sure the voltages go through the quadrature sequence shown above as expected.



          On Sat, Oct 5, 2013 at 10:57 AM, Dave C <davec2468@...> wrote:


          A friend owns a Mercedes and being pretty good at many things he did a fair diagnosis of the audio system's problem, but his solution didn't work.

          Any ideas I can pass along to him?

          Thanks,
          Dave

          Sent from my phone.
          Please forgive auto-fill errors and typos.

          Begin forwarded message:

          The Becker radio-head in my '96 E320 (W210) has performed well through the turn of the century, but it developed a tic.  When I tried to turn the volume up or down, it lost the tendency to change in small, incremental steps.  Instead, the volume would make a series of random jumps, trying momentairily to blow out speakers or whisper with no predictable trajectory.

          It had to be a malfunciton the rotory switch that controls the volume.  It
          taps different switch contacts when you turn it left or right, causing
          digital pulses which a counted up or down as a binary number.  The resulting number is then passed to a D-to-A converter, and it becomes the digital approximation of an old-fashioned potentiometer.

          I carefully removed the offending component and soldered in a new one. Resassembled radio-head, installed it, and entered the code.

          No difference!  The thing still doesn't turn the volume up and down in a
          logical way. Puzzled, if fooled with the control, trying to at least shame
          it into operating properly, and I discovered something:  It works perfectly if I change the volume very slowly.  Click......Click.....Click.

          It acts as if the main processor is desperately trying to service an urgent interrupt request that is higher up on the priority chain.  Volume control is way down there, so it can only see to it occasionally as it struggles with something up high, and the real-time quality of the knob turning is out the window.

          A schematic will not help me figure this out.  Has anyone else seen this
          problem in an M-B radio?

          Jim

        • Dave C
          No remote controls and no backup battery on the pcb. Since the only volume control is soldered to the PCB and recently replaced, I don t think connectors are
          Message 4 of 5 , Oct 6, 2013
            No remote controls and no backup battery on the pcb.

            Since the only volume control is soldered to the PCB and recently replaced, I don't think connectors are involved in this path.

            Are there dedicated quadrature controller ICs that handle such output from encoders? This could be a bad batch of such ICs?

            Other ideas?

            Thanks,
            Dave

            -=-=-=-

            On Oct 5, 2013, at 3:04 PM, Jan Kok wrote:

            Quite a puzzle!

            Is there a remote control cable, to allow changing station and volume from the steering wheel? Try disconnecting that and see if anything changes.

            The rotary volume control must be a quadrature encoder, in order for the CPU to be able to tell which direction the knob is turned. In other words, there are TWO switch contacts which produce this sequence as the knob is turned in one direction:

            contact A: 0 0 1 1 0 0 1 1 0 0 1 1 ...
            contact B: 0 1 1 0 0 1 1 0 0 1 1 0 ...

            Suppose the common connection got loose (bad connector, say). The code on the contacts could go from 0/0 to 1/1 or vice versa, and the microprocessor would think that the knob had turned, but wouldn't know which direction. Perhaps when the knob is turned slowly, there is not enough vibration or bending of boards, connectors, etc. to cause the loose connection, but faster turning does provoke the loose connection. Try tapping and wiggling the volume control in each of the four contact states, because only one of them, either 0/0 or 1/1 is subject to behaving erratically if the common connection gets loose.

            See what happens if the volume control is turned when the radio is switched off. Does the radio notice and adjust the volume when the radio is turned on?

            Check the voltages on the two contacts (A and B) and make sure the voltages go through the quadrature sequence shown above as expected.



            On Sat, Oct 5, 2013 at 10:57 AM, Dave C <davec2468@...> wrote:


            A friend owns a Mercedes and being pretty good at many things he did a fair diagnosis of the audio system's problem, but his solution didn't work.

            Any ideas I can pass along to him?

            Thanks,
            Dave

            Sent from my phone.
            Please forgive auto-fill errors and typos.

            Begin forwarded message:

            The Becker radio-head in my '96 E320 (W210) has performed well through the turn of the century, but it developed a tic.  When I tried to turn the volume up or down, it lost the tendency to change in small, incremental steps.  Instead, the volume would make a series of random jumps, trying momentairily to blow out speakers or whisper with no predictable trajectory.

            It had to be a malfunciton the rotory switch that controls the volume.  It
            taps different switch contacts when you turn it left or right, causing
            digital pulses which a counted up or down as a binary number.  The resulting number is then passed to a D-to-A converter, and it becomes the digital approximation of an old-fashioned potentiometer.

            I carefully removed the offending component and soldered in a new one. Resassembled radio-head, installed it, and entered the code.

            No difference!  The thing still doesn't turn the volume up and down in a
            logical way. Puzzled, if fooled with the control, trying to at least shame
            it into operating properly, and I discovered something:  It works perfectly if I change the volume very slowly.  Click......Click.....Click.

            It acts as if the main processor is desperately trying to service an urgent interrupt request that is higher up on the priority chain.  Volume control is way down there, so it can only see to it occasionally as it struggles with something up high, and the real-time quality of the knob turning is out the window.

            A schematic will not help me figure this out.  Has anyone else seen this
            problem in an M-B radio?

            Jim


          • Nathan McCorkle
            Does the volume switch actually rotate unlimited rotations in either direction, or do you turn it a few degrees right or left, and it springs back to center
            Message 5 of 5 , Oct 10, 2013
              Does the volume switch actually rotate unlimited rotations in either direction, or do you turn it a few degrees right or left, and it springs back to center when you let it go? If the latter, I'd say it's a problem in the counting circuit... look for a bulging cap or a resistor that's 0 ohms maybe... sounds like there could be ringing, so a busted filter or maybe snubber diode is what comes to my mind.


              On Sun, Oct 6, 2013 at 9:24 AM, Dave C <davec2468@...> wrote:
               

              No remote controls and no backup battery on the pcb.

              Since the only volume control is soldered to the PCB and recently replaced, I don't think connectors are involved in this path.

              Are there dedicated quadrature controller ICs that handle such output from encoders? This could be a bad batch of such ICs?

              Other ideas?

              Thanks,
              Dave

              -=-=-=-

              On Oct 5, 2013, at 3:04 PM, Jan Kok wrote:

              Quite a puzzle!

              Is there a remote control cable, to allow changing station and volume from the steering wheel? Try disconnecting that and see if anything changes.

              The rotary volume control must be a quadrature encoder, in order for the CPU to be able to tell which direction the knob is turned. In other words, there are TWO switch contacts which produce this sequence as the knob is turned in one direction:

              contact A: 0 0 1 1 0 0 1 1 0 0 1 1 ...
              contact B: 0 1 1 0 0 1 1 0 0 1 1 0 ...

              Suppose the common connection got loose (bad connector, say). The code on the contacts could go from 0/0 to 1/1 or vice versa, and the microprocessor would think that the knob had turned, but wouldn't know which direction. Perhaps when the knob is turned slowly, there is not enough vibration or bending of boards, connectors, etc. to cause the loose connection, but faster turning does provoke the loose connection. Try tapping and wiggling the volume control in each of the four contact states, because only one of them, either 0/0 or 1/1 is subject to behaving erratically if the common connection gets loose.

              See what happens if the volume control is turned when the radio is switched off. Does the radio notice and adjust the volume when the radio is turned on?

              Check the voltages on the two contacts (A and B) and make sure the voltages go through the quadrature sequence shown above as expected.



              On Sat, Oct 5, 2013 at 10:57 AM, Dave C <davec2468@...> wrote:


              A friend owns a Mercedes and being pretty good at many things he did a fair diagnosis of the audio system's problem, but his solution didn't work.

              Any ideas I can pass along to him?

              Thanks,
              Dave

              Sent from my phone.
              Please forgive auto-fill errors and typos.

              Begin forwarded message:

              The Becker radio-head in my '96 E320 (W210) has performed well through the turn of the century, but it developed a tic.  When I tried to turn the volume up or down, it lost the tendency to change in small, incremental steps.  Instead, the volume would make a series of random jumps, trying momentairily to blow out speakers or whisper with no predictable trajectory.

              It had to be a malfunciton the rotory switch that controls the volume.  It
              taps different switch contacts when you turn it left or right, causing
              digital pulses which a counted up or down as a binary number.  The resulting number is then passed to a D-to-A converter, and it becomes the digital approximation of an old-fashioned potentiometer.

              I carefully removed the offending component and soldered in a new one. Resassembled radio-head, installed it, and entered the code.

              No difference!  The thing still doesn't turn the volume up and down in a
              logical way. Puzzled, if fooled with the control, trying to at least shame
              it into operating properly, and I discovered something:  It works perfectly if I change the volume very slowly.  Click......Click.....Click.

              It acts as if the main processor is desperately trying to service an urgent interrupt request that is higher up on the priority chain.  Volume control is way down there, so it can only see to it occasionally as it struggles with something up high, and the real-time quality of the knob turning is out the window.

              A schematic will not help me figure this out.  Has anyone else seen this
              problem in an M-B radio?

              Jim





              --
              -Nathan
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