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    Sega Genesis 1 - No mono audio output

    Not sure if anyone will be able to help me with this, found my old sega genesis and hooked it up, video is fine but I have no audio out from either the RF or SCART connectors but the front headphone jack works.

    I found a schematic but I'm not too familiar with how audio works, it looks like the mono output goes through a LM358, could this be bad?

    The attached image doesn't zoom, here's a direct link to it:
    http://cgfm2.emuviews.com/img/mega3.gif
    Attached Files
    Last edited by caphair; 01-01-2014, 10:13 AM.

    #2
    Re: Sega Genesis 1 - No mono audio output

    First thing I would check is the switch in the headphone jack. This can go bad sometimes and cause issues like this. The switch contacts get oxidised and don't make connection very well.

    Is it possible that you could get that schematic picture in a higher resolution? Because it is hard - no, impossible to read the part numbers and values
    Muh-soggy-knee

    Comment


      #3
      Re: Sega Genesis 1 - No mono audio output

      I posted a direct link to it which is much clearer/bigger.

      here it is again:

      http://cgfm2.emuviews.com/img/mega3.gif


      How would I clean the switch? Can I just spray something in or does it need to be taken apart? Thanks

      Comment


        #4
        Re: Sega Genesis 1 - No mono audio output

        Looking at the schematic, it doesn't look like the headphone jack has a switch, looks like there are two sound chips. One for stereo going to headphone and another going to mono.

        I could be wrong?

        Comment


          #5
          Re: Sega Genesis 1 - No mono audio output

          Yeah, my apologies.

          There doesn't appear to be a switch in the headphones jack.

          Well, yes, the LM358 could be bad, or the chip IC13 could be bad.

          You probably could try connecting an audio amplifier and speaker to the output of the LM358, and see if you can get any sound. If you can, then I'd suspect IC13 is bad. However, perhaps the LM358 may be passing audio, but not enough for IC13. Check the feedback resistor in the LM358 amp circuit and make sure it is ok.
          Muh-soggy-knee

          Comment


            #6
            Re: Sega Genesis 1 - No mono audio output

            Originally posted by ben7 View Post
            Yeah, my apologies.

            There doesn't appear to be a switch in the headphones jack.

            Well, yes, the LM358 could be bad, or the chip IC13 could be bad.

            You probably could try connecting an audio amplifier and speaker to the output of the LM358, and see if you can get any sound. If you can, then I'd suspect IC13 is bad. However, perhaps the LM358 may be passing audio, but not enough for IC13. Check the feedback resistor in the LM358 amp circuit and make sure it is ok.
            Are you referring to R74 33K? It reads 32K

            I'm not exactly sure how I could connect an amp and speaker to it's output. Any voltage readings I could try?

            Also I looked up the LM358 and it's a two stage op amp, looks like only one stage is being used, could I try connecting inputs to it's second stage to see if I get audio?

            Comment


              #7
              Re: Sega Genesis 1 - No mono audio output

              inject at c30.bet its open.had a few in here for that.

              Comment


                #8
                Re: Sega Genesis 1 - No mono audio output

                Originally posted by kc8adu View Post
                inject at c30.bet its open.had a few in here for that.
                Already tried swapping that cap and still no audio, though I only had on hand a 10uf 25v one and original is 10uf 50v. Not sure if that matters? It was for a quick test

                Comment


                  #9
                  Re: Sega Genesis 1 - No mono audio output

                  Can someone help me understand how this circuit works, are LM358 and IC12 both amplifiers for the sound coming from IC11?

                  If so, why does the output of LM358 go into IC13 rather than just straight out to RF/SCART outputs?

                  Comment


                    #10
                    Re: Sega Genesis 1 - No mono audio output

                    Yes, they are both amplifiers. IC12 is the headphone amplifier, and the LM358 produces an amplified mono signal. The datasheet for IC13 (CXA1145) says that it only has a buffer inside for the audio section, so it should pass the input signal directly through without altering it.

                    C31 could be bad as well.
                    Last edited by lti; 01-02-2014, 09:55 AM.

                    Comment


                      #11
                      Re: Sega Genesis 1 - No mono audio output

                      Originally posted by lti View Post
                      Yes, they are both amplifiers. IC12 is the headphone amplifier, and the LM358 produces an amplified mono signal. The datasheet for IC13 (CXA1145) says that it only has a buffer inside for the audio section, so it should pass the input signal directly through without altering it.

                      C31 could be bad as well.
                      Awesome. Thanks. By buffer what does that mean exactly? Also anyway I can pinpoint where audio is lost with just a multimeter?

                      C31 looks like it's only before the SCART connection which also doesn't produce audio.

                      Could I run some wires from output of LM358 to the audio pin of RF module bypassing IC13 to see if I get audio out?

                      Comment


                        #12
                        Re: Sega Genesis 1 - No mono audio output

                        IC13 CXA1145 spec:
                        http://pdf1.alldatasheet.com/datashe.../CXA1145P.html
                        http://console5.com/wiki/CXA1145
                        The LM358 is probably does not have enough power to drive the modulator module and the SCART so the output of the LM358 is fed into the buffer amp inside the IC13. LM358 is used as a mixer for Left and Right signals.
                        Last edited by budm; 01-02-2014, 10:07 AM.
                        Never stop learning
                        Basic LCD TV and Monitor troubleshooting guides.
                        http://www.badcaps.net/forum/showthr...956#post305956

                        Voltage Regulator (LDO) testing:
                        http://www.badcaps.net/forum/showthr...999#post300999

                        Inverter testing using old CFL:
                        http://www.badcaps.net/forum/showthr...er+testing+cfl

                        Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                        http://s807.photobucket.com/user/budm/library/

                        TV Factory reset codes listing:
                        http://www.badcaps.net/forum/showthread.php?t=24809

                        Comment


                          #13
                          Re: Sega Genesis 1 - No mono audio output

                          Originally posted by budm View Post
                          IC13 CXA1145 spec:
                          http://pdf1.alldatasheet.com/datashe.../CXA1145P.html
                          http://console5.com/wiki/CXA1145
                          The LM358 is probably does not have enough power to drive the modulator module and the SCART so the output of the LM358 is fed into the buffer amp inside the IC13. LM358 is used as a mixer for Left and Right signals.
                          Cool I get it now. So I couldn't simply try to bypass ic13 to see if I get sound and determine it's the problem?

                          Comment


                            #14
                            Re: Sega Genesis 1 - No mono audio output

                            You can lift C30 positive leg off the board, C31 positive leg off the board, then tie those two positive legs of those 2 caps together and see if you will get sound on the SCART.
                            Never stop learning
                            Basic LCD TV and Monitor troubleshooting guides.
                            http://www.badcaps.net/forum/showthr...956#post305956

                            Voltage Regulator (LDO) testing:
                            http://www.badcaps.net/forum/showthr...999#post300999

                            Inverter testing using old CFL:
                            http://www.badcaps.net/forum/showthr...er+testing+cfl

                            Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                            http://s807.photobucket.com/user/budm/library/

                            TV Factory reset codes listing:
                            http://www.badcaps.net/forum/showthread.php?t=24809

                            Comment


                              #15
                              Re: Sega Genesis 1 - No mono audio output

                              Originally posted by budm View Post
                              You can lift C30 positive leg off the board, C31 positive leg off the board, then tie those two positive legs of those 2 caps together and see if you will get sound on the SCART.
                              By doing that will it boost the sound from LM358 enough to output sound?

                              Comment


                                #16
                                Re: Sega Genesis 1 - No mono audio output

                                We just want to verify that LM358 is putting out audio or if it is dead, you should hear the sound, if not, then LM358 is bad not the buffer amp problem in the IC13. Doing as I indicated above will bypass the IC13 to see where the problem is.
                                Never stop learning
                                Basic LCD TV and Monitor troubleshooting guides.
                                http://www.badcaps.net/forum/showthr...956#post305956

                                Voltage Regulator (LDO) testing:
                                http://www.badcaps.net/forum/showthr...999#post300999

                                Inverter testing using old CFL:
                                http://www.badcaps.net/forum/showthr...er+testing+cfl

                                Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                                http://s807.photobucket.com/user/budm/library/

                                TV Factory reset codes listing:
                                http://www.badcaps.net/forum/showthread.php?t=24809

                                Comment


                                  #17
                                  Re: Sega Genesis 1 - No mono audio output

                                  Originally posted by budm View Post
                                  We just want to verify that LM358 is putting out audio or if it is dead, you should hear the sound, if not, then LM358 is bad not the buffer amp problem in the IC13. Doing as I indicated above will bypass the IC13 to see where the problem is.
                                  Ok I'm just trying to learn as I go along. In this case what would tying those two caps do as oppose to just going straight from LM538's output to SCART?

                                  I'll try it regardless just curious.
                                  Last edited by ohmmy; 01-02-2014, 05:05 PM.

                                  Comment


                                    #18
                                    Re: Sega Genesis 1 - No mono audio output

                                    I have you lift the leg for easy way to try out, If it is SMD cap, then that will be problem, we just need to take the output of the LM358 and feed it to coupling cap and to not allow the DC at the output of the LM358 feeding into the receiving circuit, at the same time we must disconnect the input and output of IC13 from being connected to the circuit. The output pin of IC 13 has very low impedance so you cannot leave it connected so if you just jump the output of the LM358 to the SCART, the output of IC13 will load down the circuit.
                                    Last edited by budm; 01-02-2014, 05:17 PM.
                                    Never stop learning
                                    Basic LCD TV and Monitor troubleshooting guides.
                                    http://www.badcaps.net/forum/showthr...956#post305956

                                    Voltage Regulator (LDO) testing:
                                    http://www.badcaps.net/forum/showthr...999#post300999

                                    Inverter testing using old CFL:
                                    http://www.badcaps.net/forum/showthr...er+testing+cfl

                                    Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                                    http://s807.photobucket.com/user/budm/library/

                                    TV Factory reset codes listing:
                                    http://www.badcaps.net/forum/showthread.php?t=24809

                                    Comment


                                      #19
                                      Re: Sega Genesis 1 - No mono audio output

                                      Ok, tied them two caps together and still no audio from SCART.

                                      So I can conclude the LM358 is defective?
                                      Attached Files

                                      Comment


                                        #20
                                        Re: Sega Genesis 1 - No mono audio output

                                        Sounds like it if you do not even hear anything at all. Another test is to check the DC voltage at the output pin of the LM358, it should be around 1/2 of the Vcc power supply voltage.
                                        Last edited by budm; 01-03-2014, 07:52 PM.
                                        Never stop learning
                                        Basic LCD TV and Monitor troubleshooting guides.
                                        http://www.badcaps.net/forum/showthr...956#post305956

                                        Voltage Regulator (LDO) testing:
                                        http://www.badcaps.net/forum/showthr...999#post300999

                                        Inverter testing using old CFL:
                                        http://www.badcaps.net/forum/showthr...er+testing+cfl

                                        Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                                        http://s807.photobucket.com/user/budm/library/

                                        TV Factory reset codes listing:
                                        http://www.badcaps.net/forum/showthread.php?t=24809

                                        Comment

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