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    Explain how this circuit works...

    This is from a Marantz SR5012. The HDMI output dies (tv goes to No Signal) after about one minute. It works fine until then.

    I'm losing my 5v at the HDMI output connector when this happens. The voltage drops slowly (over maybe 10 seconds or so?) From 5.2v and the port drops out around 2v but the voltage continues to zero.

    The 5v coming in from the left is direct from the SMPS. The output from the MOSFET goes to the HDMI port.

    Will take measurements in the morning but was wondering in layman's terms how this circuit works ? Thanks
    Attached Files
    Last edited by mmartell; 12-04-2018, 01:04 AM.

    #2
    Re: Explain how this circuit works...

    Looks like it's a power control with foldback circuit to limit damage if the output is shorted.

    First question to ask is if CEC_POWER connected to the transistor's base is enabled or not. If that signal goes low, power will shut off.

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      #3
      Re: Explain how this circuit works...

      I'm going to meter the circuit soon and will be able to see where it's failing. But I still don't understand how the circuit does what it does and why. Obviously the CEC_POWER line is used to turn on/off the MOSFET but how is that accomplished ? And since the 5v input is used to power other circuits are at least some of the resistors used to limit current to the HDMI port ?

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        #4
        Re: Explain how this circuit works...

        That set of parallel 2R7 (2.7 ohm) resistors, equivalent to 0.9 ohm, are used as a current shunt.

        When the voltage drop across those reaches around 0.6v (corresponding to a current of about 670mA, that'll make Q1032 start conducting, and thus pulling Q1058's (P-channel MOSFET) gate (voltage) closer to its source. That will cause Q1058 to stop conducting (once its gate-to-source voltage drops below its threshold).

        CEC_POWER would need to be on / high in order to get Q1058 to conduct (by pulling its gate to ground through R1777).

        https://cdn.badcaps-static.com/pdfs/...8f35bcef54.pdf
        Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

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          #5
          Re: Explain how this circuit works...

          Thanks for that. I know this is child's play for you but could you dumb that down even further ? I'm trying to see the logic but can't get it.

          On a practical note I've tried measuring Q1058 and can see 5.2v at the source and 5.2v at the drain when conducting but when I try to measure the gate it's like it's discharging towards zero every time I put my probe on it ? Also, at times when the circuit has failed (CEC_POWER=HI, DRAIN=0v) simply trying to measure the gate turns the mosfet back on.

          ?

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            #6
            Re: Explain how this circuit works...

            Is it normal that I can't get an accurate voltage reading on the gate ? The voltage seems to be draining through the meter to ground.
            Last edited by mmartell; 12-05-2018, 02:38 PM.

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              #7
              Re: Explain how this circuit works...

              Is R1777 healthy (ie. measures 10k, as it should)? And are the traces around it intact?

              Might be worth redoing all the solder joints in that area, they're only a few.

              PS: When CEC_POWER is high, at least in theory, you should have 5v on the drain (pin 4 of Q1058). Double-check the datasheet to make sure the pin numbers are what you think they are.

              https://cdn.badcaps-static.com/pdfs/...03ed271716.pdf
              Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

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                #8
                Re: Explain how this circuit works...

                Pin 4 is Source pin. For whatever reason the image is rotated from its schematic representation in the datasheet..

                I can't measure R1777 in situ it's on the bottom of the board. Board looks new topside anyway.

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                  #9
                  Re: Explain how this circuit works...

                  Yes I get 5v on the drain pin the problem is the gate starts charging by itself and when it crosses the threshold the mosfet turns off and the 5v to the port disappears. All this while CEC_Power is high.

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                    #10
                    Re: Explain how this circuit works...

                    You'd want to measure R1777 with the device powered off and unplugged from the mains, anyway (ie. not while the circuit's active).

                    What resistance can you measure between the Q1058 drain (1-2-5-6) and ground?

                    And after that, can you measure the voltage drop between the 5v input and Q1058's source (pin 4), from power-up (when all is normal) until it dies out?

                    If there's no short-circuit on the output (either C1746, or something else downstream), then Q1057's starting to look suspicious...
                    Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

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                      #11
                      Re: Explain how this circuit works...

                      Can't measure R1777 it's on the bottom side of the board. If I have to remove the board I would likely replace Q1032 and Q1057 at that time. Seems a waste to reassemble it to run another test as it can't be run properly without full assembly due to the interconnects.

                      Q1058 Drain-Ground is 2.7k ohm.

                      Not sure what you're asking to measure re voltage drop ? 5.2v is always present at the Source pin.

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                        #12
                        Re: Explain how this circuit works...

                        Voltage drop = the voltage between the two ends of R1807 / R1808 / R1811.

                        "Can't be bothered to remove the board" is nowhere near "impossible", sorry.

                        I know it's ideal to only have to "go in" once, but it doesn't always work out that way - that's part and parcel of troubleshooting.

                        If we were looking for the easiest way to remedy this issue, i'm sure that "buy a new device altogether" would've been the first choice
                        Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

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                          #13
                          Re: Explain how this circuit works...

                          It looks like an open resistor or trace to the MOSFET's gate.

                          Can you beep out (continuity check) Q1058 G pin 3 to C1747 or Q1032 etc. I would also check G-S (pin 3-4 guess 20k).

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                            #14
                            Re: Explain how this circuit works...

                            I never said I couldn't be bothered, only wanted to eliminate any topside tests before doing so.

                            Voltage drop across those resistors is 0.009v

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                              #15
                              Re: Explain how this circuit works...

                              I have continuity from Gate to C1747.

                              G-S resistance is 20M ohm not kohm.

                              Does Drain to Ground sound right at about 2k ohm ?

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