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newbie question - burnt resistor

Discussion in 'Troubleshooting and Repair' started by kneedeepinit, Nov 24, 2013.

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  1. kneedeepinit

    kneedeepinit

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    0
    Nov 24, 2013
    I purchased a Sansui G 5500 which was not working. I bought it for the face plate and cabinet for another working unit. I would like to get it functional.

    On the Power Amplifier Circuit Board, there is a completely burnt resistor ( R46) which has also surface burned the two neighboring resistors. My understanding of resistors is that they rarely fail, something causes them to fail.

    Looking at the schematic, Transistor TR20 is right next to it. The middle leg of the transistor is directly connected to the burned resistor. Transistor failure seems to be far more common. Should I be looking for something else that cause the bad transistor or just replace it?

    My question is - should I just replace the transistor while replacing the resistor? Should I replace the neighboring resistors that look bad but may not have internal damage?

    The transistor part number is 2sa992 and they seem pretty cheap on ebay. I am hoping my local electronic shop has them.
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,270
    Nov 28, 2011
    Hi there and welcome to Electronics Point :)

    I downloaded the Sansui G-5500 service manual (from hifi-manuals.com) and I can't find either R46 or TR20. The transistors in the power supply section are numbered 600 upwards.

    Without seeing a schematic I can't say whether you just need to replace the resistor and the transistor, or whether there may be another fault somewhere else that has caused the resistor to overheat.

    When you do replace them, remove all signs of overheating from the circuit board. This may require you to actually cut or drill all the way through the board. Charred fibreglass can be conductive, and failing to remove all of it can cause problems later.

    2SA992 is not a common transistor; it has unusual specifications (120V Vce(max), typical current gain 500) and you will probably have to replace it with the same part number.
     
  3. kneedeepinit

    kneedeepinit

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    Nov 24, 2013
    thank you for the advice on cleaning the burnt debris.

    on page 4-4 of the manual, board F 2980.

    TR20 is located in column A, between rows 1 and 2
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,270
    Nov 28, 2011
    Ah, gotcha. It looks like TR20 is supposed to detect overcurrent in the output stage. It monitors the voltage across R72, which is in the emitter circuit of the NPN output transistor.

    Check R72 and R78. They are supposed to be 0.33 ohms. If they read much higher than that, some (maybe all) of the output stage semiconductors are probably damaged. Check for collector-emitter shorts on TR601 and TR602, then TR16/18 and TR08/10. The fault may even have damaged TR06, TR04 and TR02, and maybe even FET02. D601 may also be damaged; disconnect one end and measure it with a diode test range - it should measure around 2V.

    TR20 is probably not damaged but you might want to replace it anyway.

    R46 is a 2.2k resistor. But don't replace it until we know what has happened to the output stage.
     
  5. kneedeepinit

    kneedeepinit

    3
    0
    Nov 24, 2013
    thank you, I now have a plan of action. It did not make sense to me that a resistor would just burn without something else being the cause. I now have a list of components which ar emore likely to fail on their own.

    I will try to get at it this week and post the results.
     
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