Connected outputs with outputs.. broke my DAC.. help..

Discussion in 'General Electronics Chat' started by brontozaur, May 8, 2016.

  1. brontozaur

    Thread Starter New Member

    Dec 20, 2015
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    Hi All, I was wondering if someone could give me some advice. I was stupid enough to connect the RCA outputs of this DAC to the OUTPUTS of a preamp in a headphone amplifier. The result is that while the headphone amplifier seems fine, the DAC (in the pic) is powering up, but not working anymore. I've inspected it for visual burns, blown caps etc but couldn't find anything. What do you think would be the most probable cause of damage? Could it be the DAC chips (which are 8 in parallel, all removable - good old Philips TDA1543 on 8 legs in the second pic). I'm not well versed in electronics, but have a multimeter and soldering equipment. Is there something I could check for failure? Any advice would be appreciated..
    Valab_1.jpg Valab_1.jpg Valab_1.jpg Valab_2.jpg
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I would start by checking the supply voltages. Anyway, since there is no output buffer then I think the DACs are dead. As they are all parallel, you should be able to put in just one and see if any of them still works.
     
  3. brontozaur

    Thread Starter New Member

    Dec 20, 2015
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    Hmm, so I've checked the supply voltage, and it's 7.6v for all sockets. TDA1543 datasheet says input voltage min 3v, typical 5v, max 8v, so I guess it's not too high? I've tried removing all the chips and trying a few of them, with no avail. Could it be that I just fried the DAC chips, and that it would work if I replaced them with the new ones? How do you check if a chip is faulty?
     
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    7.6V is not a very typical votage. Could you take a few more shots of the bottom right side of the board, and of the bottom layer?
    Also it would be great to know the numbers on the two components on heatsinks, and of the square chip in the top right.
    I fear the power supply regulator might be toast, which could kill the rest of the stuff as well as the DACs.
     
  5. brontozaur

    Thread Starter New Member

    Dec 20, 2015
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    Thanks a lot for your help. I've attached a few more shots. The components under the two heatsinks are identical Linear LT1084CT. The square chip at the top right is C-Media CM108. On the bottom layer, there is also AMS1117. I've measured resistances of smd resistors and while some are correct, some are way off (lower), and also look a bit weird (values in the pic). Another resistor at the bottom layer must be 1.5k according to the marker on the pcb, but reads 875 Ohm.
     
  6. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    The two regulators are setup for 5V and 7.6V, so you should be able to measure 5V at one end of that 100R resistor near the regulator with the silver heatsink, and also on the top right pin on all those 74xx ICs.
    Don´t worry about the off values of the ressitors, since they are in circuit connected to other things those will skew your measurement.
     
  7. brontozaur

    Thread Starter New Member

    Dec 20, 2015
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    4.86V between one end of 100R and GND (which goes to the DAC board), and 4.9V between both top right 74xx ICs and GND.
    Does that mean that the regulators are fine? 7.6V is between V+ and GND which go to the DAC board.
     
  8. kubeek

    AAC Fanatic!

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    Yes that should be OK.
     
  9. brontozaur

    Thread Starter New Member

    Dec 20, 2015
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    So you think replacing the DAC chips could fix it?
     
  10. kubeek

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    Sep 20, 2005
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    Yes that should work. There is also a small chance that the DACs are ok but somehow they aren´t getting the data, however that is unlikely since you said you shorted the outputs.
    So I´d try getting new ones, check that the outputs are not shorted to anything else with the DACs out of the board, and then replace one to see if it starts working.
     
  11. brontozaur

    Thread Starter New Member

    Dec 20, 2015
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    Many thanks for your help. Might be a stupid question, but how do I check if the outputs are not shorted to anything else?
     
  12. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Set your multimeter to ohms range, and check if you can find anything that is connected to the ouptut trace. Anything below 1kohm would be suspicious.
     
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