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Marantz SM500dc repair, help?

Discussion in 'Audio' started by mikefc55, Dec 13, 2018.

  1. mikefc55

    mikefc55

    6
    0
    Jan 8, 2017
    I'm stuck and a little help will be greatly appreciated. There is a negative 12vdc bias on the right channel. the diagram above is where the issue lies. With Q715 out of the circuit all voltages and transistor biases are correct. With it in the circuit the 12vdc results on the collector and therefore all the way through to the output transistors. It is not the transistor though as I have swapped left and right channel. All other transistors test good out of circuit and all base/emitter potentials are correct even when the error is showing. Likewise the difference across the two parts of the output resistor (0.47ohms) is correct also. R729 even has the same drop as it's mirror on left channel. (about 75vdc), the diode(s) between the collectors of Q711 and Q715 are good as well. C715 electrolytic is new and tests good. It appears to me that Q715 is passing too much of the collector voltage but the base emitter potential is 0.6vdc as spec'd. BTW the speaker relay circuit works as it should if the logic chip sees the proper 0vdc for both channels. with the -12vdc lurking it of course stays deactivated.

    Screen Shot 2018-12-12 at 5.14.41 PM.png
     
  2. Ylli

    Ylli

    228
    46
    Jun 19, 2018
    -12 VDC at output not good. Let's get a few voltage readings and see what we can figure out.

    Please measure the voltage (reference to ground) at the following:
    Q709 pin 5
    Q709 pin 7
    Q701 emitters
    Q701 bases
    Q701 collectors
    Voltage *across* R713
    Voltage *across* R737

    Oh, and let's get the rail voltages too:
    Voltage at the junction of R709, R711 and R731
    Voltage at the junction of R713 and R737
    Collector of Q721
    Collector of Q723
     
  3. mikefc55

    mikefc55

    6
    0
    Jan 8, 2017
    I can't believe that I so fixated on the Q715 area that I totally overlooked Q709 ( OpAmp?). I will get back to you with this but I can confirm that Q701 measurements match the diagram spec. The Vcc + and - rails are lower than the spec'd diagram closer to 40v each than the called out 43v but the drops across the transistors b to e are correct i.e 0.6vdc differentials
     
  4. mikefc55

    mikefc55

    6
    0
    Jan 8, 2017
    the measurements are appended to and within your first response
     
  5. Ylli

    Ylli

    228
    46
    Jun 19, 2018
    - or + 1.5 vdc on pin 5? Since you said the output is at -12 volts, I'll assume you measure it as -1.5 volts. If that is the case, the output of this op-amp, pin 7, should have ramped down to - 15 volts or so. Might be a bad op-amp.

    Both of them the same at +37.5?

    So 1.5 mA total tail current and
    6.1 mA VAS current.

    The parts list shows Q709 as an AN6552. I'd suggest replacing that as your next step. It looks like a pretty plain jane op-amp, so for testing purposes you could use just about any dual op-amp with the same pinout. e.g. LM358, MC1458, TL072, AD822....

    edit. The 4558 appears to be an equivalent. That should be available just about anywhere from various manufacturers. Google "IC 4558"
     
    Last edited: Dec 14, 2018
  6. mikefc55

    mikefc55

    6
    0
    Jan 8, 2017
    I want to thank you for the fresh approach you gave me by looking at the OpAmp. I had a generic one hanging around and it did the trick. I also found half of the output resistor ( 0.47 ohms ) open once these issues were addressed I was able to balance the output currents. The amp has run all evening, cool as a cucumber.
     
  7. Ylli

    Ylli

    228
    46
    Jun 19, 2018
    Great!
     
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