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Harman Kardon PM655 strange value fo voltage

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  • Harman Kardon PM655 strange value fo voltage


    After the schematics (power supply zone) ; instead to have on collector +35.6v for Q5 2SD669 and -35.6 v for Q6 2SB649 i have 40.7v and -40.7v ! For the rectifiers bridges i have 44.8v and -44.8v for both...

    Is there someone who spot it too ? any idea about this difference about 5v ?

    Maybe a mistake in the schematics about values to collector for q5 and q6 ? I mean, we can find 44v in the power driver section (line from bridge rectifier D1 to power driver)... So anyway, i tell myself, how the 2 resistors of 33ohm can block nearly 9/10v (44v8 - 35v8) ?

    There's something strange ; no ?

    Thanks in advance
    Attached Files
    Last edited by Amapola; 02-27-2020, 07:37 PM.

  • #2
    5v dropped across 66 ohms? 75ma of current will do that.

    44.8 - 40.7 = 4.1v I = V/R = 4.1/66 = 62ma, sorry
    Education is what you're left with after you have forgotten what you have learned.


    • #3
      sorry, i said fact i would say the 2 resistors of 33ohm cant eat 9/10v ; i mean 44v to 35v ...


      • #4
        But you said you have 40v, not 35v on the collectors. The schematic estimates 35v but you have 40. I can't calculate current based on a real reading on one end and a schematic voltage that doesn't exist on the other. The resistors drop a voltage determined by the current. The voltages are whatever you measure.

        The voltage that matters is the emitters. That is the voltage feeding the amplifier circuits. If you have roughly 24v at the two emitters, then it is working.
        Education is what you're left with after you have forgotten what you have learned.


        • #5
          yes i've 40v on collector ; but the 10v dropped mean the 45v from d2 bridge via c8 and c9 , then via the 2*33 ohm resistors (r13 and r21) to have in final the 35v estimate like mentionned in the schematics. So, if these 2 *33ohm resistors can only drop nearly 5v max, the 40v match better with the reality in fact ... Maybe mistake about the 35v mentionned for the collector or mistake about the value for these 2 resistors.

          But if there is no more to worry about it ; i think it's done .

          Thanks for your light and sorry if my english is little bad ha ha


          • #6
            The schematic says 35v, but in the real world there is 40v there. I think you are assuming the voltage at D@ is the same in both cases. I'd suggest it is not. In the world of the schematic D2 is making about 40v, not 45. So the 35v makes sense there. But in the real world, D2 is making more like 45v, so the 40v reading makes the same sense. The voltages are all a few volts higher than the schematic.

            But whether 35v or 40v, what matters is the 24v at the output of the supply. And that is determined by zener diodes D9, D10.

            The schematic was drawn assuming average parts. They also assumed a mains voltage. What if they assumed 220v and your mains are 240v? Or maybe both you and they are expecting 240, but your mains are really sitting at 250v. That sort of thing.
            Education is what you're left with after you have forgotten what you have learned.


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