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I'm trying to repair my broken Harman Kardon AVR151.

Looking at the SMPS schematic, to my very limited knowledge I do not understand,

  • The 78L24 is a high side 24V regulator.
  • Why does the schematic say tge output is 38V?
  • Is the circuitry show below designed such a way to output a higher voltage?
  • When measuring I get a reading about 35V at VKK.

78L24 (actual part No. CVIL78L24AB)

Harman Kardon AVR 151/230C Service Manual page 123

SMPS schematic

JRE
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Jinantha Perera
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1 Answers1

6

The zener voltage ends up being added to the voltage stamped on the part, so 12V+24V = 36V.

Pin 2 of the 78L24 acts as an almost constant current source of about 4.5mA, which flows through Zener diode D929 and therefore 12V will appear across that. The 78L24 provides 24V on pin 1 relative to pin 2, which is thus 12V higher than ground.

As it happens, Izt for the zener is 5mA, close enough to the operating current.

Spehro Pefhany
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