Bias adjusting idle current class AB amp?

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Bobby Baird

Well-known member
GDIY Supporter
Joined
Jun 15, 2013
Messages
520
Location
Austin, TX
Hello, please help me understand how to bias this amp through the emitter resistors. I had to replace all output transistors which I matched based on hFE & Vbe as one gave up the ghost, also I replaced a few .47 ohm, 100 ohm emitter resistors and a few 4.7 ohm collector resistors . The amp I have is a Cerwin-Vega CV-1800 which uses the same board as the Samson SX-1800 which I found the schematic for posted here. My understanding is at idle the 2SC5200 2SA1943 pair should conduct some current even when no sound is present. Thus eliminating crossover distortion. I do not own a distortion analyzer. I see the bias adjustment pot is VR101 in the schematic. Would I use my volt meter across say R140 & R158 emitters to check mV current flow? As I only repaired one channel. I could use other channel to check it's bias current.
 

Attachments

  • Samson-SX1800-pwr-sch.pdf
    744 KB
  • IMG_7585.JPG
    IMG_7585.JPG
    4.2 MB
  • IMG_7586.JPG
    IMG_7586.JPG
    3.7 MB
  • 2SA1943 Datasheet.pdf
    208.7 KB
  • 2SC5200 Datasheet.pdf
    202.8 KB
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