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Entry  Tue Dec 14 19:29:42 2021, Paco, Update, BHD, 1Y0 rack work for LO1 
    Reply  Mon Jan 3 12:05:08 2022, Paco, Update, BHD, 1Y0 rack work for LO2 AS1 AS4 
       Reply  Mon Jan 3 16:46:41 2022, Paco, Update, BHD, 1Y1 rack work for SR2, PR2, PR3 
          Reply  Fri Jan 7 17:54:13 2022, Anchal, Update, BHD, PR2 Sat Amp has a bad channel Screen_Shot_2022-01-07_at_6.05.39_PM.png
             Reply  Fri Jan 7 18:28:13 2022, Koji, Update, BHD, PR2 Sat Amp has a bad channel 
                Reply  Mon Jan 10 13:35:52 2022, Anchal, Update, BHD, PR2 Sat Amp has a bad channel 
                   Reply  Mon Jan 10 15:59:46 2022, Koji, Update, BHD, PR2 Sat Amp has a bad channel 6x
                      Reply  Mon Jan 10 17:04:47 2022, Anchal, Update, BHD, AS1 Sat Amp CH2 had offset 
Message ID: 16555     Entry time: Fri Jan 7 17:54:13 2022     In reply to: 16540     Reply to this: 16558
Author: Anchal 
Type: Update 
Category: BHD 
Subject: PR2 Sat Amp has a bad channel 

[Anchal, Paco]

Yesterday we noticed that one of the ADC channels was overflowing. I checked the signal chain and found that CH3 on PR2 Sat Amp was railing. After a lot of debugging, our conclusion is that possible the PD current input trace is shorted to the positive supply through a finite resistance on the PCB. This would mean this PCB has a manufacturing defect. The reason we come to this conclusion is that even after removing the opamp U3 (AD822ARZ), we still measure 12.5 V at the pins of R25 (100 Ohm input resistance)

Please see the schematic for reference. We also checked the resistance between input of R25 (marked PDA above) and positive voltage rail and it came out as 3 kOhms. While I all other channels, this value was 150 kOhms.

I would like it if someone else also takes a look at this. We probably would need to change the PCB in this chassis or use a spare chassis.

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