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Entry  Thu Jan 24 22:55:50 2019, gautam, Update, Electronics, Satellite box S/N 102 investigation IMG_7294.JPGD961289-B2.pdf
    Reply  Tue Jan 29 17:19:16 2019, gautam, Update, Electronics, Satellite box S/N 105 repaired IMG_7301.JPG
Message ID: 14417     Entry time: Thu Jan 24 22:55:50 2019     Reply to this: 14421
Author: gautam 
Type: Update 
Category: Electronics 
Subject: Satellite box S/N 102 investigation 

I had taken Satellite box S/N 102, from the SRM suspension, down to the Y-end as part of debugging. However, at some point, I stopped getting readbacks from the shadow sensor PDs, even with the Sat. Box tester hooked up (so as to rule out anything funky with the actual OSEMs). Today evening, I did a more systematic investigation. Schematic with component references is here.

  1. Used mini-grabbers and a bench power supply to connect +/-24V to C57 and C58.
  2. Checked that all ICs were getting +/- 15 V to the supply pins.
  3. Debugged individual channels, checking voltages at various nodes
    • Found that the "PD K" bias voltage was anomalosly low.
    • Found that the inverting input of U3C wasn't ground.
    • The above findings are summarized in Attachment #2.
    • This suggested something was wrong with the Quad OpAmp LT1125 IC, so I elected to switch it out.
    • During the desoldering process, the pads for the "NC" pins came off (Attachment #1) - this has happened to me before on these old boards. I don't think I applied excess heat during the desoldering (I used 650F).
    • Replaced the IC, and measured the expected 10V at the "PD K" node.
  4. I then connected the tester box and verified all the shadow sensor channels (LED + PD) work as expected, using the front panel J3 and the "octopus cable".
  5. It remains to verify that the coil driver signals get correctly routed through the Satellite box before giving this box a pass certification.

The question remains as to what caused this failure mode - I can't think of why that particular IC was damaged during the Satellite box swapping process - is this indicative of some problem elsewhere in the ETMY OSEM/coil driver electronics chain?

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