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.
- Used mini-grabbers and a bench power supply to connect +/-24V to C57 and C58.
- Checked that all ICs were getting +/- 15 V to the supply pins.
- 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.
- 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".
- 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? |