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Tue Feb 24 18:42:15 2009
mc board repair
However, when we measured the MCL loop gain with several different AO path gains, the loop shape did not change at all. This led us to suspect the AO path may not be connected. The cabling from the common mode board to the MC board seemed ok. We tested the signal flow in the MC board using a signal generator and an oscilloscope. Then we found that a signal injected to the IN2 (AO path) does not reach to the TP1A (right after the boost stages), though the signal is visible in the OUT2 (monitor BNC right after the initial amplifier (B-amp) for the AO path). The signal from IN1 (MC REFL) can be observed at TP1A. This means something is broken between the B-amp and the sum-amp in the AO path. We will check the MC board tomorrow.
Today we examined the MC board. With the extension board in place everything seemed fine. Without the extension board we could replicate the problem. Jiggling the IN2 jack caused the injected signal observed at TP1A to come and go. These jacks are unfortunately mounted directly on the board. We traced the problem to a resistor in this path (R30) which looked fishy. We soldered on a new 2K resistor with OWC and it fixed the problem.