Recently, it is found that GRMC loop cannot be closed. To check what is the problem, I disconnected the slow scan of MZ and put MZ PZT offset to a value that GRMC transmission is the highest. After that, according to the original setting, I have done following checks

0. GRMC has a good alignment.

1. PDH signal has 316mV pk-pk checked from EPS1.

2. GRMC has loop sign of INV, which is as design.

3. The RF source phase is reloaded. The phase of RF source is 125deg. When it is changed to 35deg, the signal around resonance becomes flat. This indicates the RF signal phase is still a good one.

4. There is a switch which has +/- sign. This doesn't decide the sign of control loop. But when we use this type of servo for CC1/2 controls, we need to flip this switch. I tried to flip this switch, but it doesn't help to close loop.

5. GRMC transmission is checked to have 1.13V peak. This is two times smaller than the value written by Pierre.

6. Loop gain is 3 as usually used.

7. Threshold for peak identification is -0.55V. This is as required.

8. The GR power reaching AOM is measured to be 44mW, whose nominal value is 50mW.

These checks show little issues but they should not prevent the GRMC locking. More investigation is required.