Following up on alog 46240, we made the following mods to the ISS Second Loop, S/N S1700062, which is now installed again (PCB D1600298):
- Connect feedback of open OpAmp U2B to negative input and ground the positive input.
- Increase the gain of the sum by increasing R32 to 10K (from 1K).
- Swapped the corresponding OpAmp U3 from OP27 to OP37.
- Decrease the gain by 10 in the readback of ERR2 after the sum by changing R62 in the SUM signal block from 4.53K to 45.3K.
- Decreased the analog excitation input gain by 10 by increasing R33 to 100K (from 10K).
- Decreased the digitla excitation input gain by 10 by increasing R77 to 100K (from 10K).
To account for the increased analog gain we reduced the VGA gain to 0dB (from +20dB). We increased the REFERENCE_IN_MTRX_1_2 gain to 10 (from 1) and the REFERENCE_IN_MTRX_1_4 gain to 0.5 (from 0.05). The reference offset was also adjusted to 537 (from -35.14). At first glance, the output offset fluctuations are significantly reduced.
H1:PSL-ISS_SECONDLOOP_REFERENCE_IN_MTRX_1_4 is controlled by IMC_LOCK guardian, so a factor of 10 was put into that (i.e. 0.5 from 0.05 for AC coupled, and 50 from 5 for DC coupled 30W).
Also we noticed that 10Hz pole on/off indicator for 3rd loop and slow offset path on the front panel was not working (it never turned on), but the readback was working and the pole seemed to be switching.