Now the SRM Yaw bias in yaw is functional without any strage behavior.
The problem was found at the connector of the flat ribbon cable from the DAC to the cross connect.
I used the extender board to diagnose the SRM coil driver circuit at 1X4.
The UL coil input did not show any sign of voltage no matter how the bias slider was jiggled.
I opened the side panel of the rack and found the signal was absent at the cross connect which relays two flat ribbon cables
for the SRM coil driver. I checked the DAC output with a multimeter. All the bias outputs were OK at the DAC.
Then I opened the IDC connector at the DAC side of the crossconnect as the signal was already missing there.
I found that the flat ribbon cable was a half line shifted from the supposed location.
This resulted a short circuit of the DAQ +/- pins for the SRM UL coil.
I recrimped the connector and now the SRM Yaw slider is back.
This changed the nominal position of the SRM. The new slider values were saved.