Yesterday 21st of October 2020 I checked the BF condition again.

Some conditions and what I did:

• Suspension in NEUTRAL mode.
• Current keystone nominal position 100 um.
• I moved the keystone down using coil-magnet actuation in the test channel.
• I measured BF transfer functions in some positions.

What I found, see the picture:

• When the keystone is at 50 um an oscillation at about 40 mHz appears with a small amplitude.
• As the kestone keeps going down the oscillation increases.
• I measured the BF transfer function three times, and the only healthy one is the one with the keystone above 100 um.

What this suggests:

• The problem doesn't seem to be a hard physical stop. The keystone is always moving.
• I tend to think a cable pulling is producing coupling with another body.