Recent report from LLO (LLO alog 73576) showed that the test that was sent from LHO to LLO broke because the hard stop was set to the low position and the bobbin/mirror assembly hit the stop hard.
While we were testing our "dirty spare" unit in the EE shop today, however, we have found that the same bobbin/mirror assy can break even when we're quite sure that it didn't hit the stop.
We set up the testing apparatus using the dirty spare FS, a spare driver D1400078 and FAST Shutter test board D1400208. We set the HV to 250V. We made sure that the hard stop was at its highest position.
The first 4 fast shutting motion was good, Fil and Betsy shot the slow-mo video and we are convinced that the bobbin/mirror assy didn't hit the stop. On 5th attempt, while nobody was shooting video, the mirror and a PEEK plate backing broke off and was shot into the air. On a closer inspection we see partial delamination of the glass and PEEK, but it's not like the delamination is the main reason why the PEEK failed. Also a small chunk of the glass was ripped apart from the mirror and left inside the bobbin where the glass was glued to the bobbin and the coil.
Betsy and Fil will post pictures/video later.
Attached is the video of the dirty unit firing CLOSED but before the episode where the mirror broke off. Also a snapshot capturing the frame when the Bobbin assembly (D1003319) was in the most UP/CLOSED position in it's bounce. It does not look like it rammed into the hard stop viton bars.