Whilst performing the weekly DBB scans, Jason noticed that there was something amiss with the ISS.
Namely that the output of the ISS photodiodes was 0 and it had been that way since just after we
performed the Tuesday maintenance routines.  It was noticeable that on the ISS MEDM screen the
reported DC value for photodiodes A and B were significantly less than the usual 10V.  Oddly enough
all signs indicated that the loop was locked.

    The output of both photodiodes was checked at the ISS box.  They both indicated over 10V output
as they should.  We measured the same output at the end of the cables going to the ISS servo card.
We injected a DC voltage into the cable going to the AA filter chassis and the MEDM screen reported
no real change.

    Fil noticed that one of the power LEDs of the AA/AI boards was dim, so he power cycled the chassis.
Afterwards all the readings were as they should be.  The chassis was pulled and and frequency response
and noise tests were performed on the board that the photodiode signals are connected to.  The
measurements passed the test criteria.  The chassis was re-installed.

    The strange thing is that the system still thought the ISS was locked.  The only difference on the
display is that the maximum and minimum diffraction percentage was quite separated from the mean value.
There was no sign of an oscillation.  Both the PMC and ISS were happy throughout.



Fil, Peter, Jason