Yesterday Craig asked if there were any recent spectral measurements of the FSS mixer output - there weren't any.
This morning I set out to do the measurements.

    With the input modecleaner locked, a transfer function measurement was made (CG20FG9.jpg, data file has .txt
extension).  There was nothing unusual with the measurement.  A noise measurement was made (mxr.png, data files
are mxr0?.txt).  Whilst the measurement was done the spectrum was moving as if a sweep was being done, even though
the network analyser was physically disconnected (see DSCN0648.MOV).  I had noticed that the IMC MEDM screen
flashed the message that the IMC WFS were not centred.  Even with the IMC in the down state this behaviour did not
stop.

    Not long afterwards I noticed at the PZT monitor strip chart on the FSS MEDM screen was wall to wall black
and that reducing the common gain had no effect.  The mixer monitor signal was constantly greater than 1.2 Vpp.
I then remembered that work was being done on HAM2 and thought what I was seeing might be related.  However when
that work was completed, the behaviour remained.  I then noticed that the metal block over the PA85 was stone cold,
whereas it is normally warm to the touch.  A quick check of its output voltage indicated that it was almost always
close to zero suggesting a blown PA85.

    The spare FSS was deployed.  It locked readily enough, however for unknown reasons the common gain could not be
increased much beyond -6 dB without the PZT railing.  Clearly there is some difference between the two units,
although I seem to remember bringing them to the same hardware level 4 years ago.  The PA85 was replaced in the
original FSS, and things locked up straight away.  The PZT was not oscillating, gain sliders were restored.  The
transfer function with the repaired FSS is in PA85.jpg (data file PA85.txt).

    It was there I left things, since the input modecleaner needed to be back online by 08:30 am local.

    It is not obvious why the PA85 died.  One possible reason is due to overheating.  With the FSS located under
the PSL table not as much air gets blown over the PCB.  With the lid off the FSS box, the conducting surface area
is reduced somewhat.  Unfortunately accessing the test points requires removing the lid.



  Peter / Richard / Ed