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Hello Alistair,
It is a little tricky to say for sure but my hunch would be condensation around
the crystal and tank circuit. The crystal has 33pF caps hanging off it and it wouldn't
take much condensation to stop the crystal or pull it off frequency. Pulling it off
frequency wouldn't kill the heart beat LED but might kill the USB comms. The 4MHz
from the crystal is input to a PLL with a 96MHz ouput, then a /2 post scale to 48MHz
which forms to base clock for the USB module. If the 4MHz is off frequency or has
significant phase noise the PLL may not lock as required.
I am currently on holiday in Venice, will be back in a few days and will get back to you
in more detail regarding the temp compensation. The basic idea is you log the temperature versus position over a nights session while focusing manually at
intervals. This data allows you to work out the coefficent of expansion of the scope
in units of steps/degree. The expansion coefficent is then input to your software of
choice to perform compensation automatically.
Cheers,
Dave
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