After cleaning the DAC board, the noise went away, and has been gone
for six hours.  Or maybe it was the phase of the moon.  ;-)

Process was plentiful rainwater, tiny amount of detergent, toothbrush,
followed by a spray of isopropyl alcohol, and three hours drying in
sunlight on a concrete slab at 33=B0C ambient with 30 knot winds at 15%
RH.  Ouch.

Some of the green PCB lacquer was discoloured by the isopropyl alcohol.

--=20
James Cameron
http://quozl.netrek.org/
--=20
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