Thanks for the replies, I'll plan on cleaning the DAC board.

Since my post, progress has been;

- removed the DAC board from the three board card cage, and inspected
  the board; the spray oil is along one edge, about an inch (10%) of
  the board width; which suggests the attacker didn't try to squirt
  directly into the cooling holes of the card cage,

- isolated the noise source to the DAC board; by disconnecting
  the analog outputs on their way to the mixer board; which excludes
  possibility of the mixer feeding back a noise signal,

- checked again that there's no change in the main clocks or power
  supply before and during the noise,

- captured the noise signature on oscilloscope; different channels of
  the DAC show the noise at different times; minutes apart,

- applied a freezer spray to either PSU or DAC; doesn't seem to do
  anything, but plan to give it another go; while in the card cage it
  is almost impossible to address individual components.

I'm also thinking of soldering a few diagnostic leads to the DAC
packages themselves to see if they are rendering bad samples or
generating the noise locally.

They are packages about three inches by two inches, with a 0.1" pitch
SIL pinout.

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