After a few evenings chasing this intermittent fault, I thought I finally had solved it when I sprayed some Detoxit gold on a couple of the channel 2 input preamp selector switches and it snapped back to life. The next day while moving back to bring up of the O-scope interface, channel two failed again, this time appearing to be completely dead. This was actually good news, because a solid failure can be easier to debug than an intermittent failure.

This morning I figured out that there was a short between an input capacitor case ground and chassis ground.

O-scope Intermittent Short

Scraping the debris from between the capacitor ground and nearby chassis ground seems to have rectified the issue, hopefully for good. Having a good working channel to compare the bad channel with, made this repair easier than it otherwise might have been.