Continuing to hunt for the cause of the intermittent 0.8 MHz sinusoidal ripple remaining on the negative 15 volt rail of CH1, I replaced the electrolytic capacitor at the output of the 1 uf negative 15 volt regulator on the rectifier/supply board with a low ESR 10 uf that I had on-hand

Finally, that fixed the problem!

There is some random ripple remaining and so I'm going to look at replacing the ceramic caps on the board. Not sure if i'll stick with MLCC (ceramic) caps or 'upgrade' to polyester film. The ceramic caps on the computer board are standing off a bit which allows for some strain relief when the board flexes or expands. The MLCC caps on the main board are fully inserted and may be prone to cracking as a result of thermal cycling stress. My attempts to (gently) de-solder four have left none intact and I'm increasingly suspicious that one or more failed which demanded more suppression from the electrolytic caps.