This cleared the problem. 20cm receiver damaged when compressor A C102988 failed. Awaiting new receiver. New receiver installed but problems persist. 1 arc-minute, with the extreme errors appears at low and high elevations.
Tiger team reports cracks in dichroic surface.
Awaiting dichroic panel replacement. Problem resolved when az drive cabinet interface board replaced. Receiver still useful - no plans to swap. Problem fixed with receiver swap. Cable cal measurements show scatter.