HK 730 right channel quit working (all inputs and outputs including headphones - preamp?)

I've had my new-to-me HK 730 powering my also-new-to-me Infinity RSIIIBs for a few weeks now, and have been extremely happy with the sound. This morning it wouldn't play out the right channel except very (VERY) quietly out of the mid and tweeter. Aux 1, FM and AM all the same; same with using Speakers 1 and 2 outputs and with...