Over the weekend I finally got around to repairing this Ramsa Mixer. The symptoms included that the solo light was on in one place and not on at another and that the overload LED's were lit with no input. I opened it up to look inside and then everything started working correctly.
I started wiggling things and when I touched the power supply the mixer stopped working. So I removed it and examined it for bad connections. Sure enough one of the voltage regulators was no longer soldered in.