Strange behaviour but it is familiar to me with my '818 and has been a nagging issue for several years, commencing about one month after the warranty expired. I key the supplied Yaesu mic. and the RF collapses, and in my case it takes 6-8 hours before the radio is functional once again. It is possibly due to RF pick up via the morse key's lead, even into a dummy load and on off-grid power, and I still haven't found a solution that doesn't revert to this behaviour. Have you established whether the zero output from the amplifier is caused by zero drive from the radio? Maybe try the amp with a different radio as well.
--
Steve, G4VRR.
Steve