The early Krell Evo 707 had problems with HDMI handshake. It may work with some HDMI sources and not with others or it may simply not work with any HDMI connection. Try switching off the main power switch on the back lower left of the unit for a minute then switch back on. This will reboot the unit and it may help. I advise against opening the unit. This has nothing to do with unclean contacts.
The 707 is a hand built ultra high end pre/processor. It can be quirky. It is a bit slow (short delay) to pick up full digital audio signals via HDMI.
I have owned my 707 for over 12 years. I have had to bring it back to Krell for repairs a few times, which is a pain in the neck. However, it is a fantastic sounding processor; super fast, life-like, vibrant, clear, clean with tight deep clean bass. It is the best upgrade I have ever made to my music/home theater system in all my 45+ years in this hobby. I doubt there is a better sound processor on the market. I have upgraded my unit with the external Krell 4K switcher. It works great with my new 4K Sony 83" A90J OLED. My guess is you need a new video board. Call Krell support.
How long have you owned your Evo707?