You might find it isn't really a "tray", in that it won't just slide out. That lower flat piece of metal is an integral part of what holds the whole thing together to support the heavy glass CRT. It looks like everything is still cabled to it. There might be a way to remove it, but you might be better off starting from the top to test things anyway.
See what you can remove, like the neckboard or other parts, that gives you access to the PSU area. Or, even better, try the connector points on the PSU board in place, while it's on, or even the points the PSU connects to on other boards, and test voltages. A lot of the time those points will have voltages marked on them. Just don't touch the yoke, rings or any neck assemblies until you have absolutely no other choice, because then you'll have to set up the convergence and purity again.