I had a Magnavox that did the same thing. What I did notice is it
wasn't when I put the back on, rather once I unplugged it, it would not
power back on unless I removed the back, fooled with the chassis and
tried it again. Are you having to unplug the set to put the back on? If
so, I found a bad resistor in the standby voltage supply. If I happened
to touch the circuit (set unplugged), this somehow would reset or drain
a transistor that was keeping the set shut down. Replaced the resistor
and no more problems. Good luck.