[Coco] coco3 won't boot

[Robert Hermanek]

(Warning:  I have no electronics knowledge, and no time in the near 
future to learn!)  I have a coco3 that has been working for thirty 
years.  It has recently stopped booting.  The bare coco3, no MPI, no 
carts, the warranty sticker was intact, so about as stock as you can 
get.  On power up it displays a blank green screen, exactly as if it is 
about to display the extended basic banner, but it hangs at this stage.  
The only thing I've done is opened the case to see if any components 
looked damaged, and gave the socketed chips a little push to make sure 
they were fully inserted, but no help there.

Anything else simple I should be checking?  I know there is no shortage 
of "poor dead coco" threads on the list, but thought I'd ask anyway.  
Thanks in advance!

-RobertH