Originally Posted by
Turbo SOB
Here I am eatin' crow again.
There's some pretty major differences between the '86 and the '87 TII style LMs, specifically in the EPROM area. The '86 has 2x 8k EPROMs and the '87 TII has 1x 16k EPROM and an EF6840 "programmable timer module". I have no idea what the timer module would do in an '87 that it wouldn't need to do in any other LM, but there it is, with some significant trace routing differences to accompany it. I did find a spec sheet for it, though the only place to find replacements is eBay. I guess one thing to try is to remove it and see if the '87 LM still works.
Either way, I think my first priority should be to try to repair the dead '87 TII LM. If I can figure out what's wrong with it maybe others can benefit.