Glad to help - or try to. There doesn't seem to be as lot on the F-11 set. I looked in the DEC semiconductor handbook, and it's not there - although perhaps it had been dropped by the one I looked at (which was mostly uVAX stuff) as obsolete? If you look in the KDF11-A and KDF11-U Tech Manuals, there is a chapter on the F-11 chip set, as used in those cards, and that's better than nothing - it talks a fair amount about the low level details of how the various chips operate and interact, etc. Looking at the KDF11-U prints, I finally found that LED (it's pretty low level - I was worried that it might be a bit in a register, and driven by software, but it's not, it's actually driven directly by the the CPU's internal clock signal; it's on page K1 of the prints, 'Clock, State Decode', in the very upper left corner). (The source of the CPU's internal clock is just an RC circuit, in the lower middle of that page, and the trim pot that's part of it - along the upper edge of the board - can be adjusted to set the clock speed 'properly', per the note at the back of the prints on the page which lists the configuration switches. The 2MHz crystal along the upper edge drives the baud rate generator.) If ODT isn't running, the problem is almost certainly in one of those two areas. I'm not so sure. Those boards mostly just want +5V; looking a bit more, the CPU chips do seem to use +12V. The RS232 drivers will use +/-12V. I'm afraid that if i) it used to show activity, but no longer does so, and ii) the main voltages (+5V, +12V) look good, something on the CPU card has failed. But it will take a bit of digging to i) verify that, and ii) identify the fault. I have yet to trace how the LTC signal is used in the KDF11-U, but on the KDF11-A, it not being there is a total NOP. (In fact, in the BA11-N/S type mounting boxes, there's a 'Clock Enable' switch on the front panel which turns the LTC signal off - and the machine runs fine with it off - except for the TOD clock not ticking.) That clock signal - totally different from the main CPU clock - is only used as an input to what is in effect a peripheral. Not a good sign, alas. If you have a scope of some kind, and want to keep poking, I'd recommend that you start by seeing if the clock is running, and move forward from there. The KDF11-U prints are online, as is the KDF11-U Tech Manual. Skim the chapter on the CPU (4, I think), and then grovel around in the prints for a bit. Don't try to totally understand it all, just skim through it, so you know roughly where most things are. Noel