Finally found time to get to this one... > From: Rob Jarratt
> However, there is a puzzle. On the CPU I found that the track from the > pull up resistor to E70 has been cut. I don't know about the "pull up resistor" part, but I have several KDF11-U's, and _all_ of them have the trace on the bottom of the card connected to pin 1 of E70 cut, in the exact same place (about 1/8" from the pin). This suggests that it's not a local mod (as you suggest below), but an 'official' DEC ECO. > This would suggest that E70 pin 2 is floating, which I think means that > K2 BUF ACLO H is also floating The input (pin 1) will be floating, but not the output (pin 2); TTL doesn't work that way, I think. I may have this wrong, but I think open TTL inputs float high, so BUF ACLO should be low. I looked, and I don't see any other traces (e.g. on the top side) going to pin 1. So I'm a bit puzzled that DEC allowed that input to float, as open inputs can lead to erratic operaton; they're usually tied high, or to ground. > K2 BUS ACLO L however has been patched to E52 pin 4, which is the > output of a gate on sheet K6. Can't say I understand why. Me neither; that's an unused (on the prints) driver in the DS8641 (center bottom of the page) - although that gate seems to have been fully wired up (wires to pins 4, 5 and 6) as part of the ECO. There is an ECO list on pg. 167 of the -11/24 prints (2 pages after K14), but I don't see an E52 in it anywhere. The puzzle here, if E70p1 is cut, and the output is low, is why the CPU clock isn't running. The -15V on BUS ACLO shouldn't have taken out any other semiconductors; it's not attached to anything else. (It will have run C6, on the lower right of the card, the wrong way, but i) I think that's a non-polarized item - and 100V rated, per the prints, and ii) I don't think that goes anywhere else, even if it's not.) So what's stopping the clock from running, then? Noel