pdp-8/l supply

On Mon, May 05, 2008 at 04:18:57PM -0400, Brad Parker wrote: Nice. That was my first model of -8. I still marvel at what they packed into that package (and how far you can expand it with an extra box or two). . . . You might characterize at what load it droops. I had an arcade game that arrived dead because the regulator transistor was faulty - the supply did put out +5.0 V with no load, but more than a few mA of load and the supply just wouldn't put out anything close to +5V. OTOH, you mention 4.5V, which is at the bottom end of the spectrum for TTL. Obviously closer to +5.0 is good, but what you are reporting below might or might not be power related. Obviously it's good to be confident about your power, but if you get that issue addressed and still see odd symptoms, I'd grab the printset from bitsavers and keep a careful eye to any 7474 and 7440 chips that are in a path that could affect the behavior of whatever front-panel activity you are trying. In my experience with repairing several -8/Ls and an -8/i, those two chip types fail at a remarkably higher rate than all the other ICs combined. For testing simple boards (M111, M113, M117, M216), I have rigged up a squeeze-on 16-pin DIP clip to a hand-held IC tester. There's enough power from the tester battery to power 3-4 TTL ICs, and none of the signals on the simple boards interact on the board. It only takes a few seconds to check all the 7474s on an M216 that way. It may not find subtle chip problems, but in practice, I've frequently found a chip or two that's wonky. Unfortunately, this trick won't work on the M220 CPU register modules, nor I/O like the M706/M707 console interface boards, but as easy as it is to check 80% of the boards in the machine and eliminate them from your scoping, I think it's quite a valuable technique. The other issue that comes up is backplane oxidation. What has done wonders for me in the past is to take manila folders, cut them into strips the width of the card fingers, fold into a 2-thickness layer, then wet down with 1,2-propanol (isopropyl) and insert/remove a few times. You will probably see dark grey streaks on the paper the first couple of times you do it. Power is a great place to start, though. I can't really give much advice there; I've never had to delve into the PSU of an -8/L. -ethan -- Ethan Dicks, A-333-S Current South Pole Weather at 5-May-2008 at 21:30 Z South Pole Station PSC 468 Box 400 Temp -71.1 F (-57.3 C) Windchill -97.9 F (-72.2 C) APO AP 96598 Wind 5.7 kts Grid 62 Barometer 697.4 mb (9979 ft ) Ethan.Dicks at usap.gov http://penguincentral.com/penguincentral.html