Althoguh there were a few things I forgot (!) to mention last night... There's a bit more to it than that. There's a power transsitor (Q15, TIP41A) on the PSU board. The emitter goes to the relay coils (and then to logic groudn), the base goes to this 12V power output from the computer and the collector goes to the power supply rail for the chopper control circuitry. When the PSU is not running, this transitor acts like a couple of diodes. It passes the 12V from the comptuer to the relay coils (b-e junction), turning on the mains. It also passes the 12V to the chopper control IC (via the b-c junction). The chopper controller starts up, the choppers chop (if you see what I mean), and the PSU produces the correct output voltages. One of these ouptuts (about 15V I think) then powers the chopper control circuit. At this point, the transisotr acts like a conventional emitter follwoer, powering the relay from this PSU output, controleld by the computer 13V line. When the comptuer turns off, the relays are thus de-energised and the whole thing shuts down. One neice feature of this supply is that the chopper control IC is on the isolated (output) side, the outputs are coupled by pulse transformers ot the chopper transsistors. It's a relativeily benign PSU to work on. Here's the pinout : 1 : Blue ground (screen of cable for pin 9) 2 : Green groudn (screen of cable for pin 10) 3 : Logic ground (not used) 4 : Logic ground 5 : -12V from computer (not used) 6 : Logic ground 7 : +12V from computer (PSU switching/startup) 8 : Overall screen ground 9 : Blue signal (wire with coax, screen to pin 1) 10 : Green signal (wire with coax, screen to pin 2) 11 : Horizontal sync 12 : Red signal (wire with coax, screen to pin 13) 13 : Red ground (screen of cable on pin 12) 14 : Vertical blanking 15 : Vertical sync You don't have to wire pins 3 and 5, but it doesn't hurt if you do. All other pins must be wired straight-through, note commentso n screens (shields) etc. It seeemd ot be there (under HP9826) when I checked earlier. Of course, I never need to look at them there, I haev a copy of the 'originals' for my workbench. Note there are 2 'page 3s' for the 9836 motherbord. They are electrically the same (same point are ocnencted) but he signals are different because hte graphics boards are different. You want ot look at the one that says 'HP9836C' (not 'HP9836A'). In general, anything that says 'HP9826/HP9836' applies to all amchinees, or at least could do (like CPU boards, disk cotnroller, PSU, etc). 'HP9836' means eitehr all HP9836s (mono or colour) or just monochrome ones, it should be obvious. 'HP9836C' is colour machines only. -tony

You may have seen the picturs on the HPCC web site from the 2010 mini-cofernece. There are a couple of me with a 9836CU in a lot of bits. Don't worry, it did all go back together again... The unit will only turn on the EHT to the CRT (acutally, will only energise the flyback transformer) when there are currents flowing through both sets of defleciton coils (the horizotnal output stange and EHT gernator are separate). This prevents the scrren from getting a line burn t on it if somethign fails. So when everytyhing's cabled up, when you turn the comptuer on, the monitor PSU should start (and I think it then degauses). Then the CPU initiallises the 6845 on the text video board, the scans start, and the EHT then comes up (I think the CRT heaters are fet from that transformer too, BTW). Ther'es a little green LED on the deflection board in the monitor which turns on when the thing has enabled the EHT generator. I posted a pinout last night, too. Good!. Do you get any errors in the power-on self test>? Does it find the graphics board and 2 floppy drives? How much memory do you have, and do you have any i/O cards? Any -U model is goign to have a bootROM 4.0 I think, which means you can boot from an HPIB disk drive (floppy or hard), EPROM board, bubble memory board, or whatever. It took me quite a long time to produce that.... -tony