On Sat, Mar 24, 2018 at 4:00 AM, Josh Dersch via cctalk <[email protected]> wrote: > Hi all -- > > I've got an 11/03 backplane here (designated as an "11/03 EA" originally but > marked as "Model changed from EA to AA") inside a small chassis w/power > supply, model "OH780-A." (So it was at some point in its life the front-end > to a VAX-11/780.) As far as I can tell, it has a standard H9270 backplane > (4-slot, quad width) in it. > > The chassis appears to work fine when I have an actual LSI-11 in there (I've > been experimenting with an 11/23, but I do have an 11/03 boardset that works > in it). > > What I want to do with the chassis is use it with a UNIBUS->QBus bridge I > have, as a Qbus backplane for a SCSI controller (and maybe other things) for > my VAX-11/750. I have encountered an oddity in doing so, and I've looked at > the documentation and I remain puzzled. The oddity is this: If I don't > have an LSI-11 CPU board in place in the chassis, the power supply won't > come up properly. (The +5V appears to pulse, the kind of behavior I'd > expect with a shorted or heavily loaded supply rail somewhere). > > Here's a basic rundown of what I've observed: > > - If I put an 11/23 or 11/03 CPU in, everything works fine.
My first thoughts are one of two things : Insufficient load on one of the supply lines (may not be the +5V one), the CPU board does load that line. Lack of pull-ups on ACLO, DCLO. Possibly pulled up (or at least loaded) on the CPU board) I would start with tryng to get the power supply running on its own. Disconnect it from the backplane. Add dummy loads. Put a pull-up from 5V to ACLO and DCLO. Once the PSU will run on its own, connect the backplane and try again. -tony
