Somewhat random QBus backplane / chassis question

[Tony Duell]

On Sat, Mar 24, 2018 at 4:00 AM, Josh Dersch via cctalk
<cctalk at classiccmp.org> 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