Somewhat random QBus backplane / chassis question

[Josh Dersch]

On 3/23/2018 9:55 PM, Tony Duell via cctalk wrote:

> 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.

Ah ha! -- thanks for pointing that out, I'd neglected the +12 in this 
scenario.  Turns out all of the boards I'd used to load the system use 
only +5 by some stroke of luck.  I put a DHV11 in the backplane and the 
supply now works fine without a CPU.  I suppose I could use a few extra 
serial lines anyway, so I guess I'll leave it in there :).

- Josh

>
> 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
>