Racking a PDP-11/24

[pbirkel at gmail.com]

A top-mount corporate cabinet looks like this:
http://www.cosam.org/images/pdp11-23/front2.jpg  The "DECDatasystem"
front-bar in the photo is over the 1U strengthener that braces the upper
portion of the rack ... since there is no brace at the top (as yours has).
Your cabinet will work fine; in my experience RL02's are always tight and
fiddly any place but the top-spot.

The "play" is because those immense-head screws are the pivot-points that
allow the chassis to be pivoted up 90 degrees (when the slides are extended)
in order to more easily access the underside of the backplane.  They are
intentionally slightly loose in order for the pivot to work.  You really
don't need, or want, to play with those screws.

-----Original Message-----
From: Rob Jarratt <robert.jarratt at ntlworld.com> 
Sent: Saturday, February 26, 2022 3:04 PM
To: pbirkel at gmail.com; rob at jarratt.me.uk; 'General Discussion: On-Topic and
Off-Topic Posts' <cctalk at classiccmp.org>
Subject: RE: Racking a PDP-11/24

Thank you for the reply.

> -----Original Message-----
> From: pbirkel at gmail.com <pbirkel at gmail.com>
> Sent: 26 February 2022 08:48
> To: rob at jarratt.me.uk; 'Rob Jarratt' <robert.jarratt at ntlworld.com>;
'General
> Discussion: On-Topic and Off-Topic Posts' <cctalk at classiccmp.org>
> Subject: RE: Racking a PDP-11/24
> 
> The conventional mounting for RL02 drives in a corporate cabinet puts 
> one
at
> the top with the 6U CPU in the middle *but* that assumes that you have 
> a corporate cabinet designed for that purpose -- in which case the top 
> is missing so that the RL02 disk-pack can be directly accessed, and 
> there is a 1U
> divider below the top RL02 that reinforces the rack (and in effect
replaces the
> 1U lip on your rack top).

I saw something somewhere that suggested the RL02 should be at the top with
the disk accessible without pulling out the drive. I don't think the cabinet
I have was intended to do that, but I will have a closer look.

> 
> AFAIK you should be able to make your rack plan work; it's just the 
> case that
> RL02 are always "top snug" (at least in my experience).  Have you 
> tried raising
> the 6U CPU as high as possible *before* tightening the bolts/screws on 
> the mounting flanges on the slides (to the rack, not to the chassis)?
> There's usually ~1/8" of play there.  From your photo perhaps you have 
> already done that as the CPU-front looks to be snug to the rack-top.  
> If anything it looks as if your RL02 instead need to move down.
> 

I did try raising the CPU as high as possible. Will have another look to see
if the RL02s can be dropped a bit lower, but I don't think they can go lower
in terms of using lower slots, not unless I want to leave a gap and possible
interfere with the space at the bottom where there will be some cables I
think. I would like to avoid filing though!

> I would start by moving the bottom RL02 down as far as possible, then
repeat
> with the second RL02.
> 
> Your observation that there is "very little clearance between the CPU 
> and the
> RL02 at the front but more at the back" suggests to me that you need 
> to fiddle move with the front and /or rear mounting flange positioning 
> on the various slides.  Don't assume that wherever "gravity drops 
> them" is going to
> be correct.
> 
> If none of the suggested adjustments are working then I would consider 
> getting out a rat-tail file and enlarging the slots on the mounting
flanges on
> the RL02 slides so as to let them drop a little lower (assuming that 
> you have
> clearance at the bottom!).
> 
> Regardless you shouldn't need to play with the slide-to-chassis 
> attachment points -- those are always "just so" and not really free to 
> fiddle with (no
> "slop").

There *is* a bit of play in the big black sheet that is screwed onto CPU
enclosure.

> 
> In my experience "racking" is a fiddling-time and clearances can be
*really*
> tight.  But ... it can be done :->.  Good Luck.
> 
> -----Original Message-----
> From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Rob Jarratt 
> via cctalk
> Sent: Saturday, February 26, 2022 3:15 AM
> To: General Discussion: On-Topic and Off-Topic Posts 
> <cctalk at classiccmp.org>
> Subject: Racking a PDP-11/24
> 
> I am wondering if I have racked my 11/24 correctly.
> 
> As you can see here:
> https://robs-old-computers.com/2022/02/10/pdp-11-24-progress/ I have 
> put the CPU at the top and the two RL02 drives underneath.
> 
> The problem is that the CPU enclosure catches on the RL02 underneath.
> There is a bit of play in the mounting bracket:
> https://rjarratt.files.wordpress.com/2022/02/cpu-mounting-bracket.jpg.
> With a bit of manipulation I can get the CPU to slide in. However, I 
> am wondering if I have racked it correctly? I don't think there is 
> room to
move
> the RL02s down and it would presumably leave a bit of a gap below the CPU.
> There seems to be very little clearance between the CPU and the RL02 
> at
the
> front but more at the back, but I am sure that the rails are mounted 
> horizontally. Is it just a matter of tightening the big screws that 
> hold
the
> mounting brackets to stop the play? If so I am not sure I have a big
enough
> screwdriver!
> 
> Regards
> 
> Rob