Racking a PDP-11/24

Sat Feb 26 16:04:23 CST 2022

> -----Original Message-----
> From: pbirkel at gmail.com <pbirkel at gmail.com>
> Sent: 26 February 2022 20:22
> To: rob at jarratt.me.uk; 'General Discussion: On-Topic and Off-Topic Posts'
> <cctalk at classiccmp.org>
> Subject: RE: Racking a PDP-11/24
> 
> 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.

Ah! Thanks for that advice.

> 
> -----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