BA11-K mounting box
The BA11-K mounting box was for many years the standard hex system unit backplane mounting box for the H960 series of 19"-wide racks produced by DEC, one of the BA11 mounting boxes. It could contain up to 5 standard size (4-slot) system units.
The H960 racks provided 63" of vertical mounting space, divided into units of 10-1/2", which were further subdividable into a pair of 5-1/4" spaces. The 10-1/2" tall BA11-K mounting box was designed for these spaces.
BA11-K's were used in both 'basic box' and 'expansion box' roles. The latter was used to hold extra devices, and had a blank front panel. The former contained the CPU, and had a front panel of some sort - either a programmer's console, or an operator's console.
In the PDP-11/04 and PDP-11/34 (the models which used this mounting box to hold the CPU) the front panels were the KY11-LA Operator's Console and the KY11-LB Programmer's Console (the same for both CPU's).
The system units were mounted longitudinally, facing upward (unlike the BA11 mounting box, where they had been facing down); they were generally mounted with the A-row connectors at the back, next to the power distribution panel.
Cooling fans mounted on the front face of the power supply passed air both over the boards, and then through the power regulators.
The BA11-K's dimensions were 10-1/2" high, 17-1/8" wide, and 17-1/4" deep without the power supply; with the H765 added, it was 26-1/2" deep, and weighed 87 lbs (without any backplanes or boards).
The BA11-K was also used in custom configurations to hold systems such as the MJ11 memory system and MK11 memory system, with a custom backplane and power harness. Technically, these boxes were not BA11-K's, but they share physical hardware, basic power system, etc with the BA11-K.
The BA11-K came with the H765 Power System, which was a separate unit, mounted transversely at the rear of the box; it contained up to four DEC standard modular regulators. The power harness from it ended in a power distribution panel containing standard DEC power distribution connectors.
One the back it had a pair of connections to the standard DEC Remote Switching Control Bus, so it could either turn other units off and on with a power switch on its front panel, or be remotely controlled from another box.
Like many other PDP-11 boxes, it was rack-mounted using multi-part slides. The 'inner' part of the slide is bolted to the box; the 'outer' is attached to the rack. The inner slides into the outer; a releasable latch prevents it being fully withdrawn accidentally. Many BA11-K units are now found with the inners still in place, but the outers have been discarded (often left with the rack they were mounted in).
Older units had 'Chassis Trak' slides from General Devices, coated in a zinc-phosphate 'Pakerize' coating (show in the image to the right). If the inners are there, General Devices still makes another slide whose outers are compatible to those used with the BA11-D, and those are readily available (albeit fairly expensive). The General Devices 'Chassis Trak' part number for that is C-230-S-122 (22"); the -124 (24") is sometimes slightly cheaper, and can easily be modified to fit an H960 rack, by drilling two extra holes. The safety latch location does not match, but the hole in the outer for it can be modified to fit.
Newer units had silvery zinc-plated slides (shown to the left), which used a smaller rotational controller; the manufacturer is unknown.
If the outers are missing, the Chassis Trak slides above can be used; their inners do not have the rotational capability, but the mounting holes in the inners match the locations of pre-mounted swage nuts in the BA11-K's side sheets, so the inner can be simply bolted straight on. The slide will be in a different location, vertically, though.
Note: The two types of slides use large pivot bolts/pins that appear to be identical, but they are not! The shoulder on the ones for the later kind of slides are not as deep as those on the earlier kind.