H742 Power Supply

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The H742 Power Supply, and later revision H7420, is a modular power supply system from DEC, used with the BA11-F Mounting Box. Among other uses, that mounting box is used in PDP-11/40, PDP-11/45 and PDP-11/70 systems to hold the CPU.

The H742/H7420 runs off standard wall AC; it can be jumpered to use either 110V or 220V supply.

The main transformer in the H742/H7420 provides 20-30V AC to several DEC standard modular regulators (up to five, in spaces numbered 'A' through 'E') mounted in the H742/H7420, which provide other voltages such as +5V to the backplanes in the mounting box.

Generally the power harness supports only limited options as to which regulator can be in which spaces; e.g. the later -11/40 harness mandates that A, B and C hold H744 +5V Regulators, D holds an H745 -15V Regulator, and E may hold either an H745 or H754 +20V, -5V Regulator.

A 5409730 board in the H742, replaced by an 5411086 board in the H7420, supplies a modest amount of +15V DC power to various backplanes mounted in the BA11-F, along with a line time clock signal, and AC and DC 'power OK' signals.

The harness from the H742/H7420 (the same for both, it appears) used DEC power distribution connectors to provide power to the backplanes; it normally used the DEC standard power wire colour coding.

The H742/H7420 did not support the Remote Switching Control Bus; the H742/H7420 was generally plugged into an 861 Power Controller, which did have the remote control capability.

A later version of the basic H742 is the H742A; the difference is not yet known.

For the H7420, in addition to the board above, the differences seem to be mainly mechanical: a larger cooling fan; a change in the orientation of the +15V supply board from horizontal to vertical (perhaps to make room for the fans); and the sheet metal enclosure holding the fan, board and transformer is a single unit, not two bolted together, as in the H742.

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