DEC 871B Power Controller

The 871B Power Controller is a single-phase 240VAC power controller found in some PDP-11 systems. It has 6 switched power sockets.

It has a small ON-OFF-ON double-throw centre-off toggle switch. With this switch in the OFF position, there will be no mains power at the power socket.

Power can be switched on manually (to all sockets simultaneously) by moving the small switch to the “LOCAL ON” position.

The “AUX ON/OFF” switch on the front panel of a BA11 cabinet can also be connected to the J1 or J2 connectors on the 871B. If this is done, power can be switched ON/OFF from the front panel of the BA11, if the 871B’s small switch is in the “REMOTE ON” position.

Inside the 871B is a small DC power supply, which is always on whenever the circuit breaker is in the ON position. This power supply closes the large relay inside the 871B, to switch mains voltage to the 6 power sockets.

Power Sockets

The power sockets on the 871B seem to be DEC’s standard connector for single-phase 240VAC equipment. Note that the two power pins are turned at 90 degrees, relative to the standard U.S. 120VAC power connectors.

DEC’s LSI-11 Systems Service Manual shows (at page 60) that the part numbers are as follows:

Socket: NEMA 6-15R, DEC 12-11204-01
Plug: NAME 6-15P, DEC 90-08853

Neon Indicator

As with every DEC Power Controller that I have, the neon indicator was dead.

This indicator is wired to show the presence of mains power at the input to the Power Controller. It is supposed to illuminate even if the circuit breaker and/or power switch are in the OFF position.

I found a suitable replacement: Chicago Miniature Lighting Part Number 1051C1. This is available from element14 as Order Code 138-5759 for A$5.03.

Fitting the new indicator was a simple process. I crimped a 5mm lug to each lead. Then it was just a matter of connecting it in place of the old indicator, by removing and replacing two screw terminals on the circuit breaker.

The new indicator works well. It has a nice red glow to it. It is a fairly solid red colour, not washed out as appears in the photo above.

The only small gripe I have with this indicator is that the silver-coloured collar is not particularly well formed, resulting in a very small and irregular gap between the panel and the collar. It is not noticeable unless you look very closely.