Replacing severed power cord - part 1, the removal of the line filter

POWER CORD WAS SLICED OFF AT SOME TIME IN THE PAST

When the IBM 1130 was rolled into my shop, its main power cord was missing. The cable was sliced off close to where it entered the line filter at the base of the machine. Sometimes this takes place when a machine is decommissioned and either given away or sold to a salvage operator, to limit liability for any injury if someone were to power up the system. It may also be done in a museum if they wish to eliminate the risk that someone will plug in a system that is not known to be in safe working condition.

In any case, this means that I have to install a new cable suitable for 240V. The 1130 main circuit breaker is 20A and the connector onto the line filter for the main power cord is a Russell and Stoll RS3720, a 20A connector with three round pins and a ring that screws onto the receptacle on the line filter. 

The power consumption of the 1130 itself (the processor box is an 1131; when considering the 1131 plus all attached peripheral boxes, the system becomes an 1130) is 1500W thus under 7A of draw. However, the peripherals receive their AC power from the 1131 which adds the requirements of card readers, line printers and other devices which is why the cord and circuit breaker support 20A total. 

There are a number of different plugs that support 240V 20A or 30A that can be attached to the building side of a power cord, the other end having the RS3720. I tend to use either NEMA L6-20P or L6-30P depending upon the outlet into which I will plug the 1130 system. 

A line filter sits between the power cord and the rest of the 1130. Its purpose is to filter out any electrical noise generated by the 1130 system so that it doesn't feed back into the building circuitry and cause interference with other electrical devices. 

LINE FILTER CORROSION ON FITTINGS AND DIFFICULT TO OPEN

The RS 3720 connector onto the line filter box was so corroded that I couldn't full unscrew it and remove the stub of the power cord. I had to disassemble the line filter partially to reach in and snip off the wires from the female RS3720 into the filter, just to remove the male connector and cable stub. 

The line filter box has plates on the top fasted with six small 1/4" bolts each, but with the bolts removed the plates did not budge. Likely they are corroded in place. The overall line filter box is mounted to a welded plate on the bottom of the 1131, using studs with nuts on both ends. One end of the stud fits through the welded plate and I could reach those nuts. The other end apparently terminates inside the line filter box rendering those nuts inaccessible with the box still sealed.

I attempted to unscrew the nuts from the bottom but the studs begin turning once they become slightly loose. Worse, for two of the four studs, turning it to try to unscrew it seems to be twisting internal wires around the stud, probably ground connections, because I can feel the increasing resistance of the wire stopping my rotation. 

I applied some percussive maintenance to the cover plates and they finally came loose. I was able to remove the wires and the line filter box from the machine. It appears to be in good shape, with the actual filter components hermetically sealed in the center section. 

Delightful patina

CLEANING BEFORE REINSTALLATION

I did some initial cleaning of the box but the gunk, a mix of dirt and rodent excretions, is stubborn. I have some wire brushes coming tonight and will use those to attack the worst of the coatings tomorrow. It will be an easy matter then to install the new line cord. I have to buy a cord fitting that fits the opening of the existing box, to secure the cable I bought.