The brown boxes are "zero set switches", each with a "zero set amplifier" just above. The zero set equipment balances out tube differences in the operational amplifiers. Each "zero set switch" corrected or balanced 12 operational amplifiers. One of the "zero set switches" did not zero the operational amplifiers, but instead performed non-precise amplification, such as used on the plotting boards.
The black dots on the gray panel are really holes for test points to aid in trouble shooting (very rarely needed).
Behind these two racks are many relays used to control many functions in the computer, (test modes, timers, the burst command, etc.). Also in the racks behind are modulators and power amplifiers to drive the servo motor potentiometers and the plotting board motors.
There are two other racks. These contain:
- rack 3 - contains seven motor driven potentiometers
- - - - - the largest is the time pot (18 inches in diameter)
- rack 4 - contains power supplies