Each plug-in bay in the 129 has an output circuit card. The deflection signal output on pins 17 and 21 of the plug-in drive the output circuit card. There are two varieties of output circuit card: active (cathode-follower) and passive. To use sampling plug-ins in the 129, active output circuit cards are required.
The power supply of the 129 is linear and consists of a power transformer followed by silicon rectifiers, electrolytic capacitors, and then regulators. Not all of the supply voltages are regulated. A ZZ1000 tube is used as the voltage reference. The regulators (feedback amplifiers and output tubes) are all tube, except for the high-current −12.2 V regulated supply. This one uses bipolar transistors for the feedback amplifier and output device. The −12.2 V supply is independently fused. The other supplies are not independently fused. The 129 has a 6.25 A fuse on the primary of the mains transformer.