It is a rack-mount unit with a power supply that resembles the power supply of an early 500-series oscilloscope: a 50/60 Hz transformer, selenium rectifiers, a single voltage reference tube controlling all output voltages, and 6080 pass tubes for the regulated outputs.
The 127 targets applications where one wants to use Tektronix plug-ins as front-end amplifiers for other instrumentation. Since letter-series and 1-series plug-ins have relatively high output impedance, their outputs (i.e., differentially between pins 1 and 3 of the plug-in-to-scope interconnecting plug) are not suitable for directly driving long cables.
The 127 provides 20 MHz unity gain amplifiers that can drive long cables and other low impedance loads. Each of the two amplifiers in the 127 is fully differential and outputs are on the front and rear panels. The 127 has unity gain if the output is taken differentially and the load on each output is 170 Ω.
The 127 is a 5U (8.75"/22.2cm) rackmount unit and weighs 37 pounds (16.9kg). Maximum power consumption is 450 W.