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The TU-3 consists of dummy loading resistors, two cathode followers, and a bistable multivibrator.
The dummy loading resistors are connected to each regulated supply of the oscilloscope when the LOAD switch is set to HIGH, to test for the maximum power supply loading condition.
The cathode followers decouple the input signal from the oscilloscope vertical amplifier and elevate the vertical amplifier input to the required voltage (about +50 volts).