It triggers on either the internal trigger pick-off from the vertical plug-in or from an external trigger signal input. Triggering is faster and more sensitive with an external input.
The 3T77 can operate in any of four modes: external sweep, manual sweep, normal, or single sweep. In external sweep mode, an external control voltage is applied to a BNC connector on the front panel of the 3T77. This voltage determines the X-position of the spot on the CRT and, correspondingly, the delay from the trigger event to the sampling point. Manual mode is the same as external sweep mode, but the control voltage is generated internally based on the position of a potentiometer on the front panel of the 3T77. In normal mode, an internal ramp generator produces the control voltage, repeatedly redrawing the trace on the CRT. In single sweep mode, the internal ramp generator is started by a button on the front panel, makes on horizontal trace, and then waits for the button to be pressed again.
While still tube-based, the 3T77 uses 2N976 transistors and 1N3129 tunnel diodes. It uses a 6688 tube in the staircase generator circuit, and an RCA 7586 nuvistor as a ramp-current generator. The 3T77 mixes "classic" chassis/ceramic strip construction with the trigger circuit built on a PCB.
Difference of 3T77 and 3T77A
Time-Postiton instead Delay-Knob, with more range of delaying by slow sweep rate. Recovery-Time-Knob can be pulling for HF-sync.