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Tektronix 3T77
Sampling sweep plug-in
Tektronix 3T77 front

Compatible with 560-series scopes

Produced from 1962 to 1973

Manuals – Specifications – Links – Pictures

The Tektronix 3T77 is a sampling sweep plug-in for 560-series scopes. It was introduced in 1962, along with the 567 and 3S76.

Bob Webb and maybe Jim Geddes and possibly Cliff Moulton designed the Type 77, which evolved into the 3T77.

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 one 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 POSITION knob instead DELAY knob, with more range of delay by slow sweep rate.

RECOVERY TIME knob can be pulled for HF-sync.


please add


3T77 Photos

3T77 Schematics

3T77A Photos

Some Parts Used in the 3T77

Page Class Description Used in
6688 Vacuum Tube (Pentode) miniature high-slope VHF/UHF pentode P80 3T77 526 TR-4602
7586 Vacuum Tube (Triode) Nuvistor triode M 1A1 1A2 1A5 10A2 10A2A 11B1 11B2A 321 321A 3A1 3A3 3A5 3A6 3A7 3A8 3A74 3S76 3S76A 3T77 3T77A 4S1 6R1 6R1A 82 86