T7840

The Tektronix T7840  (P/N 154-0713-00, 154-0713-04) is a dual-beam CRT used in the 7844 scope, introduced in 1975. It was designed by Conrad Odenthal and Ken Hawken.

It has distributed vertical deflection plate pairs and separate horizontal deflection plate pairs. Both beams can write to the full 8 cm × 10 cm screen area.

Writing speed of a tube with blue P11 phosphor was specified as 1.7 cm/ns on high-speed film (Polaroid type 47, 3000 ISO).

The T7840 contains a dome-shaped metal mesh between the deflection plates and the screen, acting as a scan-expansion lens that increases the CRT's deflection sensitivity at the cost of some increase in spot size and intercepting some beam current. The latter loss is compensated by increasing the acceleration voltage to 24 kV.

In the T7840, the two electron gun structures are placed side by side to eliminate the vertical overlap problem present in earlier dual-gun designs such as the T5560 that place the guns on top of each other. The resulting horizontal overlap problem is solved by a pair of small auxiliary horizontal deflection plates between the vertical and horizontal deflection plate pairs of each gun. A DC voltage is applied to the extra plates to center the beams. See Patent US 3819984A.