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Convergence Drift Compensation

IP.com Disclosure Number: IPCOM000049063D
Original Publication Date: 1982-May-01
Included in the Prior Art Database: 2005-Feb-09
Document File: 2 page(s) / 38K

Publishing Venue

IBM

Related People

Beanlands, P: AUTHOR [+2]

Abstract

A circuit is used to compensate for convergence drift in a color cathode ray tube during warm up.

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Convergence Drift Compensation

A circuit is used to compensate for convergence drift in a color cathode ray tube during warm up.

A problem with all color display cathode ray tubes (CRTs), whether of the delta gun type or the newer in-line gun type, is that of static convergence drift during warm-up. This drift is of the order of 0.2- 0.3 mm, and is caused by small alignment changes in the electron guns whilst they are heating up from switch-on. The stabilization takes 10 to 15 minutes from cold, and also occurs if the display is switched off for a short time, and then switched on again. This problem is solved by the injection of compensating signals

into the convergence coils.

There are four parameters to be considered in a delta gun CRT:

(1) Red Radial Convergence.

(2) Green Radial Convergence.

(3) Blue Radial Convergence.

(4) Lateral Convergence.

The drift is a function of gun temperature and time, and so the compensating circuit must match the thermal time constant of the guns.

Since the gun temperatures are a function of the cathode temperatures (which are large compared to ambient), then an appropriate temperature and time constant analogue can be obtained by using a temperature sensor (such as a thermistor) in a metal block of appropriate mass to match time constants in contact with the high wattage series resistor of the heater circuit. It is not possible, of course, to obtain the gun temperatures directly unless a temperature sensor is placed inside the C...