Genius 2793, ISO XFR-75W, 2093 Blanking, Master Gain, And Fault Circuit, Function, Description

BLANKING, MASTER GAIN, AND FAULT CIRCUIT, FUNCTION, DESCRIPTION.

SIMPLIFIED GAIN CONTROL CIRCUIT:

VIDEO INTERFACE

C5346

+12V

+

To

GAIN SELECT

Video

241

+12V

Amp.

CRT

RESISTORS

1K

MASTER GAIN

1K

VIDEO GAIN LINE

3.6K

200

Ω

+7.5V

485

062

One of three input circuits.

HORIZONTAL BLANKING

PN2222

3

+3.4V

FLYBACK PULSE

FAULT CIRCUIT

1/4

0VDC

Hs

SIGNAL

56Vpp 62,D6

CONDITIONING

LM324

To P/S OVP

2

CIRCUIT

072

Total

033

beam current

BEAM CURRENT LIMITER

From FBT

High Temp. +12V

BIAS ACTIVE

VERTICAL BLANKING

+6V

HIGH Z +2.5V

6

MPSA64

Beam Limit

28.0K

1N4148

3

+

1N4148

1/2

7

PN2222

020

LM393

D

Vertical Bias O/S

1/2

1

253

5 +

+

018

100K

LM393

210

036

10uF

T

+2.5V

2

@ 25°C

210

071

014

.047uF

180

207

Blanking in this monitor is accomplished by reducing the video gain to zero during the vertical and horizontal blank time. During video time, the gain is set by the master gain control which is located on the remote control PCB. If the overall beam current exceeds

.75mA or 1.5mA (depending on model) for more then ten frames, the beam current limiter circuit will reduce the video gain to protect the FBT. A high temperature sensor, close to the FBT, will also reduce the beam current if the high temperature limit (70°C) is exceeded.

The fault circuit senses the temperature or beam current line and will turn off the monitor if either of these signals exceeds the beam current shut off value. If an abnormal condition exists in the monitor or the cooling system of the enclosure fails, the high temperature sensor will activate the fault circuit at 80°C. The fault circuit is also turned on when the beam current becomes large enough to damage the FBT. This condition will happen if the video bias supply (V+ –9V) on the video board fails. An OP Amp. is used to sense the fault condition and a transistor is used to transmit the fault signal down to the power supply.

The video P-P voltage amplitude at the cathodes, is the video input signal amplitude times the master gain control setting times the video amplifier gain. The gain select resistors set the maximum video gain via the master gain line. For a greater range of brightness, (highlighting) the video system is allowed to supply high peak video currents which could damage the FBT if sustained. The beam current limiter circuit insures that the long term maximum beam current is not exceeded.

Horizontal blanking is achieved by amplifying the flyback pulse (FBP) with transistor 072 . Vertical blanking starts as soon as the LA7851 starts the vertical retrace sequence and is terminated by the auto bias, bias active signal. A comparator is used to sense the vertical bias O/S, at pin 16 of the LA7851, which goes low when vertical retrace starts. Capacitor 207 holds the vertical blanking active, between the vertical bias O/S pulse, and the bias active pulse. When the bias active line goes high, the capacitor 207 is reset and vertical blanking ends, after the bias active line returns to it's high impedance state.

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