Genius ISO XFR-75W, 2093, 1493, 2793, 3693, 1793 CRT Auto Bias and Auto Bright Circuit Description

CRT AUTO BIAS AND AUTO BRIGHT CIRCUIT DESCRIPTION.

The beam current feedback circuit uses a PNP video

transistor 954 to direct most of the beam current to the auto bias circuit while passing the voltage waveform, from the video amplifiers to the CRT cathodes. Diode 958 and capacitor 956 insure that no video waveform

distortion occurs. An additional benefit of this circuit is that it protects the video amplifiers from the destructive

arc energy. Resistors	948	and	955	divide energy due
to CRT arcing, between	the	video amplifier transistors
and the beam current feedback transistor						. The
					954
beam current is filtered by capacitor					and	resistor
				941

C10 and is buffered by an operational amplifier, which translates the beam current into a low impedance voltage. This voltage is applied to a coupling capacitor

	921	through a 200 ohm resistor			C8	.
	The 200 ohm and the 68.1K resistor							forms the
							C3
program value which sets the black level								voltage via the
action of the program pulse.
	Capacitor		922	is used to stabilize the

transconductance amplifier which is used at the channel input of the auto bias IC 927 . The auto bias IC stores the bias voltage of this channel in capacitor 895 at pin

21.This voltage is buffered by an internal amplifier, with output at pin 20, which is connected to the Blue video amplifier bias control input.

Resistor 908 , 910 , and 911 are part of the auto bright circuit. They are used to sum the bias voltage of each of the three channels via a voltage node at the auto

bright amplifier,			920		pin 9. The resulting output
voltage then controls				the screen grid via transistor							.
										850
Resistor		protects the CRT grid from excessive
	881
current	during	arcing.			Capacitor			supplies a low
						878
AC impedance to GND to insure that the CRT gain is
constant during each horizontal line.							Resistor			and
									858

914defines the current gain of, and stabilizes, the auto bright control loop.

Load resistor	909	eliminates crossover distortion from
the OP Amp.		. Resistor		and		protects the
	920		852		856

transistor and OP Amp. from damage due to CRT arcing.

PNP transistor 928 is used as a voltage translator to

direct the grid pulse from the auto bias IC to G1. The voltage on G1 is normally -15 to -27 volts depending on which CRT is used. When the grid pulse at pin 11 is

low, the current from resistor

933

is conducted to

resistor

and produces a 10 volt pulse on the minus

874

G1 line.

Capacitor

and resistors

&

871

855

873

protect transistor

from CRT arcing.

928

The auto bias IC

(CA3224E) is designed for a supply

voltage of +10V and since the video amplifier requires

+12V, three diodes	903	,	905	, and		906	are used to
supply this IC. Resistors					and		form a voltage
			C4			C7

divider which supplies the, auto bright, bias voltage to the LM324 920 . The green and blue channel circuits are identical to the red channel and are controlled by the timing logic in the same way.

Refer to the waveforms at the bottom left of page 34 for the timing relationship. The vertical retrace boost pulse, from the LA7838, (15KHz models) or the delayed vertical sync pulse from the sync delay circuit (25 & 31KHz models) starts the 21 count auto bias state counter. This pulse is applied to the auto bias IC through resistor 891 . The negative going flyback pulse which is used to heat the filament also supplies the horizontal sync to the auto bias IC via diode 884

and resistor 888 . The grid pulse becomes active between the 15 and 18 horizontal cycle and the program pulse is active between the 18 and 21 horizontal cycle. These two pulses in conjunction with the internal control of the transconductance amplifier output switch are what establish the timing for the measurement and setting of the video bias.

CRT AUTO BIAS, VERTICAL SYNC CIRCUIT DESCRIPTION.

The auto bias vertical sync comes from from a buffer circuit for 15KHz operation. For 31KHz operation this signal is generated by a delay counter. For both cases, the vertical boost pulse is "and" connected with the Vertical Osc. O/S to provide flicker free operation and laser beam protection. In the case of vertical deflection failure, the loss of the boost signal causes the auto bias vertical sync to stop, which stops the auto bias function, and blanks the screen via the vertical blanking circuit, thus providing for laser beam protection.

The vertical oscillator one shot (LA7851 pin 16) supplies the start timing for the auto bias vertical sync. This signal is conducted to the emitter of

379by jumper 089 . The base of 379 is connected to the retrace boost pulse by resistor divider 383 and 384 . Combining these signals in this way produces a collector waveform which has the vertical oscillator one shot timing and is dependent on the retrace boost pulse.

			+12				CRT AUTO BIAS
	6.8K	Hfo=15KHz
							VERTICAL SYNC
	099		510Ω
PN2222
	1
2			006
							For Hfo=25-31KHz
	3		+12
100					1N4148
	P		V						1N4148
			62K	62K
			098			006				2H Dly.
4				099						024
					1	CL	Q1 12
0Ω	PN2222								1N4148
100A					2	KL	Q2 11			4H Dly.
					8,10,13	NC	Q3	9		025
7
					14	Vdd	Q4	6	1N4148
200K	379		0Ω		7	Vss	Q5	5		8H Dly.
					3	Q7	Q6	4		027
CS=.45"			CS=.74
						CD4024 100			1N4148 16H Dly.
383			389
				200K	.1uF	Autobias
	22K								028	15.8K
	384			201	101	Delay			220pF	381
			GND				GND
Retrace Boost				Filament					091	+24V

Vertical Osc. O/S, LA7851 pin 16

For the 15KHz case, transistor	100	inverts the
vertical oscillator one shot signal to produce the
CRT auto bias vertical sync signal.	Resistor		099

is the pullup and resistor 006 reduces the chance of arc damage to the transistor.

For the 31KHz case, the vertical oscillator one shot signal is directed to the clear of the 7 bit counter 100 . This O/S time out must occur after the autobias delay time out and come before the bias active pulse from the CA3224E. When the clear is low, the counter counts horizontal pulses, by the clock connected voltage divider 099 and

201 . When the counter outputs ones at each diode

connected output, further counts are inhibited by diode 006 and pullup 381 . This diode "or" signal is also used for the CRT auto bias vertical sync. The delay is set to locate the grid pulse generated 3 faint lines at the top of the screen with full vertical deflection. Capacitor 091 produces a

delay to avoid a race condition between the counter clock and the auto bias horizontal sync.

72