Zenith 4 SERVICING Diagram

SERVICE INFORMATION

SECTION 3

SERVICING

GA Shutdown Circuits

GA1

ICX2200

P29

RL

DX3003

C

GND

FROM_RESISTOR_DIVIDER

DX3002

GENERAL INFORMATION

NOTE: 19" and 20" GA1 chassis is a line connected

chassis. When servicing chassis, use an isolation
transformer to prevent possible shock hazard!!
The 25" and 27" GA2 and GA3 chassis has an
isolated supply.

Servicing the GA is the same as with other single board
chassis. If the set is dead, check the power supply first.
First the standby voltages, then the switched voltages.
If the switched voltages do not appear check the power
“On” circuit.

If the power supply is OK and the set will turn on, then
the horizontal sweep needs to be verified next. Is the
horizontal drive available from the video processor? If
the sweep system does not start-up, sweep derived volt­ages will not be generated.

If sweep and high voltage is OK and video or audio are
missing, then those circuits and the tuner need to be
analyzed.

If the receiver is working but some feature or accessory
is not working, check the Service Menu.

WARNING: If the power supply is operational and the

set still goes into Shutdown or shuts off, this
means that the vertical sweep or X-ray
protection is being activated by the micro.
Check for vertical pulse at the micro (IC6000)
pin 2, or the HV Shutdown Circuit. The set will
not turn on again unless the micro is reset by
unplugging the AC line cord.

The Vertical CRT Deflection Circuit must work in order to
generate the Vertical (Pulse) Signal, if not, the Vertical
Protection Circuit will be activated by the micro and
the set will shutdown. In case of X-RAY protection acti­vation, check the +130V source first and then the shut­down network. To turn it on again, reset the
microcontroller or unplug the set.

If the sweep and high voltage is OK and video or audio
are missing, then those circuits and the tuner need to
be checked.

GA2 & GA3

ICX2200

P29

ZDX3004

FROM_RESISTOR_DIVIDER

RL1

RL2

DX3005

C

GND

HIGH-VOLTAGE AND X-RADIATION PROTECTION

A potential source of X-rays is the picture tube, if the
high-voltage (HV) is out of specification. When the HV
is operating properly, there is no X-radiation. For the
GA chassis family, the HV has no adjustments. A HV shut­down circuit is used to prevent excessive HV and X-ray
emissions.

THEORY OF OPERATION (SHUTDOWN CIRCUIT)

The HV shutdown circuit appears on sheet 4 and sheet 5
of the schematics for 9-1869/71, 9-1789/91 and 9­1996/97 respectively. The basic HV shutdown circuit is
illustrated below.

This circuit basically monitors the sweep, pulse voltage
derived from the sweep transformer (TX3202 pins 7 & 9
in GA 19/20”; GA1 or TX3204 pins 5 & 6 in GA 25/27”;
GA2 and GA3. This voltage is rectified and applied to a

9.1 V zener diode (DX3003 in GA 19/20”; GA1 or ZDX3004
in GA 25/27”; GA2 & GA3). When the HV reaches its
maximum allowed value (see table below) the zener di­ode conducts, the voltage on pin 29 of the video pro­cessor (ICX2200) increases until it reaches the thresh­old voltage of 3.5 VDC, and shuts down the TV.

If the shutdown circuit has operated, the microcontroller
will prevent the TV from being turned on again, without
first having to unplug and plug the AC cord to reset the
microcontroller.

TIP: Make a momentary short circuit between the

OUTPUT and GND of the IC6002 (RESET CIRCUIT),
this will reset the microcontroller IC6000.

NOTE: In GA 19/20” or GA1 only, the R3212 is used for

the CRT heater and the shutdown circuit. So if
the video output board is unplugged from the
CRT, the voltage on pin 29 of ICX 2200 will be
increased, and the shut down circuit will activate.
If it is required to test the module without the
CRT or without the video output connected, then
add a load of 9 OHMS 5% 10W WIREWOUND
RESISTOR between pins 9 & 10 of the wireless
socket at the video output board.

CM150 3-1 GA - SERVICING

SERVICING (continued)

(

)

HV MEASUREMENT PROCEDURE

Apply signal through the video generator, or tune the
TV color receiver to crosshatch pattern. Connect an ac­curate HV meter between the picture tube anode and
chassis ground. Access Video Menu and adjust Bright­ness and Contrast controls for minimum screen luminance.
Wait until the Video Menu or display disappear. Read HV
meter.

The HV maximum value for each chassis is specified in
the following table:

SCREEN B+ VDC HV NOM HV MAX

SIZE

19 130 +/- 0.8 26.5 27
20 130 +/- 0.8 26.5 27
25 124 +/- 0.5 28.5 29
27 124 +/- 0.5 28.5 29

0 BEAM

COMPONENT LEVEL REPAIR

KV KV

SERVICING THE MODULE

After a module is serviced, access the Service Menu and
check that all items are set properly. When the factory
mode is off only the first seven items in the Service
Menu are available. Place factory mode to 1 to see all
Service Menu adjustments. Be sure the factory mode is
set to 0, which is Off, when the Service Menu is exited.
Also make sure that item 04 LEVEL is set to match the
Level as indicated on the model number sticker on the
back of the set. The Level is changed by placing a short
across pins 3 and 4 of connector 4G9 on the main mod­ule. Then using the remote control, bring up the Service
Menu and select item 04. Press the adjust button to
make a change. Be sure to remove the shorting jumper
when the adjustment is completed. Confirm that item
#25 is set correctly to match the set being serviced! If
not, and No Audio condition will arise.When in the Pri­vate label mode, the set will only respond to IR code

121. .

SERVICING THE POWER SUPPLY GA1 19”/20”

NOTE: Use an isolation transformer when servicing 9-

1869, 1870, 1871, 1871-01 and 1950 chassis !!

STANDBY VOLTAGES
Check the Standby Voltage at the following points:

POWER ON (9-1869/70/71 AND 9-1950)

Keyboard input at IC6000 pin 7 and 8
IR input at IC6000 pin 15
Power “On” output at IC6000 pin 32
Base of QX3402 power supply switching transistor (about

0.7 volts).
+5 volts IC6000 pin 27

SWITCHED VOLTAGES

Turn the set On and check the Switched voltages at the
following points.

+130 volts DC at FX3402
+150 volts DC at CX3406
+180 volts DC at + of C3207

SWEEP DERIVED VOLTAGES

+5 volts DC at cathode of ZD3206
+9 volts DC at pin 3 of IC3201
+12 volts DC at + OF CX3213
+14 volts DC at + side of C3222
+25 volts DC at D3202 cathode
+180 volts DC at C3207

SERVICING THE POWER SUPPLY

GA2 & GA3 25”/27” 9-1789/90/91 & 9-1831
Standby Voltages
+150 volts DC at RX3404
+123 volts DC at CX3420
+15 volts DC at emitter Q3403
+5 volts DC at cathode of ZD3401

SWITCHED VOLTAGES

Turn set On and check the Switched voltages at the fol­lowing points:

+5 volts DC at cathode of ZD3402
+9 volts DC at pin 3 of IC3431
+15 volts DC at collector of Q3404
+15 volts DC at collector of Q3403

+13 volts DC at CX3411
+12 volts DC at pin 1 of ICX3402

SWEEP DERIVED VOLTAGES

+25 volts DC at RX3242

+5 volts DC at pin 3 of ICX3402

CM150 3-2 GA - SERVICING

SERVICING (continued)

+35 volts DC at CX3272
+215 volts DC at junction CX3296 and RX3277
CRT filament at pins 1 and 2 of connector 2F5

VIDEO PROCESSOR ICX2200

Key Operating Signals:
Composite Video out at pin 47
Video in at pin 37
Luminance in pin 43
Chroma in pin 45
Composite Audio out at pin 2
Horizontal Drive at pin 32
Vertical Drive at pin 24
Video Output Blue at 2C5 pin 3
Video Output Green at 2C5 pin 2
Video Output Red at 2C5 pin 1
B+ 9 volts pins 9, 46 and 48
Serial Data, Serial Clock pins 27 and 28

+25 volts pin 6
Vertical out pin 2

HORIZONTAL CIRCUIT

Horizontal drive to base of Q3201 predriver
Driver transformer output base of Q3202
Shutdown voltage anode ZFX3004

SERVICE ADJUSTMENTS (Mechanical)

1. Video Gain (19"/20" Models) Adjusts R9738 for 1

VERTICAL CIRCUIT

Vertical drive pulse pin 4

CM150 3-3 GA - SERVICING

SERVICE ADJUSTMENTS

SERVICING (continued)

volt P-P at emitter of Q9706.

2. Audio Balance (19"/20" Models) Adjusts R9745 for
equal output at collectors of Q9701 and Q9703.

3. HORIZONTAL WIDTH (25"/27" Models) Adjusts R3249
for 1/2" overscan on both sides of the picture.

FOCUS ADJUSTMENT

Connect a color bar pattern to the receiver and adjust
the user controls for a good picture. Adjust the Focus
control, then push the menu button to video menu then
proceed until there is no distortion in the letters of the
menu.

GA1/GA2 RGB CUTOFF (1164 Micro)

The G2 control must be adjusted before RGB drive.

1. If the main module or CRT has been replaced, set
items #23 G GAIN and #24 B GAIN, in the Service

GA1 GA2

Tube Size 19” 20” 25” 27”
Customer Contrast 0 0 0 0
RF Bright/Aux Bright 36 36 26 28
Customer Brightness 31 31 31 31
Red Cutoff 0 0 10 5
Green Cutoff 0 0 10 5
Blue Cutoff 0 0 10 5
Green Gain 90 90 90 90
Blue Gain 90 90 90 90

Menu, to the default setting for the correct screen
size as listed in the Service Menu (see above table).

2. Set color level to minimum and Tint to midrange.

3. Connect the output of an NTSC generator to the
antenna input of the receiver. Set the generator to
a pure white signal, chroma off.

4. Access the Service menu to gain access to the cut­off adjustments, (Factory mode item 00 must be set
to 1 to gain access to these adjustments).

5. Set items 22 R Cut, 23 G Cut and 24 B. Cut to the
values shown in the above table.

6. Carefully observe which color is predominant on the
CRT - DO NOT adjust the cutoff control for this color.
Adjust the other two cutoff controls for the best
white screen display.

7. Set the generator to a color bar patter and turn the
chroma off. Check that the set displays a good gray

scale from black to white. If black level is too high
readjust the item #9 RF Brightness.

8. Return the color level control to normal.

GA3 OF RGB CUTOFF (1305 Micro)

The G2 control must be adjusted before RGB drive.

1. If the main module or CRT has been replaced, select

GA3

Tube Size 25” 27”

Customer Contrast 0 0
RF Bright/Aux Bright 40 40
Customer Brightness 32 32
Color Temp Cool Cool
Red Cut 1 15 15
Green Cut 1 8 5
Blue Cut 1 12 12
Green Gain 1 95 100
Blue Gain 1 70 80
Color Temp Warm Warm
Red Cut 2 15 16
Green Cut 2 10 8
Blue Cut 2 17 16
Green Gain 2 70 75
Blue Gain 2 25 32

“Cool” in the user video menu under Color Temp.
Then set items #25 Gain 1 (Green gain cool) and
#26 B Gain 1 (Blue gain cool), in the Service Menu
to the default setting for the correct screen size as
listed in the Service Menu (see table above). Set
Color level to minimum and Tint to mid range.

2. Connect the output of an NTSC generator to the
antenna input of the receiver. Set the generator to
the Pure white signal, chroma off.

3. Access the Service Menu to gain access to the cut­off adjustments, (Factory mode item 00 must be set
to 1 to gain access to these adjustments).

4. Set items 22 R Cut, 23 G Cut, 24 B cut to the values
shown in the table.

5. Carefully observe which color is predominant on the
CRT. DO NOT adjust the cutoff control of this color.
Adjust the other two cutoff controls for the best
white screen display.

6. Set the generator to a color bar pattern and turn

CM150 3-4 GA - SERVICING