Zenith B25A76R - User Manual

IMPORTANT SAFETY NOTICE
This manual was prepared for use only by properly trained audio-visual service
technicians.
When servicing this product, under no circumstances should the original
design be modified or altered without permission from Zenith Electronics
Corporation. All components should be replaced only with types identical to
those in the original circuit and their physical location, wiring and lead dress
must conform to original layout upon completion of repairs.
Special components are also used to prevent x-radiation, shock and fire hazard.
These components are indicated by the letter “x” included in their component
designators and are required to maintain safe performance. No deviations are
allowed without prior approval by Zenith Electronics Corporation.
Circuit diagrams may occasionally differ from the actual circuit used. This way,
implementation of the latest safety and performance improvement changes into
the set is not delayed until the new service literature is printed.
CAUTION: Do not attempt to modify this product in any way. Never perform
customized installations without manufacturer’s approval. Unauthorized
modifications will not only void the warranty, but may lead to property damage
or user injury.
Service work should be performed only after you are thoroughly familiar with
these safety checks and servicing guidelines.
GRAPHIC SYMBOLS
The exclamation point within an equilateral triangle is intended
to alert the service personnel to important safety information in
the service literature.
The lightning flash with arrowhead symbol within an equilateral
triangle is intended to alert the service personnel to the presence
of noninsulated “dangerous voltage” that may be of sufficient
magnitude to constitute a risk of electric shock.
The pictorial representation of a fuse and its rating within an
equilateral triangle is intended to convey to the service personnel
the following fuse replacement caution notice:
CAUTION: FOR CONTINUED PROTECTION AGAINST RISK OF FIRE,
REPLACE ALL FUSES WITH THE SAME TYPE AND RATING AS MARKED
NEAR EACH FUSE.
SERVICE INFORMATION
While servicing, use an isolation transformer for protection from AC line shock.
After the original service problem has been corrected, make a check of the
following:
FIRE AND SHOCK HAZARD
1. Be sure that all components are positioned to avoid a possibility of
adjacent component shorts. This is especially important on items trans-
ported to and from the repair shop.
2. Verify that all protective devices such as insulators, barriers, covers,
shields, strain reliefs, power supply cords, and other hardware have been
reinstalled per the original design. Be sure that the safety purpose of the
polarized line plug has not been defeated.
3. Soldering must be inspected to discover possible cold solder joints, solder
splashes, or sharp solder points. Be certain to remove all loose foreign
particles.
4. Check for physical evidence of damage or deterioration to parts and compo-
nents, for frayed leads or damaged insulation (including the AC cord), and
replace if necessary.
5. No lead or component should touch a receiving tube or a resistor rated at
1 watt or more. Lead tension around protruding metal surfaces must be
avoided.
6. After re-assembly of the set, always perform an AC leakage test on all exposed
metallic parts of the cabinet (the channel selector knobs, antenna terminals,
handle and screws) to be sure that set is safe to operate without danger of
electrical shock. DO NOT USE A LINE ISOLATION TRANSFORMER DURING THIS
TEST. Use an AC voltmeter having 5000 ohms per volt or more sensitivity in
the following manner: Connect a 1500 ohm, 10 watt resistor, paralleled by
a .15 mfd 150V AC type capacitor between a known good earth ground
water pipe, conduit, etc.) and the exposed metallic parts, one at a time.
Measure the AC voltage across the combination of 1500 ohm resistor and
.15 mfd capacitor. Reverse the AC plug by using a non-polarized adaptor
and repeat AC voltage measurements for each exposed metallic part. Voltage
measured must not exceed 0.75 volts RMS. This corresponds to 0.5 milliamp
AC. Any value exceeding this limit constitutes a potential shock hazard and
must be corrected immediately.
X-RADIATION
1. Be sure procedures and instructions to all service personnel cover the
subject of x-radiation. The only potential source of x-rays in current TV
receivers is the picture tube. However, this tube does not emit x-rays when
the HV is at the factory-specified level. The proper value is given in the
applicable schematic. Operation at higher voltages may cause a failure of
the picture tube or high-voltage supply and, under certain circumstances
may produce radiation in excess of desirable levels.
2. Only factory-specified CRT anode connectors must be used.
3. It is essential that the service personnel have available an accurate and
reliable high-voltage meter.
4. When the high-voltage circuitry is operating properly, there is no possibility
of an x-radiation problem. Every time a color chassis is serviced, the
brightness should be run up and down while monitoring the high voltage
with a meter, to be certain that the high voltage does not exceed the
specified value and that it is regulating correctly.
5. When troubleshooting and making test measurements in a product with a
problem of excessively high voltage, avoid being unnecessarily close to
the picture tube and the high voltage power supply. Do not operate the
product longer than necessary to locate the cause of excessive voltage.
6. Refer to HV, B+, and shutdown adjustment procedures described in the
appropriate schematics and diagrams (where used).
IMPLOSION
1. All direct view picture tubes are equipped with an integral implosion
protection system; take care to avoid damage during installation.
2. Use only the recommended factory replacement tubes.
TIPS ON PROPER INSTALLATION
1. Never install any receiver in a closed-in recess, cubbyhole, or closely
fitting shelf space over, or close to, a heat duct, or in the path of heated
air flow.
2. Avoid conditions of high humidity such as: outdoor patio installations
where dew is a factor, near steam radiators where steam leakage is a factor,
etc.
3. Avoid placement where draperies may obstruct venting. The customer
should also avoid the use of decorative scarves or other coverings that
might obstruct ventilation.
4. Wall- and shelf-mounted installations using a commercial mounting kit
must follow the factory-approved mounting instructions. A product mounted
to a shelf or platform must retain its original feet (or the equivalent
thickness in spacers) to provide adequate air flow across the bottom. Bolts
or screws used for fasteners must not touch any parts or wiring. Perform
leakage tests on customized installations.
5. Caution customers against mounting a product on a sloping shelf or in a
tilted position, unless the receiver is properly secured.
6. A product on a roll-about cart should be stable in its mounting to the cart.
Caution the customer on the hazards of trying to roll a cart with small
casters across thresholds or deep pile carpets.
7. Caution customers against using a cart or stand that has not been listed
by Underwriters Laboratories, Inc. for use with its specific model of
television receiver or generically approved for use with TVs of the same or
larger screen size.
8. Caution customers against using extension cords. Explain that a forest of
extensions, sprouting from a single outlet, can lead to disastrous
consequences to home and family.
PRODUCT SAFETY SERVICING GUIDELINES FOR AUDIO-VIDEO PRODUCTS
i
A.C. Voltmeter
1600 OHM
10 WATT
Place this probe
on each exposed
metal part.
Good Earth Ground
such as the Water
Pipe, Conduit, etc.
0.16uF
CM151 CA - SAFETY

Summary

Important Safety Notice

Fire and Shock Hazard

Details precautions to prevent fire and electric shock hazards.

X-Radiation

Discusses potential X-radiation sources and safety measures.

High Voltage and X-Radiation Protection

Safety Circuit Test Procedure

SC3492 Z Model Remote Control

Programming the MBR3447 and MBR3457 Z Remotes

Auto Find

Describes the Auto Find feature for programming remote codes.

Programming Codes

User Menus 221-1384

Channel;Add;Del

Allows selective marking of channels as Added or Deleted.

User Menus 221-1384 (continued)

Age Block

Restricts content based on age-appropriateness.

Set Password

Allows setting a password for Parental Control features.

User Menus 221-1385

EZ Program

Describes the EZ Program feature for automatic channel search.

User Menus 221-1385 (Continued)

Advanced TV Control

Accesses advanced TV control and parental settings.

Sex Scenes Block

Blocks content containing sex scenes.

User Menus 221-1386

Auto Program

Explains the automatic channel search and programming feature.

User Menus 221-1386 (Continued)

General Block

Blocks programs by general age rating or children rating.

Reentering Parental Control Menu

User Menus 221-1387

Auto Program (A34)

Channel Add;Del;Surf

Manages channels by adding, deleting, or surfing.

User Menus 221-1387 (Continued)

Video Menu

Allows adjustment of video parameters like contrast, brightness, and color.

TV Parental Guideline Rating System

Explains TV rating system for parental controls.

Children Block

Blocks channels for children based on age ratings.

Content Block

Blocks content by category like dialog, language, and violence.

Motion Picture Association of America (MPAA) Rating System

Details MPAA ratings for content restriction.

Advanced TV Control Menu

Accesses advanced TV control and parental settings.

MPAA

Sets content restrictions based on MPAA ratings.

Lock On;Off

Enables or disables the Advanced TV Control feature.

Installers Menu 221-1387

Service Menu Adjustments

Describes how to access and navigate the service menu.

Technical Settings

Factory Menu Recommended Settings

Lists recommended factory settings for service adjustments.

Installers Menu 221-1384 (continued)

Installers Menu 221-1385

Installers Menu 221-1386

Servicing

High Voltage and X-Radiation Protection

Discusses high voltage and X-radiation safety precautions.

Theory of Operation (Shutdown Circuit)

Explains the function of the high voltage shutdown circuit.

Servicing the Power Supply

Provides guidance on checking power supply voltages.

Servicing (continued)

Component-Level Servicing

Service Adjustments

Service Adjustments (Electrical)

Details electrical service adjustments like G2 and vertical size.

Focus Adjustment

Service Adjustments (continued)

Purity Adjustment

Details the procedure for adjusting CRT purity tabs.

Improving CRT Corner Purity

Convergence Adjustment

Explains how to perform static and dynamic convergence adjustments.

Modification

IF and Audio Alignment Procedure

Video Detector

AGC Delay

MTS and Stereo Decoder Alignment

Aligns registers for MTS and Stereo decoder ICs.

IF and Audio Alignment Procedure (continued)

Stereo, SAP VCO Adjustment

Adjusts VCO register for stereo/SAP signals.

Stereo, SAP dbx Filter Adjustment

Adjusts dbx filter for stereo/SAP signals.

Separation Adjustment

Adjusts stereo separation for MTS and Stereo models.

Schematics

Interconnect Diagram

Shows the overall interconnection of major components in the chassis.

Power Supply

Provides the schematic diagram for the power supply section.

Audio Processor Stereo

Shows the schematic for the stereo audio processor.

Audio Processor MTS

Provides the schematic for the MTS audio processor.

Audio Processor Mono

Shows the schematic for the mono audio processor.

Main Microprocessor

Displays the schematic for the main microprocessor section.

Video Processor

Provides the schematic diagram for the video processor.

Horizontal Vertical Deflection

Shows the schematic for the horizontal and vertical deflection circuits.

Jack and Switch Circuit

Picture In Picture

Video Output

PCB Layout

SMD Layout

Zenith B25A76R Specifications

General

Brand	Zenith
Model	B25A76R
Category	TV
Language	English