Fisher TX 420 Service Manual

Page 1

PRICE $1.00

SERVICE MANUAL TX'420

SERIAL NUMBERS
BEGINNING 10001

The Fisher®
TX-420

Stereo To
Four-Channel Converter

WORLD LEADER IN HIGH QUALITY STEREO

Page 2

CONTENTS and SERVICE TIPS

PAGE

REQUIRED TEST EQUIPMENT
MECHANICAL DISASSEMBLY
HARMONIC DISTORTION TEST
CHASSIS LAYOUT

SIGNAL FLOW

2 + 2 DECODER

CONTROL AMP

POWER AMP

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PAGE

PRE-AMP/POWER SUPPLY 1O

CARTRIDGE PLAYER
FUNCTIONAL OPERATION 12
PREAMPLIFIER 14
ADJUSTMENTS 15
DISASSEMBLY 16
INDEX/PARTS LIST 17
TROUBLESHOOTING GUIDE 18

CAUTION I

This precision high-fidelity instrument should be serviced only by qualified personnel,
trained in the repair of transistor equipment and printed circuitry.

Many of these items are included only as a reminder 7 they
are normal procedures for experienced technicians. Shortcuts
may be taken, but these often cause additional damage to
transistors, circuit components, or printed circuit boards.

SOLDERING: A well-tinned, hot, clean soldering iron tip
will make soldering easier, without causing damage to the
printed circuit board or the components mounted on it.
Regular use of a sponge cleaner will maintain a clean
soldering surface. The heat available at the tip, (not the watt-
age of the iron) is important. Some 50-watt irons reach tem-
peratures of 1,0000 F, while others will hardly melt solder.
Small-diameter tips should be used for single solder con-
nections, pyramid and chisel tips for large areas.

Always disconnect the AC power cord from the line when
soldering. Turning the power switch OFF is not sufficient.
Power-line leakage paths. through the heating elements of the
iron. may destroy transistors.

PARTS REMOVAL: li a part is not being returned for in-
warranty factory replacement, it may be cut in half (with
diagonal cutting pliers) to make removal easier. Multiple
terminal parts, such as lF transformers, or electrolytic capaci-
tors, should be removed using special de-soldering tips made
especially for this purpose. Removing solder from terminals,
reduces the possibility of breaking the printed circuit board
when the part is removed.

ACCIDENTAL SHORTS: A clean working area, free of metal
particles. screws, etc., is an important preventive in avoiding
servicing problems. Screws, removed from the chassis during
servicing, should be stored in a box until needed. While a set
is operating, it takes only an instant for a base-to«collector
short to destroy a transistor (and others direct-coupled to it).
In the time it takes for a dropped screw, washer, or screw-
driver, to contact a pair of terminals (or terminal and chassis),
a transistor can be ruined.

SOLlDASTATE DEVICES: Integrated Circuits contain the
equivalent of many circuit parts, including transistors, diodes,
resistors, and capacitors. The preferred troubleshooting pro-
cedure requires isolating the trouble to one stage using AC
signal tracing methods. Once the suspected stage is located
the DC voltages at the input and output leads are measured to
give an accurate indication of the operating conditions of the
1C. DO NOT use an ohmmeter, to check continuity with the
IC mounted on the printed circuit board. FonNard biasing the
internal junctions within the lC may burn out the transistors.
Do not replace a defective lC until all external resistors,
capacitors, and transformers are checked first. to prevent the
replacement IC from failing immediately due to a defect in
the connecting components. Solder and unsolder each lead
separately using a pliers or other heat sink on the lead to

prevent damage from excessive heat. Check that the leads are
connected to the correct locations on the printed circuit
board before turning the set on.

Whenever possible, a transistor tester should be used to de-
termine the condition of a transistor or diode. Ohmmeter
checks do not provide conclusive data, and many even de-
stroy the junctionlsl within the device.

Never attempt to repair a transistor power amplifier module
until the power supply filter-capacitors are fully discharged.

If an output or driver transistor becomes defective (opens or
shorts), always check ALL directAcoupled transistors and
diodes in that channel. In addition, check the bias pot., and
other parts in the bias network, before installing replacement
transistors. All output and driver transistors in one channel
may be destroyed if the bias network is defective. After parts
replacement, check bias for specified idling current.

In some applications, replacement of transistors must be
made from the same beta group as the original type. The
beta group is indicated by acolored marking on the transistor.
Include this information when ordering replacements.

When mounting a replacement power transistor, be sure the
bottom of the flange, mica insulator, and the surface of the
heat sink. are free of foreign matter. Dust and grit will pre-
vent flat contact, reducing heat transfer to the heat sink.
Metallic particles can puncture the insulator, cause a short,
and destroy the transistor.

Silicone grease must be used between the transistor and the
mica insulator and between the mica and the heat sink for
best heat transfer. Use DowACorning D08, or an equivalent
compound made for power transistor heat conduction.

Use care when making connections to speakers and output
terminals. To reduce the possibility of shorts, lugs should be
used on the exposed ends, or stranded wire should be tinned
to prevent frayed wire ends. Current in the speakers and out-
put circuitry is quite high -- poor contacts, or small wire,
can cause significant power losses. For wire lengths greater
than 30 feet, 16 AWG, or heavier, should be used.

VOLTAGE MEASUREMENTS: All voltages are measured
with the line voltage adjusted to 120 volts. All measured
voltages are t 20%. DC voltages are measured to chassis
with a VTVM, with no signal input unless otherwise noted.
AC signal voltages are measured under the conditions speci-
fied on the schematic.

ALlGNMENT PROCEDURES: DO NOT attempt realignment
unless the required test equipment is available, and the align-
ment procedure is thoroughly understood.

© COPYRIGHT 1972 FISHER RADIO ALL RIGHTS RESERVED PRINTED IN U.S.A.

Page 18

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18

SYMPTOM

TROUBLE SHOOTING GUIDE

CAUTION: IN ANALYZING THE CAUSE OF COMPLAINT, DO NOT INSERT ADDITIONAL TAPE CAR-
TRIDGES UNTIL THE POSSIBLE CAUSE AND REMEDY HAVE BEEN REFERRED TO IN THIS GUIDE.

CAUSE

REMEDY

Tape breaks or winds up
around capstan shaft.

Squealing or excessive wow.

Drive belt inps off flywheel '
(also slow tape start-up).

Tape pulls out of center of
cartridge when cartridge is
removed from unit.

Poor tape drive (also
wow and flutter).

Crosstal k,

Motor does not run when
cartridge is inserted.

Intermittent audio during
tape operation.

Improper head indexing,
intermittent head indexing.

Unit does not change Program

Slow speed (also see
"Nor Tape Drive").

1. Defective tape cartridge.

1. Defective tape cartridge.

2. Defective capstan shaft
and/or bearings,

1. Defective Drive belt.

2. Motor bracket bent (drive
shaft not vertical).

1. Undersized or burred
tape guide slot.

1. Flywheel and/or motor
pulley dirty or greasy.

2. Drive belt Improperly
instalIed.

3. Drive belt defective
(stretched, eth,

4. Defective capstan shaft,
and/or bearings.

l. Defective tape cartridge.

2. Tape head out of
adjustment

3. Head mounting hardware
binding.

4. Tape guide bracket not
positioning tape correctly
with respect to head.

5. Defective cam and shaft
assembly.

6. Tape head not indexing
correctly.

I. Microswitch not actuated.

1 . Intermittent tape-capstan,
or tape~to-head Contact.

2. Intermittent head-
connecting cable.

I. Inadequate tension on

solenoid plunger return spring.

2. Defective plunger.

3. Solenoid plunger endstop
screw not properly adjusted.

4. Cam tension spring out of
hole, or missing.

5. Defective cam and shaft
assembly.

I. Solenoid not operating.

2.|Tape head not indexing
properly.

1. Binding tape cartridge.
2, Defective drive belt.

3. Defective capstan shaft,
arid/or bearings.

Replace cartridge with knowu good cartridge.

Replace cartridge with known good cartridge.
Replace flywheel assembly, and/or capstan housing.

Replace drive belt.

Straighten bracket. If belt rides high on puIIey,
insert screwdriver between motor and flywne
(in circular slot of chassisl and pry motor awa
from flywheel until shaft is aligned. If belt
rides low, insert screwdriver in slot an op-
posite side of motor and pry toward flywheel
until aligned.

CAUTION: DO NOT APPLY PRESSURE TO
PULLEY.

Deburr slot or replace guide bracket.

Use isopropyl alcohol to clean the outer edge
of flywheel, motor pulley, and capstan shaft.

Make certain rough surface of belt is on inside.
(Rough surface drives flywheel.) 0n belts with
stripes, install belt with stripes on outside.

Replace drive belt.

Replace flywheel assembly, and/or capstan
housing;
Replace cartridge with known good cartridge.

Check head height and azimuth adjustments as
outlined on page 15.

Check for obstructions or bent mounting rods.
Index head to lower position, then lift up
head. It should have sufficient tension to spring
back without binding.

Check distance between bottom edge of tape
guide bracket and chassis. lf distance is not
.095 to .I00, replace tape guide bracket.

Replace cam and shaft assembly.
See "Improper Head Indexing",

Replace switch, or bend actuating arm
enough to actuate switch when cartridge is
inserted.

See Poor Tape Drive".

Determine if cable is faulty by wiggling re-
ceptacle end of cable, or feed a signal into
input of preamplifier. if cable is faulty,
replace.

Increase spring tension, or replace spring.

Replace solenoid plunger.

Adjust solenoid end-stop screw as outlined
on page l5.
Insert spring into its hole, or replace spring.

Replace cam and shaft assembly.

Check solenoid circuit. Repair or replace
defective parts.

See Improper Head indexing".

Replace cartridge with known good cartridge.
Replace drive belt.

Replace flywheel assembly, and/or capstan
housing.