Fisher 49 A T Service Manual

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Service Manual

THE FISHER

Consoles
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77 K ' . I AMPLIFIER
F wwwww , 49|
TUNER

FISHER R'ADIO - LONG ISLAND CITY 1 NEW YORK

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CAUTION: This is a FISHER precision high-fidelity in-
strument. it should be serviced only by qualified personnel -
trained in the repair of transistor equipment and printed circuitry.

EQUIPMENT AND TOOLS NEEDED

The lollowing are needed to completely test and align modern high-fidelity instruments
such as amplifiers. tuners and receivers.

Iest Instruments

Vacuum-T e Voltohmmeter DC VTVM

Audi (A Vacuum-Tube Voltmeter (AC VTVM)
Oscill . ope (Flat to 100 kc minimum)

Audio (Sine-wave) Generator

Intermodulation Analyzer

SWeep (FM) Generator (88 to 1.08 mc)

Marker Generator .

Multiplex Generator (preferably with RF output-

FISHER Model 300 or equal).

Many of the items below are included just as a re-
minderT-they are normal procedures for experienced
techniCIans. Shortcuts can be taken but often they
cause additional damage-to transistors, circuit com-
ponents or the printed-circuit board,

Soldering-A well-tinned, hot, clean soldering iron tip
Will make it easier to solder without damage to the
printedcircuit board or the many many circuit com
ponents mounted on it. It is not the wattage of the
iron that counts-it is the heat available at the tip.
Low-wattage soldering irons will often take too long to
heat a connection-pigtall leads will get too hot and
damage the part. Too much heat, applied too long, Will
damage the printed-circwt board. Some 50-watt irons
reach temperatures of 1.000 F-others will hardly
melt solder. Small-diameter tips should be used for
single solder connections-larger pyramid and chisel
tips are needed for larger areas.

0 When removing defective resistors, capacitors, etc,
the leads should be cut as close to the body of the
circuit component as possible. (If the part is not being
returned for in-warranty factory replacement it may be
cut in half_-with diagonal-cutting pliers-to make
removal easier.)

9 Special desoldering tiplets are made for unsolder-
ing multiple-terminal units like IF transformers and
electrolytic capacitors. By unsoldering all terminals at
the same time the part can be removed with little
chance of breaking the printed-circuit board.

. Always disconnect the chassis from the power line
when soldering. Turning the power switch OFF is not
enough. Power element, can destroy transistors.

Transistors-Never attempt to do any work on the
transistor amplifiers Without first disconnecting the
Pic-power Iir'iecora_~ wait until the power supply filter-
capaCitors have discharged,

3 Guard against shorts-ii takes only an instant for
a baseato-collector short to destroy that transistor and
possibly others direct»coupled to it. [in the time it
takes for a dropped machine screw, washer or even
the screwdriver, to glance off a pair of socket terminals
(or between a terminal and the chassis) a transistor
can be ruined]

a DO NOT bias the base of any transistor to, or near,
the same voltage applied to Its collector.

- DO NOT ,use an ohmmeter for testing transistors.
The voltage applied through the test probes may be
higher than the base-emitter breakdown voltage of the
transistor.

Output Stage and Driver-Replacements for output
and driver transistors, if necessary, must be made from
the same beta group as the original type. The beta
group is indicated by a colored dot on the mounting
lange of the transistor. Be sure to include this in-
formation, when ordering replacement transistors.

PRECAUTIONS |

Miscellaneous

Adiustable-LineVoltage Transformer or
line-voltage regulator

Load Resistors (2) ~&ohm, 50-watt (or higher)

Stereo source (Turntable with stereo cartridge
or Tape Deck)

Speakers (2) Full-range, for listening tests

Soldering iron (with small-diameter tip).
Fully insulated from power line,

and check the bias idjustment, the control and other
parts in the netw a with an ohmrneter before insert-
ing a new transistor. All output transistors in one
channel will be destroyed if the base-biasing circuit
is open on the emitter end.

0 When mounting arreplacement power transistor be
sure the bottom of the flange, the mica insulator and
the surface of the heat sink are free of foreign matter.
Dust and grit can prevent perfect contact. This re-
duces heat transfer to the heat sink. Metallic particles
can puncture the insulator and cause shorts-ruining
the transistor.

0 Silicone grease must be used between the transist-
or and the mica insulator and between the mica and
the heat sink for best heat conduction. Heat is the
greatest enemy of electronic equipment. It can shorten
the life of transistors, capacitors and resistors. (Use
Dow-Corning DC-3 or C20194 or equivalent compounds
made for power transistor heat conduction.)

0 Use care when making connections to speakers and
output terminals, Any frayed wire ends can cause
shorts that may burn out the output transistors-they
are directvcoupled to the speakers. There is no output
transformer- nothing to limit current through the tran-
sistors except the fuses. To reduce the possibility of
shorts at the speakers, lugs should be used on the
exposed endswat least the ends of the stranded wires
should be tinned to prevent frayed wire ends. The
current in the speakers and output circuitry is quite
high. Any poor contact or small-size wire. can cause
power losses in the speaker system. Use 14 or 16 AWG

0 If one output trans" .or burns out (open or shorts),
always remove all 0 JUt transistors in that channel

, for long runs of speaker-connecting wiring.

DC-Voltage Measurements-These basic tests 0 the
tranSIStor CirCUitry_are made without the signal gener-
ator. Without any Signal input )measure the circuit volt-
ages-as indicated on the schematic. The voltage
difference between the base and the emitter should
be in the milllvolt range-a sensitive DC meter is
needed for these readings. A low-voltage range of lvolt,
full scalewor lower-is needed.

Audio-Voltage (gain) Measurements-The schematic
and printed-circuit board layout diagrams are used.
Input signals are injected at the proper points-found
most quickly by using layout of the printed-circuit
board instead of the schematic. An AUDIO (AC) VTVM
connected to the test points should indicate voltages
close to those values shown in the boxes on the sche-
matic. Many of the signal levels in the input stages
are only a few millivolts-they can not be read on the
AC ranges supplied on most Vacuum-Tube AC/DC Volt-
ohmmeters (VTVMs). Even with a l-volt range a signal
level of 100 millivolts (.lvolt) will be the first 1/10
of the meter scale. A reading of 1 millivolt (.001 volt)
will hardly even move the meter needle.

Page 12

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R25, 26
R27, 28
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R31, 32
R33, 34
R35, 36
R37A, B
R38
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R41
R42
R43
R44
R45, 46
R47
R48, 49
R50
R51
R52
R53, 54

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Dela, _ MISCELLANEOUS the supply voltage var-
Composition, 330, l0%, 2w 1104anan cm, 2 Diode, Sillcon Reamer SRsom-I 'emvewme "in
_ 9.1m; _ CR3,4,5,6 Diode, Silican Rectifier 51250517 (+158°F). WHENEIER
Camposivlen, to, 107., m RCZOBIFIOOK F1,2.3.4 Fuse, 2 Amvere F755-l45 CATlON SHOULD BE
_ Deb, _ F5 Fuse, 3.2 Ampere, Slo-Blo F33319 FOR THIS CIRCUIT In
Wirewound, .51 ahm, 5%. 2w szwWRsu 011.3» *Tmnsism, Powev, 35144 *TRJSMA mud, in, 50,11 .h, at"
3.9K RZDCJVZJ Ti Transformer, Driver (left! 71135-1164
Wit-wound, Dual, 15 +10, um, um RSOSOO-ZBX T2 Transformer, Driver (right) TllZS-lls-Z ' Remove the iii-ohm l
_ p.19, _ T3 Translurmer, Fewer Tnasms P2, F3, PA).

Wirewound Dual, 15+), 10%, 10W RSDSOO-ZBX

-. Deleted -

Composiflen 820, 10%, 12W
Compusirion, 1K, 10%, m
100K

Wirewound, 39D, 10%, 3W
Wirewnund 22, 5%, 2W
100K

Winwound, 390, 10%, SW
Wlnwaund, 150, 10%, 3W
Wirewuund, 1 ohm, 5%, 3W

Composition, 320K,10%, 19W

47K

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P1, 2, 3, A Pot-minimum, W. W. 10on 20%, 2W RENO-141.1

" Transisfor must be replaced with one from some Beta group,
as indicated by the color do? on the transistor, unless bath
transistors are replaced with a matched pair. (Always re-
place mica insulatar whnn replacing trunsisterj

OUTPUT-STAGE BASE-BIAS CIRCUIT MODlFlCATION

The critical adiustmanl of the base-bias adwsfing p0-

mrticmeters (P1, P2, F3, P4-in the main schematic) is elim-
inated by replacing lrtz ccnlrnls will! voltage-and-temperoture
compensating diodes, These diodes will keep the base-bias
voltage within $0.015 volt oi the selected value even when

0 Remuve the lO-ohm,
R28, R29, R30).

0 Remove the (IS-chm.
neared to the emihe'

0 install a 0175-ohm,5
part number RL300W
units removed from t

0 Install the diodes (,=
place of the lO-uelm
polarity. Make sure
red doe (on the side
in the schematic ins