Page 1
CAUT ION:
ThisisaFISH ER precision high-fidelity in-
strument. Itshould beserviced onlybyqualified personnel -
trained
inthe
repair oftransistor
equipment
andprinted
circuitry.
EQUIPMENT ANDTOOLS NEEDED
The following areneeded tocompletely testand align modern high-fidelity instruments
such asamplifiers, tunersandreceivers.
Miscellaneous Adjustable-line-Voltage Transformeror
line-voltage regulator
Load Resistors (2)-8-ohm, 50-watt(orhigher)
Stereo source (Turntable withstereo cartridge
or Tape Deck)
Speakers (2)Full-range, forlistening tests
Soldering iron(with small-diameter tip).
Fully insulated frompower line.
Test
Instruments
Vacuum-Tube Voltohmmeter DCVTVM
Audio (AC)Vacuum-Tube Voltmeter(ACVTVM)
Oscilloscope (FlattolOOkc minimum)
Audio (Sine-wave) Generator
I ntermodu lation AnaIyzer
Sweep (FM)Generator (88to108 mc)
Marker Generator
Multiplex Generator (preferably withRFoutput-
FISHER Model300orequal).
Many ofthe items below areincluded justasare-
minder- theyarenormal procedures, forexperienced
technicians. Shortcutscanbetaken butoften they
cause additional damage-to transistors, circuitcom-
ponents orthe printed-circuit board.
Soldering-A well-tinned,hot,clean soldering irontip
will make iteasier tosolder without damage tothe
printed-circuit boardorthe many manycircuit com-
ponents mounted onit.Itis not thewattage ofthe
iron that counts -itis the heat available atthe tip.
Low-wattage solderingirpnswilloften taketoolong to
heat aconnection -pigtail leadswillgettoohotand
damage thepart. Toomuch heat,applied toolong, will
damage theprinted-circuit board.Some50-watt irons
reach temperatures of1,000° F- others willhardly
melt solder. Small·diameter tipsshould beused for
single solder connections -largerpyramidandchisel
tips areneeded forlarger areas.' '
- When removing defective resistors,capacitors, etc.,
the leads should becut asclose tothe body ofthe
circuit component aspossible. (Ifthe part isnot being
returned forin-warranty factoryreplacement itmaybe
cut inhalf -with diagonal-cutting pliers-to make
removal easier.)
- Special de-soldering tipletsaremade forunsolder-
ing multiple-terminal unitslikeIFtransformers and
electrolytic capacitors.Byunsoldering allterminals at
the same timethepart canbe,removed withlittle
chance ofbreaking theprinted-circuit board.
- Always disconnect thechassis fromthepower line
when soldering. Turningthepower switch OFFisnot
enough. Power-line leakagepaths,through theheating
element, candestroy transistors.
Transistors-Never attempttodo any work onthe
transistor amplifiers withoutfirstdisconnecting the
- ~ AC-power linecord-wait until thepower supply filter-
capacitors havedischarged.
- Guard against shorts-ittakes onlyaninstant for
a base-to-collector shorttodestroy thattransistor and
possibly othersdirect-coupled toit.[In the time it
takes foradropped machine screw,washer oreven
the screwdriverrto glanceoffapair ofsocket terminals
(or between aterminal andthechassis) atransistor
can berllined.]
- DO NOT biasthebase ofany transistor to,ornear,
the same voltage appliedtoits collector.
- DO NOT useanohmmeter fortesti ngtransistors.
The voltage applied through thetest probes maybe
higher thanthebase-emitter breakdownvoltageofthe
transistor.
Output StageandDriver-Replacements foroutput
and driver transistors, ifnecessary, mustbemade from
the same betagroup asthe original type.Thebeta
group isindicated byacolored dotonthe mounting
flange ofthe transistor. Besure toinclude thisin-
formation, whenordering replacement transistors. -
Ifone output transistor burnsout(open orshorts),
always remove alloutput transistors inthat channel
and check thebias adjustment, thecontrol andother
parts inthe network withanohmmeter beforeinsert-
ing anew transistor. Alloutput transistors inone
channel willbedestroyed ifthe base-biasing circuit
is open onthe emitter end.
- When mounting a'replacement powertransistor be
sure thebottom ofthe flange, themica insulator and
the surface ofthe heat sinkarefree offoreign matter.
Dust andgritcan prelent perfectcontact. Thisre-
duces heattransfer tothe heat sink. Metallic particles
can puncture theinsulator andcause shorts-ruining
the transistor.
- Silicone greasemustbeused between thetransist-
or and themica insulator andbetween themica and
the heat sinkforbest heatconduction. Heatisthe
grea,test enemyofelectronic equipment. Itcan shorten
the lifeoftransistors, capacitorsandresistors. (Use
Dow-Corning DC-3orC20194 orequivalent compounds
made forpower transistor heatconduction.)
- Use care when making connections tospeakers and
output terminals. Anyfrayed wireends cancause
shorts thatmayburn outthe output transistors -they
are direct-coupled tothe speakers. Thereisno output
transformer -nothing tolimit current through thetran-
sistors except thefuses. Toreduce thepossibility of
shorts atthe speakers, lugsshould beused onthe
exposed ends-at lea,st theends ofthe stranded wires
should betinned toprevent frayedwireends. The
current inthe speakers andoutput circuitry: isquite
high. Anypoor contact orsmall-size wire,:,~~ncause
power losses inthe speaker system.Use14ojl16 AWG
for long runs ofspeaker-connecting wiring.
DC-Voltage Measurements-These basictestsofthe
transistor circuitryaremade without thesignal gener-
ator. Without anysignal inputmeasure thecircuit volt-
ages -as indicated onthe schematic. Thevoltage
difference betweenthebase andtheemitter should
be inthe millivolt range-asensitive DCmeter is
needed forthese readings. Alow-voltage rangeof1volt,
full scale -or lower -is needed.
Audio-Voltage (gain)Measurements-The schematic
and printed-circuit boardlayout diagrams areused.
Input signals areinjected atthe proper points-found
most quickly byusing layout ofthe printed-circuit
board instead ofthe schematic. AnAUDIO (AC)VTVM
connected tothe test points should indicate voltages
close tothose values showninthe boxes onthe sche-
matic. Manyotthe signal levelsinthe input stages
are only afew millivolts-they cannotberead onthe
AC ranges supplied onmost Vacuum-Tube AC/DCVolt·
ohml!'eters
(VTVMs).
Evenwithal-volt range asignal
level of100 millivolts (.1volt) willbethe first 1/10
of the meter scale.Areading of1millivolt (.001volt)
will hardly evenmove themeter needle. '
Page 2
DIALSTRINGING PROCEDURE
- Hook oneendofthe spring overthebottom eorinthe
front --nd drive-drum (withthedrum rotated toits extreme
counterclockwise position).
- Stretch thetension springuntiltheloop onthe free end
sticks outofthe slot inthe edge ofthe drive-drum. Now
Insert alength ofstiff wire, about l-inch long
(0
piece of
straightened-out paperclipwill donicely) through theloop
. to keep thespring stretched whilestringing thedial cord.
Place thepiece ofstiff wire inthe outer groove ofthe
drive-drumi bridgingtheslot inthe drive·drum.
- Tie asmall, non-slip, loopinthe end ofthe dial cord.
i
Thread theloop inthe dial cord through theopening in
the driveo/lrum slot,under thespring, andhook theloop
over thetopear inside thedrive drum.
- Wrap thedial cord around theclrive-drum (counterclock-
wise) about
*
of aturn, inthe inner groove, ondthen around
the top guide pulley.
- Stretch thedial cord tothe left end ofthe dial, around
th.nwo guidepulleys andthen back tothe fly-wheel drive
shaft.
- Wind 3full turns ofdial cord around the.drive shaft(as
shown inthe upper detail drawing).
- While keeping th"dial cord tautrotate thedrive-drum to
TOINNEA~
GROOVE
o
TO OUTER
GROOVE
its extreme clockwise position andfitthe clioIcorcl into
the remaining pulley.
- Set thedial cord inthe outer groove ofthe front-end drive-
drum andthread itthrough theloop inthe end ofthe tension
spring. (Seedetail drawing.ot lowerright.)
- Pull allslack dialcord through theloop inthe tension
spring -
- Check allpulleys forproper threading ofthe dial cord.
- Tie asmall knotinthe dial cord tosecure
it
to the loop
in the tension spring.(Useatweezer withasmall tipto
help tiethe knot.) Keepdialcord astout aspossible while
tying theknot.
- Apply adrop ofquick-drying cementtothe knot toprevent
it from slipping orbecoming undone.
- After thecement hasdried completely pulloutthepiece
of stiff wire andgently letthe spring contract t9~Q.pplytension
to the dial cord. . ..':'
.
.
:~.
- Rotat. thefront·end drive-drum taits extreme'tbunter-
clockwise position.
- Set
the
clial pointer tothe zero (0)calibration onthe log-
ging seale ofthe sIi de --.uIe cliol.
- Attach.the pointertothe dial cord andcement itin place
with adrop ortwo ofquick.dryin,ll cement.
If replacement partsareout ofstock, locally, theymaybeobtained
directly fromtheParts Department ofFIStlER RadioCorporation.
They willbeshipped "bestway", either prepaid orC.O.D. unless
otherwise specified.
For instrument-operation informationandtechnical assistance write
Richard Hamilton, CustomerServiceDepartment,FISHER Radio
Corporation, LongIsland City,NewYork 11101.
