Marantz 2275 User manual

sees eserneets
TABLE OF CONTENTS
SECTION PAGE
VEACFOGUCTIONN oe ieibececcdieesseerientscesciecavceasasscttecdeusctsdacdadcaccceticacsesatecdtvancdescscbencsevdndaacnocounassescaeacde 1
Service Notes --s--ceseececccncccucccccseccccssccecccessceenetensseeceaentecesseesccesenesesnenaceseerennee neue eunseescenesesennes 1
PAM Tuner — coccoccvarccccencccccccccnnsanssanssscancnsuccesesenecepecerersunrersaescsceusesesessene neces ses nanenncuessenranncese neg 1
PIV Tinh ectdestisesecccaiteanscatecds cucecsee ecoadeasuceiteusceuscecscedsconscusctcstocacsecsscueecestsedsaddsccevaeesideceecse 2
Phono and Tone Amplifiers a Wie u Waveho vnhek Gite be sedis vis veueu Fs URUHN donee Da NNN ErOEs cone Rie bayes Resets Ncdeweseieancass 5
Power Amplifier ---+:---+--+sssssreseseneescensnarescennansnensceessansesenscsucensenessesasseceuesnsseusenecantoneasseesenses 5
Power Protection Circuit — <sressecescceceneceensccecnnensnenececensarensssenscunsnenansncussaucsananereseseaueaaucuseasaes 6
Speaker Protector Relay Circuit ----+ssssesssessesseesesesscnssesseseeesstenenasenssssssnseusesenuensanesaeenegs 6
Suggestions for Troubleshooting the Power Amplifier —-------secsssseseseeeeceenseneresenenaesenserersens 6
Test Equipment Required FOr SErViCiAG ---re-reecee seven enceenstecteceeetseeeneeeeseneneesenceenssnnconcneenseneane 7
AM Alignment Procedures «:+:ssesesseessecsseecenteesnensncnercenenrensnscnenenennseecacecaeauanen seusneeauaneguecenneges 7
FM Alignment Procedures — -+:-:+++++sssceeceeeeneesenscceeessrasenesnecceecansscerneeneeseensecaeenssenseeseagecssensrents 8
Stereo Separation Alignment --+++--sscecceerseceeteeeceeeenennenensenceneneneeeensennasssecsecencaneneeneeeaeeaoannenens 8
Muting Circuit Alignment —-ss::cscssssccscseecseseeneesnsnreeeseenerecanseesenaneeaeancensassnectenseacsonenansgenereeeas 8
FM Dolby Level Adjustment ----scccssssseccsccsseerecensssnceeenseeceeenenseanseenanascenaansnsenaneeeaaeaseseone nets 9
AUCIO ACJUSTMENTS —-vserereececeeceeerecansenentaencnenstecaseeatetensneceuaccustenseusasensceececaseustenaasensersranssees 9
Parts List: © <isdvseiccececceccen cies dens sonst cee cnc ccecn sped sad sedacvenccqccseneseceeceatescstac etccetasniceostecueeswcasteesses 20~32
Technical Specifications .---+-s.secseccccsseecrenereesessesnssssnessnensnecensnnenerensasenntanencaneasensaccenenoaenenees 33~34
Servicing Informations for European Model — ----ssssesscectesesseesseeseeesenssneseeeeeeennstaanenees 35~37,39
LIST OF ILLUSTRATIONS
FIGURE PAGE
1. Block Diagram OF the: HA1156 . © -wcisseveccs dice eeseeate caress dccsecelavedesssovedennsandievasaseveaveadeuess 4
2. Dial Stringing --scrccceesscccceenccccceesnseeecnensanuecnececcueeescuanecesarasuceuecaseeseeeenensnteesssesnssenenapeeteneaes 10
3. Front Panel Adjustments and Component LOCATIONS: Seseewssse cs ches ccscenelecvecetaiic cocdeceactscasede 11
4. Main Chassis Component Locations (Top VICW) -onsecsenceseenccsecceccnerenseentcnseeeeseucnscesenrenneras 11
5. Rear Panel Jacks and Component LOCATIONS: ssesetecide secs desaccscecsaacessws dcctenducevcccvecaneaensesenes 12
6. Main Chassis Component Locations (BOttOM View) — -sscccccsersscnsercersnsecesecesensucesonseeseeeeoar 12
7. FM Front End Assembly P100 Component POCATIONS: © cieesesdsccesesces cecccscdteesi cies egacvesseess 13
8. AM Tuner Assembly P150 Component LOCATIONS sseseesisees dens ccceecdvccsccnavecvecadetsevaasireeane 13
9. FM IF Amplifier Assembly P200 Component Locations — --+::::sseseeserseeteensserererenenensanees 13
10. MPX Stereo Decoding Amplifier Assembly P300 Component Locations —-+--+++-+seseeseeees 14
11. Dolby FM Level Amplifier Assembly PCO1 Component Locations —-ssreeceeesesseeserstsensees 14
12. Phono Amplifier Assembly P400 Component Locations — «.+++.::ssseeeeseestsesrenssrnsenenannennees 14
13. Power Amplifier Assembly P700 Component Locations — ----+sssseseeesssceeeseeeceeseneeneneneeenes 15
14. Power Supply and Protection Relay Circuit Assembly P800 Component Locations 15
15, Pre and Tone Amplifier Assembly PEO1 Component Locations «--++-++ssessseesssenseneeeterenees 16
16. Dolby FM, Mono L, R, Multipath and Tape Monitor Switches Assembly
PSO1 Component Locations —.......ccccecceesssconseeceeneceeeseeensseseceseecnensseesoacaaccecsssseeeeeassesessaees 16
17. Muting, Speaker, Loudness, Power, Hi and Low Filter Switches PT0O1
Component Locations — ...ccccceececessseceereeeceseseseseessauenececaueenseseseeeaentecseeseeasesseescusngesseseesees 16
18. Function Lamps Assembly PY01 Component Locations —........cecscccessesecscseceeseeseesesssnacs 17
19. Muting Level and Antenna Attenuator Assembly PUO1 Component
LOCATIONS” _-eessevebek. cctsteeese elie laste ated adaciidecc onda cde edad veclaas vwedaswardecdeuabieesidvaaibovaenedacesesndens 17
20 _ Dial Scale Illuminator Assembly PZ01 Component Locations —.........scssssessesceeeeeeseseneeee 17
21. Exploded Mechanical Diagram — ...........cecssesesessescncnsnescceecceseceeaasaaessssssessessrsasessuesseses 18~19

22. Fuse Assembly PRO1 Component Locations for European Model 35
23. Rear Panel Jacks and Component Locations for European Model ...........005 itvteeteeseene 36
24. Main Chassis Component Locations (Bottom View) for European Model .......... Sudvsecsites, 36
25. Voltage Conversion Chart for European Model — .......scssseseseeresseres nPescvpauiebaukpestsGres easnetac ne.
26. Schematic Diagram for U.S.A. Model _ ...... saeukawecesencess a sudeessecbegevasaaiectsasddacwessseieeeczenrie~ “SO
27. Schematic Diagram for European Model sssdwasdveceuteocevevevs sivsccas. 39
TABLE PAGE
1. Test Equipment Required for Servicing idenveduececsdudsiguscetacauaesenks Setvetiacesecsssseceriseses west, OD,

INTRODUCTION
This
service
manual
was
prepared
for
use
by
Authorized
Warranty
Stations
and
contains
service
information
for
the
Marantz
Model
2275
Stereophonic
Receiver.
Servicing
information
and
voltage
data
included
in
this
manual
are
intended
for
use
by
knowledgeable
and
experienced
personnel
only.
All
instructions
should
be
read
carefully.
No
attempt
should
be
made
to
proceed
without
a
good
understanding
of
the
operation
of
the
receiver.
The
parts
list
furnishes
information
by
which
replacement
parts
may
be
ordered
from
the
Marantz
Company.
A
simple
description
is
included
for
parts
which
can
usually
be
obtained
through
local
suppliers.
1. SERVICE NOTES
As can be seen from the circuit diagram, the chassis of the Model 2275 consists of the
following
units.
Each
unit
mounted
on
a
printed
circuit
board
is
described
within
the
square
enclosed
by
a
bold
dotted
line
on
the circuit
diagram.
LT. EM Front Endl cccsscsssssrsserceececcecccecesccceeceseeersesseaannececaeeessecescenssans Mounted on P.W. Board P100
2. FMIF Amplifier sssesseseescesssesesreeseeseeeeteenseennensessessaneneeseeteenees Mounted on P.W. Board P200
3. AM Turner Unit cccescessececeeceeeeeeeceeceeseeeteeessseeseriseeesensensesseereeeressenees Mounted on P.W. Board P150
4.
MPX
Stereo
Decoding
Amplifier
—
s:-:scessssesseseeseserereenes
Mounted
on
P.W.
Board
P300
5. Phono Amplifier -:--ceeeccesccesseessreesesscestneeteessatstesetsenneseeesseenees Mounted on P.W. Board P400
6. Power Amplifier ---::cesseseseeesessesseteseeseseneenetetsereeneeenseeaenees Mounted on P.W. Board P700
7. Power Supply and Protection Relay Circuit +--+: Mounted on P.W. Board P800
8B. Pre and Tone Amplifier -----::ss:seesesetetseeretteetertseesssereetessteaes Mounted on P.W. Board PEO'
9. Dolby FM Level Amplifier --:--:ss:ssessreceersteterteetretsstseesseresees Mounted on P.W. Board PCO1
10. Muting, Speaker, Loudness, Power, Hi and Low
Filter SWITCHES — cceccceecccceresseeesceeeeceeeeetceesetsernssersuseansceanrncoseetans Mounted on P.W. Board PTO!
11. Dolby FM, Mono L, R, Multipath, and Tape
Monitor SWitChes -vrvrsesscecsrceeeeecserececeeteecetensnsaeseneuenanenesecsaneeees Mounted on P.W. Board PSO1
12. FUMction Lamps cvvvsssrscesseseesesetsetsecseneesencsesenenerennassenstneecatneengs Mounted on P.W. Board PY0O}1
13. Dial Larmps -vvesssesessesseseessestenneeseeseneretteeenreseteesecncsnesreseaensesaeatees Mounted on P.W. Board PZ01
14. Muting Level and Antenna Attenuator -erresrettreseescereees Mounted on P.W. Board PUO'
2. AM TUNER
The AM Tuner section in the 2275 consists of one IC, including an RF amplifier, local
oscillator,
mixer,
IF
amplifier,
and
detector,
and
three
transistors,
one
of
which
comprises
a
signal strength indication amplifier and the other two comprise a detected audio signa! amplifier.
All components except the tuning capacitor and ferrite bar antenna are mounted on the
printed circuit board P150.
The AM signal induced in the ferrite bar antenna is fed to the RF amplifier input (Pin (2) and
amplified to the level required for overcoming conversion noise, thus giving good S/N
performance. The tuned circuit inserted in each of the output and input circuits of the RF
amplifier assures very high image and spurious rejection performance.
The amplified and selected AM signal is then applied to. the Mixer input. The local oscillator
voltage is injected to the other Mixer input (Pin (@) through a capacitor C157. Then, both AM
signal and local oscillator output voltage are mixed and converted into the 455kHz intermediate
frequency. The resulting IF signal is applied to the IF transformer L153 consisting of one ceramic
filter and two tuned circuits.
The output of L153 is fed to the IF amplifier input (Pin @) through a coupling capacitor
C162 and amplified to a sufficient level to drive the detector. The detected audio signal derived
from pin @) is filtered and amplified, and the final audio output is obtained from the collector
of H153 and applied to the TAPE MONITOR OUT jacks through the function switch S001.
The DC component of the detected IF signal is used as an AGC voltage to control the emitter

current of the RF amplifier through the AGC amplifier incorporated in the IC. A part of the DC
component is also fed from J157 to the signal strength indication amplifier H154. The output
appearing at the collector of H154 is level adjusted by R178, indicated on the signal strength
meter MOO2.
2.1 Suggestions for AM Tuner trouble shooting
Check for a broken AM bar antenna. Next, attempt to tune stations by rotating the fly-wheel
tuning knob slowly and observe the AM signal strength meter for deflection. If the signal strength
meter gives a deflection at several frequencies received, a probable failure exists after the AM
detector stage. Next, connect an oscilloscope to J161 and check for audio signals with the tuning
meter deflected. If the signal strength meter does not deflect, check the local oscillator circuit.
Normal loca! oscillator output voltage at the hot end of the oscillator tuning capacitor is about
1.5 to 3 volts, varying with the tuning capacitor position. When measuring the local oscillator
output voltage use an RF VTVM, no common circuit tester will give a correct indication, due to
loading. |f the local oscillator output voltage is normal, check all voltage distribution in the AM
circuits by using a DC VTVM and compare the measured values with those given in the schematic
diagram.
3. FM TUNER
The FM Tuner section in the Model! 2275 is divided into four functional blocks: FM Front
End, |F Amplifier & Detector, Muting Control and Multiplex Stereo Decoding Circuit.
An FM signal from an FM antenna is fed to antenna coil L101 from the balun coil. The
signal is then applied to the FET RF amplifier which in turn feeds its output to the FET Mixer
H102 through a triple tuned, high selectivity circuit. The FET Mixer converts its input signal into
a 10.7MHz intermediate frequency and amplifies it at the same time. H103 is the local oscillator,
the output of which is injected into the source of the FET Mixer. The injection voltage is
approximately 700mV. The 10.7MHz front end output is fed to the IF amplifier from J105.
The IF amplifier unit consists of eight IF amplifier stages, one AGC amplifier stage and an
audio buffer amplifier stage. Eight ceramic filters are also used to obtain high selectivity, and four
symmetrical diode limiter stages are also employed for best limitting characteristics, improved
capture ratio and good AM suppression.
A part of the FM Front End output is fed to and rectified by the AGC amplifier H209, and
the rectified output is fed back to the gate of the FET RF amplifier from J208 to decrease the
gain with increased signal strength.
The signal required for multipath indication is obtained from five IF amplifier stages through
coupling capacitors C211, C214, C223, C252 and C228 respectively and is rectified by four full
wave rectifiers diodes H221 through H228. These AM components of the FM signal are mixed
and amplified by transistors HSO1 and HSO2 and the output is again rectified to obtain DC
current required for actuating the signal strength meters, which is used for multipath indication
as well.
The IF signal is fed to the Detector Amplifier H208. The detected audio output is fed to the
buffer amplifier H210 and its output is fed to: (a) the noise amplifier H310 through resistor
R378 and capacitor C333, (b) the QUADRADIAL OUTPUT Jack on the rear panel through
resistor R379, and (c) the MPX stereo decoding IC (H321) through R301 and H301.
The DC current developed at the third winding of the discriminator transformer is directly
cnnected to the FM center tuning meter.
3.1 Audio Muting and Stereo mode auto-selecting circuit
100% solid state muting is incorporated in the Model 2275. Three inputs control the muting
function. The first is related to signal strength, the second to the noise level at the detector, and
the third is derived from the DC component of the detector output. These inputs are properly
matrixed and gated to provide muting free from noise and transients.

The first input of DC voltage obtained by rectifing a part of the IF signal from H205 and
H206 is fed to the base of H308 and turns it on. This level is predetermined by the muting
threshold level control. When H308 is turned on, H309 is turned off, allowing the
emitter-collector resistance to be increased and the collector voltage to be raised to about 9V.
The raised collector voltage increases the gate bias voltage and turns on switching FET H301,
decreasing the source-drain resistance to near zero and allowing the audio signal applied at the
source to flow to the decoding IC, pin@).
When the input signal is lower than the predetermined level, the DC output obtained is small
and can not turn on H308, thus H308 remains off. This turns H309 on, decreasing it’s collector
voltage and turning H301 off. Thus, no audio signals can pass through FET H301. This is the
fundamental principle of the muting operation but for more elaborate muting, the second and
third inputs are necessary.
The second input is used to protect the muting operation and MPX stereo lamps from
misoperation due to undesirable noise. High frequency noise included in the detected audio signal
is separated by a small capacitor, C333, and amplified by noise amplifier transistor H310. Its
output is rectified by the two diodes. The rectified DC output is proportional to the noise
component in the audio signal.
When there is excessive noise in the audio signal such as obtained with a station incorrectly
tuned, the rectified DC output turns transistor H311 on, decreasing the emitter-collector
resistance to zero. This lowers H309 collector voltage to 0. Therefore, H301 is turned off and any
audio signal having excessive high frequency noise can not go through the FET’s source-drain
path. Transistor H317, also, turns off when transistor H309 or H311turns on. This turns on
transistor H303 connected to pin (@) on the MPX decoding IC. Pin (8) is therefore grounded and
puts the IC in the monaural mode of operation. This prevents stereo misoperation due to
undesirable noise when the FM tuning is incorrect.
The third input is obtained from the FM discriminator circuit. The DC output, so called ‘’S’’
curve, is applied to the gate of H312 through resistor R281 and voltage divider network (R361 &
R362). The DC output is zero with a station correctly tuned in, but will vary from a negative toa
positive value, or vice versa, when the tuning point is deviated toward either a higher or lower
frequency from correct tuning.
When the DC output is increased to a greater positive level than predetermined, the increased
source potential of H312 turns transistor H315 on. (This means the collector of H309 is
grounded, H301 turns off, H317 turns off, and H303 turns on. This grounds pin of the MPX
stereo decoding IC, therefore the decoder is set in the monaural mode of operation and the stereo
indicator lamp turns off. When the DC output is increased to a greater negative level than
predetermined, the decreased source potential turns off H313 which turns H314 on. (This means
the collector of H309 is grounded). The subsequent changes are exactly the same as that just
described above.
Thus, when the tuning is shifted or deviated to certain frequencies at which undesirably noisy
side responses are produced, both muting and monaural/stereo switching transistors H303 are
operated automatically to provide muting
With the station correctly tuned, the bias current of FET H312 is adjusted so that both
transistor H314 and H315 are not turned on, resulting in no effect on transistor H309.
3.2 MPX Stereo Decoding Circuit
The stereo composite signal from the buffer amplifier undergoes a phase compensation by
R301 and C301, is fed through the muting FET H301 to the input terminal pin @) of the MPX
stereo decoding IC H321. This 1C uses PLL (Phase Locked Loop) technology and decodes the left
and right stereo signals, which become available at pins @ and (5), respectively. These decoded
left and right stereo audio signals are introduced through a low pass filter consisting of L301 to
L304 and C311 to C320 for elimination of undesirable residual switching signals and then
through a de-emphasis network consisting of R325, R326, C321 and C322 to a npn-pnp direct

coupled audio amplifier, where the signals are amplified and fed to output terminals J311 and
J313. From these terminals, the audio signals are fed to the TAPE MONITOR OUT jacks through
the function switch. Figure 1 presents an internal block diagram showing the functions of the
PLL MPX stereo decoding IC HA1156. The input stereo composite signal, amplified by the audio
amplifier, is delivered to the phase detectors PD-1 and PD-2. A part of the stereo composite signal
is also delivered to the stereo decoder section. The VCO (Voltage Control Oscillator) produces a
free run oscillation of approximately 76kHz with the time constant determined by capacitor
C305 and resistors R311 and R312 connected on the outside to pin (4 . The VCO output has its
frequency divided down to 19kHz through the two frequency divider stages (DIV-1, DIV-2), and
is returned to the phase detector PD-1, which contains two input terminals designed to produce
an output in proportion to the product of the two input signals. The signal fed to one PD-1 input
is a 19kHz square wave formed through frequency division of the 76kHz VCO output signal by
the two frequency divider stages DIV-1 and DIV-2, and the 19kHz pilot signal included in the
stereo composite signal, as a reference signal is fed to the other PD-1 input. Therefore, the output
of PD-1 which has gone through the low pass filter, LPF-1, provides DC output voltage in
proportion to the phase variance between the two inputs. This DC output voltage is amplified by
the DC amplifier, and is supplied to the 76kHz VCO as a contro! voltage. This means that the
output frequency and phase of the VCO have been phase-locked to the input pilot signal. The
38kHz sub-carrier reproduced by the PLL, as stated above, is delivered through the stereo switch
to the stereo decoder section as a switching signal, thus driving the decoder stage. One of the
inputs of PD-2-is given the 19kHz resulting from the frequency division completed by DIV-1 and
DIV-3, whereas the other input receives the 19kHz output contained in the composite signal, and
the output is provided with a DC component in proportion to the amplitude of the pilot signal.
This DC output is furnished through LPF-2 to the trigger amplifier which drives the stereo
indicator lamp. and stereo switch. Therefore, insufficient supply of the pilot signal results in
failure to light the stereo indicator and to turn on the stereo switch located in the path of the
38kHz switching signal, thereby avoiding a wrong stereo operation.
H303 located off pin is a switching transistor for automatic monaural-stereo mode
selection. When the intensity of an incoming signal from an FM station is weaker than a
predetermined level, H303 is turned on and pin is grounded, thereby developing a condition
for monaural reception. For forced monaural operation, switching the MODE switch to
“MONO”, turns on H303 with the positive bias voltage applied to the base, and pin (@) is
AUDIO 19KHz
AMPLIFIER
STEREO
COMPOSITE
SIGNAL
INPUT
LPF-1 76KHz 38KHz
+2 +2
(DIV-1) (DIV-2)
STEREO
SWITCH
PHASE
DETECTOR
(PD-1)
DC
AMPLIFIER
19KHz, <90°
TRIGGER
19KHz
STEREO
INDICATOR
LAMP
LPF.2
PHASE
DETECTOR
(PD-2)
DECODER
AUDIC OUTPUT
STEREO COMPOSITE
SIGNAL
38KHz SQUARE WAVE
Figure 1. Block Diagram of the HA1156

grounded, thereby establishing monaural operation. Transistor H302, connected externally to pin
(, is intended to kill the 76kHz VCO (oscillator) which interferes with AM signals during the
reception of an AM stations. When the function switch is set to “AM” position, a positive bias is
applied to the base of H302, turning it on and pin is grounded. Thus, the oscillation of the
VCO is stopped, ending the interference with AM reception.
3.3 Suggestion for Troubleshooting the FM Tuner
3.3.1 Symptom: No FM Reception
Rotate the fly-wheel tuning knob slowly and observe the FM signal strength meter. If the
signal strength meter deflects at several frequencies received, the tuner and IF circuits preceding
the discriminator circuit are functioning. When no reading is obtained on the meter, check the
FM local oscillator circuit, using an RF VTVM. The normal local oscillator voltage is
approximately 500mV at the tuning capacitor, depending on the tuning capacitor position. If the
local oscillator voltage is present, then check all voltage distribution in the FM Front End and IF
amplifier unit comparing them with those shown in the circuit diagram. When the signal strength
meter deflects but no sound is obtained, check the audio circuits using a high sensitivity
oscilloscope.
3.3.2 Symptom: No Stereo Separation
First be sure the ‘‘MODE” switch is in the normal 2 CH position. Connect an FM RF signal
generator with output modulated by a stereo pilot signal to the rear FM antenna terminals,
and check for stereo light operation. If not turned on, check for 19kKHz VCO output signal
(J310), using an oscilloscope and frequency counter.
4. PHONO AND TONE AMPLIFIERS
Program source signals from the PHONO jacks on the rear panel are fed to the input circuit of
the Phono Amplifier through the selector switch, and the output of the Phono Amplifier is fed to
another section of the selector switch. This amplifier provides a gain of 40dB.
All signals selected by the function switch (SO01-2R, 4R) are fed to the balance and
volume controls through the MONO (L, R) and Hi-Blend switches.
Signals properly attenuated by the volume control are led to the tone amplifier and are
subjected to tone control by the bass, mid and treble controls, and high and low cut filters.
These processed audio signals are then fed to the PRE OUT jacks on the rear panel.
5. POWER AMPLIFIER
The signal from the tone amplifier is applied to the differential amplifier (base of H701)
through the coupling capacitor C701. The differential amplifier provides a high input impedance,
and its collector output (H702) is connected to the base of H703 which in turn feeds its output
to the following stages: H711 through the network of R720, C711 and R721, and H712 through
the network of R720, C712 and R722. The outputs of H711 and H712 are fed to H713 and
H714, respectively. HOO1 and HOO2 are power transistors used in a complementary configuration
and mounted on heat sinks.
To maintain overall amplifier stability and linearity, degenerative feed back is utilized
throughout the amplifier. This feedback is also necessary to reduce distortion to withina
specified limit. The RC network of R724 and C709 conditions the feed back signal for audio
signals. R723 and C708 also comprise a feedback loop provided to obtain a stable zero DC offset
voltage at the speaker output terminals. R741 is a potentiometer resistor to adjust the DC offset
voltage to zero.
A dynamic bias is applied to the bases of driver transistors H713 and H714. This dynamic
bias circuit is comprised of H709, H710 and R742. This provides a variable base bias for the
driver transistors that automatically maintains the proper base voltage with temperature change.

The temperature sensitive biasing components of the dynamic circuit are thermally coupled to
the heat sink which mount the power transistors.
6. POWER PROTECTION CIRCUIT
A protection circuit for the amplifier is provided by sensing resistor networks and two
switching transistors. When the output transistors are over-driven, the current increase through
the power output transistors cause an increased current flow through R740. This increased
voltage potential is applied to the base of H708 through resistor R736 and H705 turning H708
on. Since the emitter of H708 is connected through R727 to the base of H713, the base of H713
is by-passed to the common cneter point through the emitter-collector path of H708. Thus, the
input signal to H713 is restricted to the value which maintains the operation of the output stage
within the safe area. Resistors R730 and R729 with H717 works as a sensing network. When the
center voltage (collector voltage of power transistors) is excessively increased to a positive value
by certain malfunctions, the voltage applied to the base of H705 turns H708 on, thus removing
the drive from H713 and the power transistor. For the other half cycle of the driving signal, the
same operating principle is applied as described above, through H706 and H707.
7. SPEAKER PROTECTOR RELAY CIRCUIT
The speaker protection circuit consisting of H805, H806, H807, and associated parts protects
the speaker systems against turn “ON” and “OFF” transients. This circuit is so designed that no
sound is heard for the first three to five seconds after the power switch is turned on by the time
constant circuit consisting of C809 and R813. This circuit also protects the speaker system
against difficulties due to poor DC balance between the speaker system terminals by instantly
operating the relay and cutting off the speaker system from the circuit. When a positive DC off
balance voltage is developed between speaker terminals by possible defects such as defective
power transistors, short-circuits, or a broken potentiometer .R741 protection is instantly
available. Since the base of H805 is connected to the speaker terminal, it is turned on by this
offset voltage developed, turning transistors H806 and H807 off, thus cutting off the relay and
disconnecting the speaker from the output circuit. When a negative offset voltage is developed,
this voltage directly turns H806 and H807 off, giving the same protection as above.
The circuit also protects the speaker system from possible damage when the amplifier is
over-driven by very low frequencies such as 7Hz or less.
8. SUGGESTIONS FOR TROUBLESHOOTING THE POWER AMPLIFIER
8.1 Excessive line consumption
a. Check for shorted rectifier HOO5; also check CO06 and C007.
b. Check for shorted transistors H713 and H714, HOO1, HOO2, HOO3, and HO004, and check
H709. Check bias diode H710. Check L004 for shorts.
CAUTION: BECAUSE THE DRIVER AND OUTPUT STAGES ARE DIRECT COUPLED,
SEVERAL COMPONENTS MAY FAIL AS A DIRECT RESULT OF A SINGLE
INITIAL COMPONENT FAILURE. IF A SHORTED TRANSISTOR OR
ZENER DIODE IS FOUND, OR CONTROL OR BIAS DIODE, BE SURE TO
CHECK THE REMAINING DRIVER AND OUTPUT COMPONENTS FOR
SHORTS OR OPEN CIRCUITS BEFORE REENERGIZING THE AMPLIFIER.
8.2 No Line Consumption or Zero Bias
a. Check line cord, fuse, transistors H709, HOO1, HOO2, HOO3, and HO04, and bias diode H710.
b. Check for open rectifier HOO5 or open L004.

TEST EQUIPMENT REQUIRED FOR SERVICING
Table 1 lists the test equipment required for servicing the Model 2275 Receiver.
Item
Manufacturer and Model No.
oe
Use
AM Signal Generator
Signal source for AM alignment.
Test Loop
—+
Used with AM Signal generator.
FM Signal Generator
Less than 0.2% distortion
Signal source for FM alignment.
Stereo Modulator
Less than 0.2% distortion
ate . .
Stereo separation alignment and
trouble shooting.
Frequency Counter
MPX Oscillator adjustment
(VCO).
Audio Oscillator
a!
Weston Model CVO-100P, less
than 0.02% residual distortion
is required.
Sinewave and squarewave sig-
nal source.
Oscilloscope
High sensitivity with DC hori-
zontal and vertical amplifiers.
Waveform analysis and trouble
shooting, and ASO alignment.
VTVM
With AC, DC range RF type
VTVM Circuit Tester
AC Wattmeter
Simpson, Model 390
AC Ammeter i
Line Voltmeter
Commercial Grade (1-10A)
Commercial Grade (0-150VAC)
Variable Autotransformer
(0-140VAC, 10 amps.)
Powerstat, Model 116B
Voltage measurements.
Trouble Shooting.
Monitors primary power to
Amplifier.
Monitors amplifier output un
der short circuit condition.
Monitors potential of primary
power to amplifier.
Adjusts level of primary power
to amplifier.
Shorting Plug
Use phono plug with 600 ohm
across center pin and shell.
4
Shorts amplifier input to elimi-
nate noise pickup.
Output Load
(8 ohms, 1%, 100W)
Commercial Grade
Provides 8-ohm load for ampli-
fier output termination.
Output Load
(4 ohms, +1%, 100W)
Commercial Grade
Provides 4-ohm load for ampli-
fier output termination.
Table 1. Test Equipment Required for Servicing
10. AM ALIGNMENT PROCEDURES
10.1 AM IF Alignment
Connect a sweep generator to J153 and an alignment scope to the test point © .
2. Rotate each core of IF transformer L153 for maximum height and flat top symmetrical
1.
response.
10.2 AM Frequency Range and Tracking Alignment
Set AM signal generator to 515kHz. Turn the tuning capacitor fully closed (place the tuning
pointer at the low end.) and adjust the oscillator coi! L152 for maximum audio output.
Set the signal generator to 1650kHz. Place the tuning pointer in the high frequency end
and adjust the oscillator trimmer on the oscillator section of the tuning capacitor for
1.
2.
maximum audio output.
Repeat Step 1 and 2 until no further adjustment is necessary.