Sherwood SEL-400 User manual

XL4oo
SHER\ATOOf) MODEL
SEL-4OO
\-,
I. SELECTOR:
Selaets Iistening source: llic, Phcro 2,
Phono | 1 Tuner, Aux l, or Aux 2.
2. !,98:
Providee for eelqctron of Steteo,
Stereo Reverse, tlono, Left, ot
Right Channels.
3- tQEE&i['
Tells lou that th! arplificr is th.
4. FUilCTI0ft LI6HT5:
Tells you the poaition of tha Salector
control.
5. .SSKEE:
Selactg llain epaakers, Rcrote, Both,
or Off.
6. L0UIINESS/V0LUHE:
l4aetcr Porer ON/OFF and Volurc Control.
AIeo autonstically adda bass colpense-
tion at loy lsvcle rhan thc Loudnaes
eritch ie depreaaed.
7. !I9.:
Input jacke for Left and Right channel
nicrophoncs.
8. S,g,:
Increases Bass tones up to +16d8,
decreases to -l6dB.
e. EM!,9E'
Incrcaaes llidrange tonca up to +6d8,
decreaaee to -6d8.
r0. f!!!!!:
Increaees Treble tonca up to +16d8,
decreages to -l6dB.
STEREOAMPLIFIER
SERVICEMANUAT
1r. @EQ_8!.9!,E9.:
Two
jacks for ptivate listening with
ona or two seta of stereo headohonee.
1
2. BALANEE:
EEf$E?s tha ralative voLumelavals of
the LEft and Right speakera.
r3. !1ow_Ett!EE:
Fot removal of undeeircd low fraquancy
materi,a.} such as nrunbJ.er fron older
recordinga.
I4. EI.E!-E!EE.E:
Cute off scratch from records and back-
ground nhiesi from noiey Flrl etationE.
15. TAPE III]NITOR:
Perrnits monitoring during tape racord-
ing.
15. .[Q!E-EE@L:
Dieablae the Tonc Controls and auto-
matically se';s preanp responae charac-
teriatic to iFlatn.
I7. LOUDI{ESSCOHPENSATION:
Activates Loudnegs control comogngation
circuitry.
.I
8. DYI{AQUAD:
Activatae Dynaquad 4-channel Matrix
circuitry.
I9.'I-CHANNEL ADAPTER:
For connacting 4-chennal decodcr. Aleo
doublea aa gacond Tapc Monitor.
20. r4,BE--P!,9E.1U,€,:
Psrnite racording to, or playing back
fron, a saeond tapc racorder. Aleo,
dubbing fron ona racordar to ths othcr.
olo
slEni/ooD sEL-400
spEctFtcATtoNs.
V -* Preomplifier
BA55 COHTROL: g6dB 0 StlHz [xith center detant].
tilIIrRAt{GE
C0!|TR0L: g6dB O 9O0Hz [with center detent].
TREBLE COIITROL: al6dB t l5,O00Hz [rith centsr detent].
IiIPUT SEiISITIVITY (and IIIPEDAITCE)
e lOO0Hz:
CR0SSTALK: Greater than -40d8, 20-20"000H2.
LtI FILTER: 3dB Q 45H2, 26dB g 10H2, 1ZdB/gctave
HIGtf FILTER: 3dB O 7O00Hz, 20dB O 20,000H2, lAdB/Octave.
TOTAL HARIIOfIIIC
DISTORTION:
Preamplifier only:
Record Output Amplifier: Leee than 0.01S O 2V.
Lese than 0.01f O 5V.
Aux l: 6V.
Aux 2: 5V.
,Tape ltlonitor: 5V.
4-CH Adapter: 6V.
FREQUEI{CY
RESPONSE:
Phono 1 & Phono 2: 3O-15,OO0Hz
glOf.
Aux I, Aux 2, l,lic, Tape l{onitor, Tape
Ilubbing, & 4-CH Adapter: 2O-2OrO00Hz t*dA.
(500 Ohm).
FlaximumOutput:5V.
HUf{
ANI}N0I5E: [signat to Noise] [IHF "Ao rdeightins]
\ _ Phono l: 65dB 75dB-
V Phono 2: 65dB 75dB
Aux | & Aux 2: 80dB BSdB
Tuner: 80dB B5dB
Volune Mininun: 90dB 100d8
* All specificationa with l20VAC line.
Phono l:
Phono 2:
llic:
Tuner:
Aux 1:
LO:
Phono 2:
l{ie:
Tuner:
0UTPUT LEVEL (and IHPEIIANCE):
Record Out: 0 VU, lmu, 775mV
Tape Ilubbing: tsUmV.
4:-CH Adapter: 1sthrv.
Preamplifier: l50mV (2.2K Ohm);
Aux 2: l5&aV (220K Ohrn).
Tape llonitor: ls{lnrv (220K 0hrn).
Tape llubbing: lsthtv (22OK
Ohsr).
4-CH Adapterr l5thV (220K Ohn).
II{PUT
CAPABILITY
FoRo.25t THI}
O lOooHz:
Phono| (rith Input Sensitivity Selector Syitch):
H
I : gtlmv.
l{EO: 'l2tlmv.
z.OnV, 4.5nV, & g.tlmv se].actabl'e (5OKtlhrr).
z.tlnv (5OK
Ohn).
2.3rnV
(5OO
Ohn).
't
stlnV (22OKohn).
lslrov (220K
Ohm).
1zthv.
9tlnv.
9tlrnV.
6v.
Amplifier
PflER &JTPUT (RilS rnininum - both channele driven) with no
rore than 9.25* Total Harmonic Dietortioni
85 uatte per channal O I cihme, from 20-20r000H2.
9O ratte per channel O I ohne, 1000H2.
120 ratta per channel O 4 ohmsl 1000H2.
IHF PflER BAIIIUIIITH: 5-40,t100H2.
IIIPUT SEIISITMTY (and IMPEDANCE)O lOtl0Hz: 125nV (270K Ohm),
ItAtfIttE FACT0R: 30 I I ohma, from 20-20,000H2.
Htxr AilD lroISE:
Signal to lloise: 90d8.
IHF rAi Heighting: l00dB.
FREQUEXCY
RESP0NSE: 7-100,000H2, +0d8, -1d8.
CRIISSTALK: Greater than -60d8, 2O-21'BOOHz.
Generolt
PflER REQUIREMENTS:
l l5-125VAC
, 6OHz.
AC IruTLETsONCHASSIS:,
Svitched - 2: 100 watts maximum each outlet.
Unsritched - 1: 100 watte maximum.
RATEI)
PttER CONSUMPTI0N: 35-450 watts.
PfrER FUSE: 5 Amp, 3AG [Fast Acting 0n1y].
DII{ENSIONS: 16-9/16 long, 5 high, 14-'l/4 deep.Iinches].
SHIPPINGhrlEI6HT: 33 lbs.

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Q711
Q710
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FIGUR'E
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SHERL'/OOD
SERVICE MANUAL MODEL SEL-4OO
AMPLIFIER SERVICE
INFORMATION
\--
v
CONTENTS:
PARTS
L0CATIoNS
Irig. 1]
AMPLIFIER
SERVICINGAND
ADJUSTMENT
POIdER
SUPPLYSERVICING
AMPLIFIERELECTRONIC
RELAYPROTECTION
ORDERING
PART5
PARTSLIST ISre SuppIements]
SCHEMATIC
DIAGRAT{
[
See Supplements
]
CIRCUITB0ARDC0tIP0NENT
LAY0UT
ISr" Supplements]
AMPLIFIER
SERVICINGAND
ADJUSTMENT:
!.9[E:
1. For simplicity only the ]eft channel and its related circuitry gre described. The
right ci-rannel is identical except for reference symbol numberi Ittt Schematic Diagre
For parts locations, refer to Figure I on the inside covers.
2. As a convience for fast component location, this manual contains detailed pictorials
of all the printed circuit board assemblies. Reference to these drawings should aic
considerably in a quick, accurate fault analysis of an existing malfunction.
USE
OF A VARIAC:
It is imperative that a variable voltage line source fVariac] equipped with a line watt-
meter to identify abnormal pobrer consumption be used when servicing power amplifiers anc
associated power supply circuitry. With the Loudness volume control set at minimum, the
power consumption should not exceed 30 watts as the voltage is increased by the Variac t
the rated 12O VAC. If the power consumption begins to exceed 30 watts, do NOT
increase
the line voltage any further. Determine if the malfunction ie in the Power supply, the
preamplifier or the amplifier section of the unit.
t
Verify that the line fuse is unopened and cheek idling power consumption. The main caus
for abnormal powet consumption, in order of decreasing occutancer ares
l. 0pen ot shorted amplifier output, driver or pre-driver transistors.
2. Open or shorted power supply diodes.
3. Shorted power transforme!.
AMPLIFIERFAULT
ANALYSIS:
If an amplifier channel is suspected, check circuit boards for burned parts and replace.
Check all the transistors with an ohmmeter for opens or shorts and replace if defective.
WARNING: Some ohmmeters may damage sensitive solid-gtate devices. Whenever possiblet
use a high resistince ringu [.t least RXl0].
Use the centerpoint voltage Imeasured from the + speaker terminal to ground] "" a guide.
The centerpoint voltage should always be Zero + 5OmV. Any deviation suggests shorted or
open deviceso ,
PAGE:
Inside Covers
1
4
ANDSPEAKER
SYSTET'I
CHECKS 4
Back Cover
\--.

sHERWOOD
SERVICE
MANUAL
If channel- operation is still faulty, verify that there are no shorted capacitors, open
besistors, etc.r ort the board.
Inspect the underside of the board for shorted pads, broken connections, etc.
V'Jhen
the board is restored, readjust Output Bias Isrr below].
DI5T_0STI0N
IN AMPLIF
IER 0UTPUT:
Distortion which exceeds the amplifier ratings Is"" Front Cover] r"y be due to the
following:
1. Mismatched output transistors.
2. Defective (low-beta) driver transistors.
3. Incorrectly adjusted output transistor bias.
OUTPUT
TRANSISTORBIAS ADJUSTMENT:
Proper output transistor operation and output bias adjustment are most important to
assure coo1, low-distortion operation of the amplifier. Bias adjustment is necessary if
the output transistors are replaced*, or if any of the transistors in the driver circuit
or the amplifier exhibits one or more of the following symptoms:
1. Overheating of the output transistors under normaL operating condi!ions.
(Normal output trangisto1: case temperatures are 30-35oC L82-95oFJ, quiescent
operati-on bt 2s
oC IZe "r] ambient. j
2. Excessive low level Intermodulation Distortion (
tMn) or Total Harmonic
Distortion (fHn) - more than 8.1% at 3.0 volts across I ohms.
* It is extremely important that the insulating washers used to separate the output
transistors from their heat sink be unbroken and instal-led properly with silicon
grease liberally applied to all surfaces in contact with each other. Make certain
the emitter and base pins of the output transistors do not contact any part of the
heat sink assembly.
NOTE: For the following testsr.an I oh6 load resistor must be connected to each of
the two power "rptifirr-[Speaker] output terminals.
SUGGESTED
AMPLIFIERTEST BENCH SET-UP SCOPE
I
-J
AUDIO
osc.
SCOPE
B
TO 8rr. LOAD
OF
AMPLIFIER
VOLTMETER VARIAC
o- t60 vo|.Ts
2
D.C.
VOLTMETER
+PROBE
ils- l25VAC
sor
Bffl'sE
ADJ.
TOINPUT
.,ACKS BEING
REGEIVER
AC O-3OO
WATTS
sw
mcx posr non
Jtt:^:.:tltt'
t.
H.D.MEASUREM-E]{T
t#x
2. I.M.DMEASUREMENT VV!'-
3. SINE WAVE MEAS.
:6.3VAC
6OdBATTE
IO
dB STEPS
\ 60Hz
V
Figure 2 \/

\z^
I-'
BIAS ADJUST!4ENT
USING AN INTERM.ODULATI0N
DISTORTI,0lrl.
(I..t'|.-D.
) ANALJZER:
l. Connect the amplifier for testing as in Figure 2.
2. Connect an Intermodulation Distortion Analyzer with a ratio of 421 using 6AHz and
7000H2 to the amplifierrs Left AUX 1 input and set the SELECTOR
switch io AUX 1.
3. Set the volume control to maximum and adjust the generator for an amplifier output o
3.0 volts across the I ohm load of the amplifier channel under test.
4. While observing the resultant distortion waveform, adjust the bias potentiometer
IVnZgt ] "o that the crossover distortion is at th; polnt of leing eliminat"J-[ii.""
uABnl. NOTE: Class trAn operation [Continued CCI{
rotation] causes the output tran
istors to draw excessive current and consequently overheat.
5. Repeat Steps 3 to 4 for the Right channel.
Figure 3
Thg following performance indicates a properly operating amplifier with both channels
driven into I ohm loads:
1. Less than 0.1S fntermodulation Distortion Itt{n] at 3.0V Itypically 0.05%].
2. 85 watts of powex per channel at no greater than o.zs* rHD.
BIAs ApJUSTitENT
UsINE HARHONIE
D.ISTORTr0N
AtyALJZER:
f. Cpnnect the amplifier for testing as in Figure Z.
2. Connect an oscillator with }ess than .O1* distortion at lKHz to the amplifierts Left
AUX I input and set the SELECT0R
switch to AUX i.
3. Set the volume control to maximum and adjust the oscillator for an amplifier output
of 3.0 volts across the I ohm load of the amplifier channel under test.
4. Using the Harmonic Distortion Analyzerr looking at the distortion of the amplifier
properly nulled, make the adjustment a9 follgws: Adjust bias for Class rfABrr
operat-
ion by turning the bias potentiometer [VnZOt] =o thal the crossover is at the point
of being eliminated. N0TE: Class nAi operation [continued CCW
rotation] ".us"s
the output transistors to draw excessive current and overheat. Refer to Figure 4
be1ow.
5. Repeat Steps 3 and 4 for the Right channel.
The following performance indicates
driven into I ohm loads:
1. Less than 0.1% THDat 3.0
2. 90 watts per channel at no
3. 85 watts per channel at no
Proper Bios
Adiuslmenf
Figure 4Proper Eios
Adiustment
a properly operating amplifier
volts Itypically 0.05%].
greater than 0.25* THD
ItfH=].
greater than O.25* THD
lZOtlz
with both channels
lmproper Bios
Adiustment
lmproper Bios
Adiuslmcnt
\-'
3-
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SHERWOOD
sERVICE MANUAL
BIAS ADJUSTMENT
USINGA LINE !'/ATTMETER:
When test equipment required for previous bias adjustments is not readily available
and adjustment is absolutely necessarVr the following proceedure may be used which
requires only an accurate line wattmeter:
1. Turn the loudness control to minimum.
2. Adjust the bias potentiometers (VR701
which the amplifier begins to cause a
consumption.
POWER
SUPPLY
SERVICING
AND
FAULTANALYSIS:
& VR801
) one at a time to the point at
very slight increase in line wattage
Power supply malfunctions are usually due to shorted or open power diodes Xl -X4, zt
diodes Z1-25, transistor Ql, or a defective povrer transforme!.
The devices may be easily checked with an ohmmeter. The transformerfs operation mE
checked by measuring secondary voltages with the associated circuit legs disconneci
AMPLIFIERELECTRONIC
RELAYPROTECTION
ANDSPEAKER
SYSTEM
CHECKS:
It is absolutely necessary in the design of a high powered amplifier to provide Prc
ion for both the amplifier ANDthe speaker load. The primary purpose of the relay
protection circuit is to disconnect the speakers if a potentially damaging situatic
should occur.
If there is no output to thg speaker, first check the speaker connections for short
wires or a shorted speaker lspeaker load resistance should not indicate less than 4
resistance on an ohmmeterJ. If the relay protection circuit is suspect, the follou
explanation of itls normal operational characteristics may be halpful.
The complementary darlington output transistors are load line limitedr with the ma:,
current limited to 7.7A peak (equivalent to 1?gW
A 4 ohms). Signal voltages develc
across the output transistorfs emittei resistors R751 and R752 forward-bias transis
Q901 and Q902, shunting the output transistorrs base drive signal.
The dual comparator, integrated circuit IC901, and associated sensing circuit is a
lridge system_which, in effect, measuxes the speaker line impedance. fhe comparatc
Ltett channelj Pints 3 and Z are biased so that Pin 3 is 0.3V positive with respecl
Pin 2. With this condition, the comparator output voltage is high L25UJi hence, tl'
bi-stable multivibrator transistors Q950 and Q951 activate the relay, connecting tt
speaker loads to the_amplifier circuit. Also, because of this turn-on ndelayn, the
Power turn-on pulse Ispeaker pops] are eliminated.
As the load impedance is reduced, with signal applied, a point is reached where thi
now rectified signal (due to diodes X912 & X940 for the Left Ehannel, and diodes Xt
and X952 for the Right Channel) drives Pip 2-positive with respect to Pin 3. At tt
point the comparator output switches low LlVJ, causing the bi-stable relay driver I
deactivate the relay, disconnecting the potentially dangerous load condition.
To reset the protection circuit, the AC Power must be turned off for approximately
seconds before again turning on the amplifier.
This protection circuit achieves variable current limiting, hence power limitingr t
inverse function of the amplifier load. The relay control circuits are separately
triggered by each channel, but both shut down simultaneously. For example: Power t
is typically limited to ,|20 watts @ 4 ohms, 9O watts-E 3 ohms, 4O watts g 2 ohms, t
wattl'E 1 ohm. Normally, in a 1 ohm load condition [without this patented variablt
current limiting syetemJ, with current limiting only, the output transistors would
required to dissipate 300 watts, resulting in their instantaneous deetruction.
Loudspeaker protection is also provided in the event that either a positive ot negi
DC voltage is developed at the amplifier speaker terminals.
Pc,sitive voltage protection: An amplifier malfunction resglting in a positive DC \
age at the sprik"" lermiqral has this voltage fed to the t-] invirting-input of coml
lCgOl througi-r R915 [geOf]. This causes lClOlts output to switch low ltypically 1V.
clamping off transistor Q951, thereby deactivating the speaker load.
Negative voltage protection: If a negative voltage were to appear at the speaker
terininal, such as would occur wherr a negative side driver or output tranEistor fai.
in a shorted mode, transistor Q910'will be clamped off. This permits a positive vt
from the 26 volt supply to bg coupled through resistor R912 L27OKJ, diode X911' an(
resistor R91 3 LAZOfj to the L-l inverting input of the comparator IC901. As was tt
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ALL RESTSTORSARE r/4 WATT ltiltrut, tO%
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FIGURE
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VIEW
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SHERbJOOD
SERVICE
MANUAL MODELS
in_the positive rloltage protection, the comparatorts output now ewitches 1ow, clamping
off transistor Q951, thereby deactivating the relay and disconnecting the speaker 1oad.
Thermal- protection: The ampJ-ifier is al-so protected against excessive and potentit
. damaging temperatures due to abnormal load or environmental conditions. This prott
is accomplished with two miniature thermal switches Ion" per amplifier channell wh:
are fastened to the output transistors._ When the output transistorts operating ter
ture reaehes approximat'efy 130oC lZTOofl, the switch is thermally activated, o[enir
itrs contacts, thereby diseonnecting the AC line voltage to the pourer transformerrt
primary, a POhIEROFF condition. Jhis open circuit condition now a1'l.ows
the temperr
to d"9p.__A! approxinrately 100oC IZtO"fl the contacts close and the'"iq,mplifier retu:
to a P0WER
0N condition.
SEDERING
PARTS:
Vr/hen
ordering replacement parts, always include part numbers Isee Parts List].
't
I
When defective
Stations only]
either credit
parts are returned for
include a list, by part
or replacement parts.
The return of entire circuit boards for replacement is
except where component failure has reeulted in physical
replacement under warranty IAuthorized Service
number and valuer of the parts returned. Request
IF
normati! not covered by wamantV,
damage to the board itself.
\.1
\r
If a set or board cannot be repaired, return the complete amplifier to Sherwood Factory
Service Laboratoryr 4300 N. California Ave. Chicago, Illinois 60618. Include a conplete
description of the malfunction.
SI{ER\AZOOD ELEcTRoNTE
LABoRAIoRrEs,
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4300NURTHCALIFoRNIA
AVENUE,CHICAG0,
ILLIN0IS 50618 31
2-478-7300