McIntosh C20 - User manual

C20
STEREOPREAMPLIFIER
(SerialNo.6L700& above)
MAINTENANCE
MANUAL

DESCRIPTION
COMPONENT
NO.
Switch,
Input
Selector
Switch,BassCompensator
Switch,TrebleCompensator Switch,
High
Frequency
Cutoff
Filter
Switch,RumbleFilter Switch,ModeSelector
Switch,
TapeCompare
Switch,TapeJack Switch,Phase SiliconRectifier,
50PIVat
1.6A
Transformer,
Power
S1 S2
S3 S4
S5 S6
S7
S8 S9 S1,S2
M199A
DESCRIPTION
COMPONENT
NO.
FilterCapacitor,
20/20/40
MFat
300/350/350V
FilterCapacitor,
40/40
MFat
350/350V
Filter
Capacitor,
1000/1000
MFat
15/15V
Control,Bass
Control,Treble Control,
Loudness(AuralCompensator)
Control,Volume
—4
Section
combined
with
power
on-off
switch
Control,Balance Control,Bass
Trim
(Lo
Bass)
on
rearpanel
C102 C103 C104
R1
R2 R3 R4 R5
R7(2
used)
Special
componentsused
intheC20are
listed
below.
These
compo-
nents
are
designed
and
builtspecifically
forthe
C20.They
maybe
pur-
chased
directlyfromCustomerService,MclntoshLaboratoryInc.When
you
Table
2.
SPECIAL
COMPONENTS
(Schematic
No.
SC119-201)
order
parts
be
sure
to
give
the
SCHEMATIC
NO.and
DESCRIPTION
from
Table
2
below,
and
serial
and
model
number
for
youramplifier.

Table1.C20VOLTAGEANDRESISTANCECHART
(SchematicNo.SC117-201)
Tube
12AX7
(V1)
12AX7
(V2)
12AU7
(V3)
12AU7
(V4)
12AX7
(V5)
12AX7
(V6)
12AX7
(V7)
6X4
(V8)
PinNo.
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
0
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
DCVolts
No.Signal
146
0
1.0
5.6
-5.6
110
0
1.0
—
146
0
1.0
5.6
-5.6
110
0
1,0
64
0
2.4
5.6
-5.6
198
62
120
—
64
0
2.4
5,6
-5.6
198
62
120
—
104
0
.7
5.6
-5.6
230
22
29
_
104
0
.7
5.6
-5.6
200
22
29
-
250
20
70
5.6
-5.6
250
20
70
0
315AC
—
6.3AC
0
__
315AC
335
Resistancein
OhmsUnitOff
130K*
1M
2.2K
5
5
360K*
56K
4K
—
130K*
1M
2.2K
5
5
360K*
56K
4K
_
120K*
<50
1.8K
5
5
14K*
1.1M
100K
—
120K*
<50
1.8K
5
5
14K*
MM
100K
—
330K*
110K
1.8K
5
5
9K
222K
22K
—
330K*
110K
1.8K
5
5
31K*
222K
22K
—
7K*
1M
47K
5
5
7K*
1M
47K
0
580
_
.1
0
—
560
*
NOTES:1.HighimpedanceVTVMisusedtomeasureoperatingvoltages.
2.Resistancemeasuredtogrid,(chassis)
3.* Denotespin7 ofV8groundedforthistest.
4.Controlpositionsfortestsareasfollows;
MODESELECTOR
BALANCE
PHASE
H.F.FILTER
LOUDNESS
INPUTSELECTOR
BASSCOMPENSATOR
TREBLECOMPENSATOR
BASS
TREBLE
RUMBLEFILTER,
TAPEJACK,
TAPECOMPARE
VOLUME
LOWFREQUENCYTRIM
STEREO
0
0
FLAT
FLAT
TAPEHD2
0
TAPE
0
0
OUT
ON,CCW
0

C20
Schematic
(Schematic
No.
SC117-201)
NOTES:
(1)
THlS
SCHEMATIC
APPLIES
TOC20
PREAMPLIFIERS
WITH
SERIAL
NUMBERS
6L700
AND
ABOVE.
(2)ALL
ROTARYSWITCHES
ARE
SHOWN
INTHE
EXTREMECOUNTERCLOCKWISE
POSlTION.
(3)ALL
PUSHBUTTON
SWITCHES
ARE
SHOWN
INTHE
"OUT"
POSITION.
(4)
RESISTOR
VALUES
AREIN
OHMS
±10%,
½WATT
EXCEPT
AS
NOTED.
(5)
CONDENSERVALUES
AREINMFDFOR
NUMBERSLESSTHANONE,
ANDIN
MMFD
FOR
NUMBERSGREATERTHANONE.
(6)IN
SOMEUNITS C104
MAYBE
FOUND
ASTWO
SEPARATE
2000
MFD/15V
CONDENSERS.
(7)A
RESISTOR
IM
SOME
UNITS
MAYBE
USED
AT
THIS
POINT
TO
ADJUST
THE
VOLUME
CONTROLS
TRACKING
CHARACTERISTlC.
THE
VALUE
IS
MATCHED
TOTHE
PARTICULARVOLUMECONTROL
USED
*
THESE
COMPONENTS
HAVE
A
CLOSETOLERANCE
ON
THEIR
VALUE,
OR
HAVE
SPECIAL
CHARACTERISTICS
SUCH
AS
LOW
NOISE
ORA
SPECIALRESISTANCETAPER.

LABORATORYINC.
2CHAMBERSSTREET,BINGHAMTON,N.Y.
Phone-AreaCode607-723-5491
2M-AA117-2015-62
BE012003
CustomerService
MclntoshLaboratoryInc.
2ChambersStreet
Binghamton,NewYork
Ourtelephonenumberis723-5491.
Thedirectdialareacodeis607.
YourC20willgiveyoumanyyearsofpleasantandsatisfactory
performance.Ifyouhaveanyquestionsconcerningtheoperation
ormaintenanceofthispre-amplifierpleasecontact: