MAINTENANCE
MANUAL
C20
STEREO PREAMPLIFIER
(Serial No. 6L700 & above)
from
DESCRIPTION
your amplifier.
for
NO. and
number
SCHEMATIC
model
the
and
give
to
serial
and
sure
be
below,
2
parts
order
Table
SC119-201)
COMPONENTS
Selector
DESCRIPTION
Input
Switch,
NO.
S1
COMPONENT
Filter
1.6A
Cutoff
50 PIV at
Frequency
Switch, Rumble Filter
S5
Tape Compare
Switch, Mode Selector
Switch,
S6S7S8
Switch, Tape Jack
High
Switch, Treble Compensator
Switch,
Switch, Bass Compensator
S3
S4
S2
Power
Switch, Phase
Silicon Rectifier,
Transformer,
S9
S1, S2
M199A
No.
you
pur-
SPECIAL
compo-
These
below.
listed
in the C20 are
2.
(Schematic
Table
may be
power
with
15/15V
300/350/350V
350/350V
MF at
C20. They
MF at
combined
MF at
for the
DESCRIPTION
20/20/40
1000/1000
40/40
Section
— 4
Loudness (Aural Compensator)
rear panel
on
Bass)
(Lo
Trim
switch
Capacitor,
built specifically
Filter Capacitor,
Filter
Filter Capacitor,
Control, Treble
Control, Bass
on-off
Control,
Control, Volume
Control, Balance
Control, Bass
and
NO.
R5
used)
R7 (2
designed
components used
directly from Customer Service, Mclntosh Laboratory Inc. When
are
Special
nents
chased
C102
C103
COMPONENT
R2
R 3
R1
C104
R4
Table 1. C20 VOLTAGE AND RESISTANCE CHART
(Schematic No. SC117-201)
Tube
12AX7
(V1)
12AX7
(V2)
12AU7
(V3)
12AU7
(V4)
12AX7
(V5)
12AX7
(V6)
12AX7
(V7)
6X4
(V8)
Pin No.
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
DC Volts
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
315 AC
—
6.3 AC
0
__
315 AC
335
Resistance in
Ohms Unit Off
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. High impedance VTVM is used to measure operating voltages.
2. Resistance measured to grid, (chassis)
3. * Denotes pin 7 of V8 grounded for this test.
4. Control positions for tests are as follows;
MODE SELECTOR
BALANCE
PHASE
H.F. FILTER
LOUDNESS
INPUT SELECTOR
BASS COMPENSATOR
TREBLE COMPENSATOR
BASS
TREBLE
RUMBLE FILTER,
TAPE JACK,
TAPE COMPARE
VOLUME
LOW FREQUENCY TRIM
STEREO
0
0
FLAT
FLAT
TAPE HD 2
0
TAPE
0
0
OUT
ON, CCW
0
Schematic
C20
SC117-201)
No.
(Schematic
AS
SUCH
CHARACTERISTlC.
ABOVE.
AND
6L700
NUMBERS
POSlTION.
SERIAL
WITH
NUMBERS GREATER THAN ONE.
FOR
MMFD
AND IN
NOTED.
AS
TRACKING
CHARACTERISTICS
CONTROLS
SPECIAL
VOLUME
CONDENSERS.
THE
USED
MFD/15V
ADJUST
HAVE
OR
2000
TO
POSITION.
EXCEPT
POINT
"OUT"
½WATT
PREAMPLIFIERS
EXTREME COUNTERCLOCKWISE
IN THE
IN THE
TO C20
APPLIES
SCHEMATIC
THlS
(1)
SHOWN
±10%,
ARE
SHOWN
ARE
ROTARY SWITCHES
(2) ALL
OHMS
SWITCHES
ARE IN
VALUES
PUSHBUTTON
RESISTOR
(3) ALL
(4)
SEPARATE
NUMBERS LESS THAN ONE,
ARE IN MFD FOR
CONDENSER VALUES
(5)
THIS
AT
AS TWO
FOUND
MAY BE
SOME UNITS C104
(6) IN
USED
MAY BE
UNITS
SOME
IM
RESISTOR
(7) A
PARTICULAR VOLUME CONTROL
TO THE
MATCHED
IS
VALUE
THE
VALUE,
THEIR
ON
CLOSE TOLERANCE
A
HAVE
COMPONENTS
THESE
*
SPECIAL RESISTANCE TAPER.
OR A
NOISE
LOW
NOTES:
Your C20 will give you many years of pleasant and satisfactory
performance. If you have any questions concerning the operation
or maintenance of this pre-amplifier please contact:
Customer Service
Mclntosh Laboratory Inc.
2 Chambers Street
Binghamton, New York
Our telephone number is 723-5491.
The direct dial area code is 607.
2M-AA117-201 5-62
BE012003
LABORATORY INC.
2 CHAMBERS STREET, BINGHAMTON, N. Y.
Phone-Area Code 607-723-5491
Loading...