Shure AUDIOMASTER 1200, Service Manual

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AC VOLTAGE MEASUREMENTS
Numbers within rectangular symbols on the circuit diagrams
denote ac voltages at that point under the following test
conditions:
1. Voltages measured with respect to chassis unless other-
wise indicated.
2. Line voltage: 120 V, 60 Hz.
3. Test signal of 0.5 mV across pins 2 and 3 of connector
J101.
4. AC voltage measurements may vary ±20% from values
shown.
5. Measurements made with ac voltmeter of 1 megohm or
greater input impedance.
6. No load on Speaker Outputs.
7. All controls (of channel under test only) full clockwise
except Equalization controls centered.
8. Reverb Return full counterclockwise.
9. Limiter switch and Phantom switch in Off position.
10. Monitor Master control clockwise.
DC VOLTAGE MEASUREMENTS
Numbers within elliptical symbols on the circuit diagrams
denote dc voltages at that point under the following test
conditions:
1. Voltages measured with respect to chassis unless other-
wise indicated.
2. Line voltage: 120 V, 60 Hz.
3. No input signal applied.
4. Dc voltage measurements may vary ±20% from values
shown.
NOTES:
5. Measurements made with dc voltmeter of 11 megohms or
greater input impedance.
6. Phantom switch in On position.
RESISTANCE MEASUREMENTS
With the ac line cord disconnected from the ac source and
the Power On/Off switch off, the following ohmmeter measure-
ments may be made.
1. Reverb pan output coil: approximately 360 ohms; input
coils approximately 40 ohms.
2. Transformers may be checked for continuity of each winding.
3. To test transistors, ICs and diodes, refer to the section on
CHECKING ACTIVE COMPONENTS.
CHASSIS-MOUNTED BOARDS AND COMPONENTS
The following printed wiring boards and components are
chassis-mounted.
SHR-11 POWER BOARD
SHR-12 SENSOR BOARD
SHR-14 MAIN BOARD
A1 REVERB PAN
T1 POWER TRANSFORMER
REVERB PAN
To remove the reverb pan assembly, turn the 1200 on its side
(power transformer down) and remove the three screws holding
the reverb shield to the chassis bottom. Turn the 1200 upright
and remove the screw holding the reverb shield to the standoff
attached to the parallel vertical interior shield. Disconnect the
phono connectors at the bottom of the reverb pan assembly.
Remove the four screws holding the reverb pan to the vertical
metal interior shield, and lift the reverb shield and pan assembly
out of the chassis holding the pan to the shield. (If necessary to
free the reverb shield, remove the cable tie(s) holding the
orange and white leads to the metal shield.)
To replace the reverb pan, connect the white lead connector
to the reverb Input and the orange lead connector to the reverb
Output. Use the previously removed screws to secure the reverb
pan to the reverb shield, and to secure the shield to the chassis
bottom and the parallel vertical interior shield. Be sure to replace
any cable ties that were removed or loosened.
SHR-11 AND SHR-12 POWER AMP BOARD ASSEMBLY
The Power Amplifier Board Assembly consists of SHR-11
and SHR-12 mounted on the large aluminum heat sink. To
separate the heat sink and its associated boards from the 1200,
remove the screws holding the sink to the power transformer
shield bracket, and those holding the sink to the metal shield
that divides the fan from the input and output connectors and
boards. Disconnect the connectors from P1101, P1102 and
P1201, and unsolder the red, green, and white leads from AA,
AB and AC, respectively, on SHR-11. (The opposite ends of
leads AA and AB are connected to SHR-14, Main Board; the
opposite end of lead AC is connected to SHR-13, Speaker
Output Board.) The Power Amp Board is now free of the 1200.
To replace parts on SHR-11, it is then necessary to remove
SHR-11 from the large heat sink. Do this by removing the four
Phillips screws holding the power transistors at the corners of
the board, and the two Phillips screws holding the board on the
brass standoffs.
1. Removal of SHR-11 from the heat sink may damage the
insulators below the power transistors. Therefore, when
servicing SHR-11, be prepared to replace all four 802639FK
insulators. If either predriver transistor Q1103 or Q1104
(mounted on the vertical heat sink fastened to SHR-11)
is replaced, be prepared to replace insulator 802681FK
beneath it.
2. If transistor Q1105, mounted on the solder side of board
SHR-11, is replaced or if the board has been removed from
the heat sink, make certain that Q1105 physically contacts
the large heat sink after reassembly of the board to the heat
sink is completed. Apply a small amount of Wakefield Type
120 Thermal Joint Compound to the top surface of the
transistor in order to ensure proper heat transfer.
To replace SHR-11 in the 1200, solder the red, green and
white leads to AA, AB and AC. Connect the cable assembly with
three leads from P1405, SHR-14, to P1101 ; and connect the
cable with a single lead from P1407, SHR-14, to P1102.
SHR-11 POTENTIOMETER ADJUSTMENT
If any components are replaced on SHR-11, trimpots VR1101,
VR1102 and VR1103 may require readjustment. Measure the
SHR-11 voltages described in the following paragraphs. If
measured voltages are not correct, make adjustments as follows.
With power off, no input signal applied, and no load on the
1200’s speaker output, turn VR1103 completely clockwise. Turn
power on and allow unit to warm up or thermally stabilize for ten
minutes prior to making the adjustments. See Figure 2 for
trimpot locations.
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