AC VOLTAGE MEASUREMENTSTo replace the reverb pan, connect the white lead connector
Numbers within rectangular symbols on the circuit diagramsto the reverb Input and the orange lead connector to the reverb
denote ac voltages at that point under the following testOutput. Use the previously removed screws to secure the reverb
conditions:pan to the reverb shield, and to secure the shield to the chassis
1.Voltages measured with respect to chassis unless other-bottom and the parallel vertical interior shield. Be sure to replace
wise indicated.any cable ties that were removed or loosened.
2. Line voltage: 120 V, 60 Hz.
3. Test signal of 0.5 mV across pins 2 and 3 of connectorSHR-11 AND SHR-12 POWER AMP BOARD ASSEMBLY
J101.The Power Amplifier Board Assembly consists of SHR-11
4.AC voltage measurements may vary ?20% from valuesand SHR-12 mounted on the large aluminum heat sink. To
shown.separate the heat sink and its associated boards from the 1200,
5. Measurements made with ac voltmeter of 1 megohm orremove the screws holding the sink to the power transformer
greater input impedance.shield bracket, and those holding the sink to the metal shield
6. No load on Speaker Outputs.that divides the fan from the input and output connectors and
7. All controls (of channel under test only) full clockwiseboards. Disconnect the connectors from P1101, P1102 and
except Equalization controls centered.P1201, and unsolder the red, green, and white leads from AA,
8.Reverb Return full counterclockwise.AB and AC, respectively, on SHR-11. (The opposite ends of
9. Limiter switch and Phantom switch in Off position.leads AA and AB are connected to SHR-14, Main Board; the
10. Monitor Master control clockwise.opposite end of lead AC is connected to SHR-13, Speaker
Output Board.) The Power Amp Board is now free of the 1200.
DC VOLTAGE MEASUREMENTSTo replace parts on SHR-11, it is then necessary to remove
Numbers within elliptical symbols on the circuit diagramsSHR-11 from the large heat sink. Do this by removing the four
denote dc voltages at that point under the following testPhillips screws holding the power transistors at the corners of
conditions:the board, and the two Phillips screws holding the board on the
1. Voltages measured with respect to chassis unless other-brass standoffs.
wise indicated.
2. Line voltage: 120 V, 60 Hz.NOTES:
3. No input signal applied.
4. Dc voltage measurements may vary ?20% from values1.Removal of SHR-11 from the heat sink may damage the
shown.insulators below the power transistors. Therefore, when
5. Measurements made with dc voltmeter of 11 megohms orservicing SHR-11, be prepared to replace all four 802639FK
greater input impedance.insulators. If either predriver transistor Q1103 or Q1104
6. Phantom switch in On position.(mounted on the vertical heat sink fastened to SHR-11)
is replaced, be prepared to replace insulator 802681FK
RESISTANCE MEASUREMENTSbeneath it.
With the ac line cord disconnected from the ac source and
the Power On/Off switch off, the following ohmmeter measure-2. If transistor Q1105, mounted on the solder side of board
ments may be made.SHR-11, is replaced or if the board has been removed from
1. Reverb pan output coil: approximately 360 ohms; inputthe heat sink, make certain that Q1105 physically contacts
coils approximately 40 ohms.the large heat sink after reassembly of the board to the heat
2.Transformers may be checked for continuity of each winding.sink is completed. Apply a small amount of Wakefield Type
3.To test transistors, ICs and diodes, refer to the section on120 Thermal Joint Compound to the top surface of the
CHECKING ACTIVE COMPONENTS.transistor in order to ensure proper heat transfer.
CHASSIS-MOUNTED BOARDS AND COMPONENTSTo replace SHR-11 in the 1200, solder the red, green and
The following printed wiring boards and components arewhite leads to AA, AB and AC. Connect the cable assembly with
chassis-mounted.three leads from P1405, SHR-14, to P1101 ; and connect the
SHR-11POWER BOARDcable with a single lead from P1407, SHR-14, to P1102.
SHR-12SENSOR BOARD
SHR-14MAIN BOARDSHR-11 POTENTIOMETER ADJUSTMENT
A1REVERB PANIf any components are replaced on SHR-11, trimpots VR1101,
T1POWER TRANSFORMERVR1102 and VR1103 may require readjustment. Measure the
SHR-11 voltages described in the following paragraphs. If
REVERB PANmeasured voltages are not correct, make adjustments as follows.
To remove the reverb pan assembly, turn the 1200 on its sideWith power off, no input signal applied, and no load on the
(power transformer down) and remove the three screws holding1200?s speaker output, turn VR1103 completely clockwise. Turn
the reverb shield to the chassis bottom. Turn the 1200 uprightpower on and allow unit to warm up or thermally stabilize for ten
and remove the screw holding the reverb shield to the standoffminutes prior to making the adjustments. See Figure 2 for
attached to the parallel vertical interior shield. Disconnect thetrimpot locations.
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.)
5 |
ACEC
