SHURE 1200 POWERMIXER
Service Manual

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 otherwise 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 otherwise indicated. 2. Line voltage: 120 V, 60 Hz. 3. No input signal applied. 4. Dc voltage measurements may vary ±20% from values shown. 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 measurements 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. NOTES: 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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