Service Instructions

SR105 Power Amplifier

AMPLIFIER SERVICE (See Guarantee)

The SR105 Power Amplifier uses components of the high- est quality, operating well within their respective ratings to assure long life.

W A R N I N G

Voltages in this equipment are hazardous to life. Make all input and output connections with ac power disconnected. Refer servicing to qualified service personnel.

REPLACEMENT PARTS

Parts that are readily available through local electronic parts distributors are not shown on the accompanying Parts List. Their values are shown on the Circuit Diagram (Figure 21, Page 25). Commercial parts not readily avail- able and uniaue arts are shown on the Parts List and may be ordered diredtly from the factory.

The commercial alternates shown on the Parts List are not necessarily equivalents, but are electronically and mechanically similar, and may be used in the event that direct factory replacements are not immediately available. To maintain the highest possible performance and reliabil- ity, Shure factory replacement parts should be used. When ordering replacement parts, specify the Shure Replacement Kit Number, description, product model number and serial number.

FUSE REPLACEMENT

To replace line fuse F2 (with no apparent problems in the unit), disconnect the line cord and remove the rear-panel fuseholder cap. Replace the defective fuse only with a 3AG-5A fuse.

NOTE: The following symbol on the rear panel denotes a fast operating fuse.

CAUTION

If trouble symptoms - overheating (thermal cycling), erratic operation, etc. -were apparent before the fuse blew, or if the replacement fuse blows, trouble- shoot the Amplifier carefully to find the source of the trouble. Do not continue to replace fuses unless the trouble has been corrected.

The Amplifier also contains two wired-in fuses, one in series with rear-panel line fuse F2 (FI, 3AG-8A) and one in the meter lamp circuit (F3, SAG-1A). If replacement be- comes necessary, replace only with identical fuses.

BOTTOM PLATE REMOVAL

To remove the chassis bottom plate, turn the Amplifier upside-down and remove the 10 screws located at the edges of the chassis.

COVER REMOVAL

To service components on the top of the chassis, the protective cover grille (MP7) must be removed. This is done by removing one screw from the top, two screws from each side, and eight screws from the back surface. To re- move, slide cover up and to the rear of the Amplifier.

LAMP REPLACEMENT

Two bayonet-base pilot lamps (PL2, PL3) illuminate the scale of the OUTPUT VOLTAGE meter. These lamps are mounted on the top surface of the chassis behind the meter as shown in Figure 13, Page 13. To replace a lamp, remove the grille cover and the nut on the lamp bracket mounting stud. Swing the bracket up and out until the lamp is ac- cessible. Push the lamp inward, twist it 1/8-turn counter- clockwise, and withdraw it from its socket. After replacing the lamp, reposition the assembly under the meter and tighten the bracket nut. Replace the grille cover.

DC OUTPUT BALANCE ADJUSTMENT

In order to minimize the idle power consumption of the Amplifier and eliminate dc offset at the direct-coupled output, a dc balance control (R4-DC BAL.) has been pro- vided. This control is located on the Main Circuit Board under the chassis, as shown in Figure 14, Page 14, and may be reached by removing the bottom cover. The dc balance of the Amplifier should be checked and adjusted as neces- sary whenever any circuit components are replaced.

The dc balance adjustment is made with the Amplifier VOLUME control (R41) full counterclockwise and no speaker load. If the Amplifier being adjusted is equipped with a 70-volt output (SR105A), the 70V-OFF Switch (S4) should be turned off. Connect a dc voltmeter (preferably with a 0.1 Vdc full-scale range) across the terminals of the DIRECT OUTPUT terminal strip (TSI).

CAUTION

The polarity of any dc voltage at the output may be either positive or negative and could be as much as 10 volts; care should be exercised in connecting the meter such that an on-scale reading is obtained.

Carefully adjust the dc balance control for 0 Vdc +-25 mV at the output.

If the 70V-OFF Switch was moved to make this adjust- ment, return it to its original position. Replace the bottom cover.

OUTPUT VOLTAGE METER CALIBRATION

In the event that the OUTPUT VOLTAGE Meter (MI) or its associated circuitry is repaired, a meter calibration check and adjustment should be performed. With the Amplifier off, check the zero position of the meter. If it is not on zero, use a small screwdriver to turn the meter zero