General, Troubleshooting, AC Voltage Measurements

SR105 Power Amplifier

NOTES TO CIRCUIT DIAGRAM

GENERAL

Shure part numbers are not shown in the Parts List accompanying the Circuit Diagram (Figure 21, Page 25) if parts are readily available through local electronics parts suppliers. In these instances, the Circuit Diagram shows only the reference designation and value of the standard parts.

All capacitor values are shown in microfarads unless otherwise designated. All non-electrolytic capacitors are 100 working volts dc or more unless otherwise specified. Electrolytic capacitors are shown in microfarads x volts.

All resistor values are shown in ohms (k = 1000). Resis- tors are 10% tolerance unless otherwise specified. Resis- tors are Vi-watt unless otherwise specified.

Transistor lead codes are as shown in Figure 20. Accep- table replacements are as shown in the Parts List.

The following round symbols denote: Chassis Ground 4

Circuit Ground &

Printed Circuit Board Ground A

FIGURE 20. TRANSISTOR LEAD CODES

TROUBLESHOOTING

A general troubleshooting process is as follows: If the Amplifier is completely "dead," check the ac power source, fuses, and power supply output (52.5V at pin F of Main Circuit board). If the indicator lamps are on but the output is distorted, low or not present, apply an input signal as described under Ac Voltage Measurements below, and

determine that the input and output voltages to each board assembly are correct. If an incorrect ac voltage is found at any board output, perform Dc Voltage Measurements on that board as described below to isolate the problem area.

AC VOLTAGE MEASUREMENTS

The numbers within rectangular symbols 0on the Circuit Diagram denote the ac voltage at that point under the following test conditions:

1.Voltage measured with respect to chassis unless other- wise indicated.

2.Line voltage: 120V, 50/60 Hz.

3.Test signal of 0.3V, 1 kHz applied across connector J1.

4.Ac voltage measurements may vary t 2 0 % from values shown.

5.Measurements made with ac VTVM of 1 megohm or greater input impedance.

6.Four-ohm non-inductive 200-watt load across DIRECT OUTPUT Terminal Strip TS1 for direct output measure- ments or (SR105A only) 33-ohm non-inductive 150-watt load across 70-volt OUTPUT Terminal Strip TS2 (70V- COM) for 70-volt output measurements, but not both.

7.Volume Control set to maximum (10).

8.SR105A Only: 70V-OFF Switch set to OFF for all circuit measurements except 70-volt circuitry (Filter Circuit board and associated components). Filter Circuit board rolls off the low frequencies; amplifier output should be -3 -c-1 dB at 30 Hz referenced to 1 kHz and -12 -c-3 dB at 20 Hz.

DC VOLTAGE MEASUREMENTS

The numbers within elliptical symbols 0 on the circuit diagram denote the dc voltage at that point under the following test conditions:

1.Voltages measured with respect to chassis unless other- wise indicated.

2.Line voltage: 120V, 50160 Hz.

3.No input signal applied.

4.Dc voltage measurements may vary t 2 0 % from values shown.

5.Measurements made with VTVM of 11 megohms or greater input impedance.

RESISTANCE MEASUREMENTS

With the ac line cord disconnected from the ac source and the POWER ON-OFF Switch in the OFF position, the following ohmmeter measurements may be made:

1.Transformers may be checked for continuity of each winding.

2.To test transistors and diodes, see Page 15.