Service manual

NOTES TO THE CIRCUIT DIAGRAM
D.C. Voltage Measurements:
Check the DC voltages before checking AC voltages be-
cause transistors Q2 through Q9 are direct coupled and
any deviation from the nominal voltages will affect the AC
voltages. The four key DC voltages are +94, +20, the
emitter of Q1, and the split voltage at the junction of R3,
R5, R7, and R8. If these four key DC voltages are correct,
then proceed with the AC voltage measurements.
The numbers within the symbol on the circuit dia-
gram denote the DC voltage at that point with the follow-
ing test condition:
1. Voltages measured at points indicated with respect to
chassis, unless otherwise specified.
2. Line voltage PM300: 120 V. 60 Hz.; PM300E: 120/240 V.
50/60 Hz.; PM300E6: 100, 120, 140, 200, 220, or 240 V.
50/60 Hz.
3. No input signal applied.
4. DC voltage measurements may vary ±20% from the
values shown.
5. Measured with a VTVM of 11-megohms input imped-
ance.
A.C. Voltage Measurements:
The numbers within the symbols on the circuit
diagram denote the AC voltage at that point with the fol-
lowing test conditions:
1. Voltages measured at points indicated with respect to
chassis, unless otherwise specified.
2. Line voltage as specified above.
3. 1,000 Hz. signal applied to one of the high impedance
parallel inputs at 300 millivolts (.3 volts).
4. Measured with an AC VTVM of 1.0 megohms or greater
input impedance.
5. Noninductive load of 8 ohms, 200 watts connected to
speaker output jack.
6. Volume control set to maximum.
7. AC voltage measurements may vary ±20% from the
values shown.
Shure part numbers are not shown in the Parts List, ac-
companying the circuit diagram, if parts are readily avail-
able through local electronic parts supply distributors. In
these instances, the circuit diagram will show the values of
the standard parts.
All capacitor values are shown in microfarads. All non-
electrolytic capacitors are to be 100 volts or more unless
otherwise specified in the circuit diagram. Electrolytic
capacitors are shown in microfarads and volts.
All resistor values are shown in ohms. Resistors are all
to be 10% tolerance unless specifically noted on the cir-
cuit diagram.
All resistors are ½ watt unless otherwise specified.
The following ground symbol
denotes chassis ground:
Ohmmeter Measurements:
With the A.C. line cord unplugged and the power switch
in the “OFF” position, the following ohmmeter measure-
ments may be made.
1. B+ to ground approximately 100 ohms.
2. Ohmmeter plus probe to the junction of R3, R5, R7, and
R8, ohmmeter minus probe to ground; greater than 120
ohms.
3. Ohmmeter plus probe to B+, ohmmeter minus probe to
the junction of R3, R5, R7, and R8: greater than 120
ohms.
4. To test transistors and diodes, see page 7.
PM300 CIRCUIT DIAGRAM
NOTES:
1. ALL CAPACITORS IN MFD AND 100 VOLTS OR MORE
UNLESS SHOWN. ELECTROLYTIC CAPACITORS SHOWN
IN MFD x VOLTS.
2. ALL RESISTORS 10%, ½ WATT, UNLESS OTHERWISE
INDICATED. R3 THRU R8 10%, 5 WATT.
3. THE FOLLOWING SYMBOLS DENOTE:
DC VOLTAGE
AC VOLTAGE
WITH RESPECT TO CHASSIS UNLESS OTHERWISE
SHOWN.
4. R
SEL
MAY BE 180 OR 82 OHMS, OR NOT USED, DEPENDING
ON POWER DRAIN (SEE PAGE 7).
FIGURE 7.
9