5: Maintenance
PRC-PS-MS
5-11
adjusting R4 returns the voltage across D2 to 12.5V but the power supply
output power (J1 connector pins A and E) is not 12.5V, skip to step 3.
2. If adjusting R4 does not return the voltage across D2 to 12.5 Vdc,
measure the resistance across R2 (100 ohms), R3 (4.7 k ohms) and R4
(5 k ohms). If any of these resistances are out of specification (greater
than 5%), replace the resistor. If the resistors are within specification,
replace the D2.
3. Measure the op-amp U1A output voltage at pin 1. If it is not 1.5 Vdc or
less, adjust R8 (CURRENT ADJUST control) until the output of U1A
changes. If adjusting R8 does not change the output of U1A, measure the
resistance across R6 (0.05 ohms), R7 (4.7 k ohms), R8 (5 k ohms) and
R37 (2.4 k ohms). If these resistors are within specification (greater than
5%) replace U1.
4. Measure the output of op-amp U1B at pin 7. If it is not 14.0 Vdc,
measure the resistance across R11 (4.7 k ohms) and R12 (10 k ohms).
Replace any shorted components.
5. If the output of U1B (pin 7) is 14.0 Vdc, but the power supply output
power (J1 connector pins A and E) is not 12.5V, measure the
collector-to-base and emitter-to-base resistance of the pass transistors
Q1, Q5 through Q8). It may be necessary to lift the legs of these
transistors to get accurate measurements. Replace any defective
transistors.
Audio Amplifier
Output
This section assumes there is no audio output at the speaker.
1. Measure the DC voltage at the audio amplifier U5 output (pin 4). If it is
not 6 Vdc, check for shorts at pins 1, 2 and 4. Also check the resistance
of R13 (3.9 ohms).
2. Turn the AUDIO GAIN control fully clockwise and measure the AC
voltage at U5 pin 1. If it is not the same as the voltage at the SET
POWER connector (J1) pin P, measure the resistance of R1 (10 k ohms).
Also inspect the wiring to R1.
3. Check the speaker by measuring the resistance from the minus (–) side
of C15 to the plus (+) side of the speaker. This should show a short
circuit. If it does not, remove the ground from the SET POWER
connector J1 pin C. Measure the DC voltage at pin C. If it is not
12.5 Vdc, measure the resistance across diode D9. If it is okay, replace
transformer K1.
4. Measure the AC voltage at the output of U5 pin 4. If it is not 2.8 Vrms,
replace capacitor C15.
