Repair Procedures - Yamaha YST-SW800 Service Manual

YST-SW800

REPAIR PROCEDURES

It is recommended to disconnect the SW power supply section and the power amplifier because it is highly possible
that the SW power supply section has also broken down due to the overload caused by breakdown in the power
amplifier.
Repair Procedure
1) Remove CB201, CB202 and CB205. (Power (1) P.C.B.)
Then the power supply circuit and the power amplifier circuit will be disconnected.
2) Repair the power amplifier as described below. (Main (1) P.C.B.)
Replace the defective parts.
Whether the major parts are defective or not can be checked as follows.
Remove 3 screws fixing the heat sink to the rear panel and 4 screws fixing the
support P.C.B. to the rear panel.
FET (Q16~19, Q22~24): Measure the resistance value between GATE and SOURCE
Remove 3 screws fixing the Main (1) P.C.B. to the support P.C.B.
TR (Q1~4):
Check the operation as follows.
a. Prepare a stabilized DC power source equipped with +30V and -30V outputs.
b. Connect +B, -B and GND output terminals of the stabilized power source to +B, -B and GND lines of the power
amplifier.
c. Connect the terminal of the oscilloscope to the circuit between q in the figure and GND.
d. Remove CB1. (Main (2) P.C.B.)
e. Connect the output of the signal generator to the circuit between the lead on the (-) side of C1 and GND.
f. Set the frequency of the signal generator to the 100Hz sine wave and the output level to the minimum level.
g. Turn on the stabilized DC power source.
h. Raise the output level of the signal generator gradually and check the output waveform on the oscilloscope.
i. If nothing abnormal is found with the waveform until the output level reaches 40Vp-p, the check result is OK.
After operation check
Connect CB1.
3) Repair the SW power supply section as follows. (Power (1) P.C.B.)
Follow the instruction described below to avoid any danger.
High voltage maintained in C209 and C210 even after the power is turned off is dangerous. It is necessary to
force this electric charge to be discharged before starting the repair work.
For such forced discharge, connect a resistance of approximately 100Ω/5W to each of both terminals of the
capacitor. Then check to make sure that the voltage between the capacitor terminals is 0V.
Replace the defective parts.
Whether the major parts are defective or not can be checked as follows.
TR (Q202~205): It is highly possible that a failure occurs due to a short
R216, R217:
5
to determine whether a failure exists or not.
If the measured resistance value is close to 0Ω, a failure
exists.
If the measured resistance value is close to the value of
the resistor connected between GATE and SOURCE, no
failure exists.
Check DIODE characteristics between BASE and EMITTER
or between BASE and COLLECTOR to determine whether
a failure exists or not.
between COLLECTOR and EMITTER.
It is highly possible that a failure occurs due to an open
circuit.
FET
G
D S
TR
B
C E
TR
B
B
C
C E
E