Philips MC-77/22 Service Manual page 58

Service Hints
The analog part of the Combi Board can be repaired without
opening the metal shielding box. In case of a 'Class D'
problem it is advised to disassemble the board first,
desolder the metal bottom of the shielding box and
assemble the board again. This takes a few minutes only.
Attention: Poor soldering of the metal shielding box results
in disturbance of the tuner.
In most cases the FETs 7109 and/or 7121 for the left
channel, respectively 7218 and/or 7231 for the right channel
will be defective. This can be checked easily with an
ordinary Ohm-meter.
L C :
EFT HANNEL
In case 7109 is defect replace following parts:
7109, 7111, 7105, 7119, 7104 and 3101
In case 7121 is defect replace following parts:
7121, 7113, 7118, 7117 and 3130
R C :
IGHT HANNEL
In case 7218 is defect replace following parts:
7218, 7221, 7209, 7228, 7208 and 3205
In case 7231 is defect replace following parts:
7231, 7210, 7235, 7227 and 3243
If none of the FETs is defective the fault is most probably
located in the modulator. To check the operation - follow the
given signals.
General description of Oscilloscope setup:
Ground Ch1 & Ch2
Y-Deflection
CH1 = 20V/DIV
Y-Deflection
CH2 = 20V/DIV
Timebase = 1µs
The following signals are measured at:
AUX in = 500mV/1kHz, Volume = -28dB
11-4
Triggerlevel =-33,6V
Triggered on Ch1
A
Measuringpoint : Output pre-amplifier
Normal analog signal measured (1kHz- Timebase 1µs). If
this signal can´t be measured - the fault is outside the
shielding box.
B
Measuringpoint : Input Modulator
The 1kHz signal not visible anymore. Reducing the
timebase to 1µs shows the oscillogram below.