Philips MCM8/21 Service Manual page 59

Service Hints
C
Measuring point :
The first stage of the modulator is an integrator. An
integrated rectangle results in a triangle.
D
Measuring point :
The oscillator signal (squarewave) is fed to the second
integrator (7122-B).
E
Measuring point :
The integrated rectangle results in a triangle. 7122-C works
as a comparator. 7122-D improves the shape of the pulses.
11-3
F
Measuring point : Output modulator
In this pulse width modulated square wave the analog
Audio signal is included. Measurements with an analog
scope will show a jitter on the falling edge.
The modulator frequency is still fixed to 500kHz. Similar to a
frequency modulation - in this case the amplitude of the
analog audio signal varies the pulse width, the frequency
defines the speed.
G
Measuring point :
The low-side driver signal <G1> (Ch2) is the modulator
output level-shifted by transistor 7128. The high-side driver
signal <G2> (Ch1) is the inverted low-side driver signal
level-shifted by transistor 7119.
Service Hints
H
Measuring point : gate-signal of the FETs
Ch1 = H1, CH2 = H2
I
Measuring point :
Digital output signal.
J
Measuring point :
The following three signals are measured after output filter
5101 with different timebases.
11-3
K
Measuring point :
After common mode coil 5102 - the audio signal is restored.