Philips MC-77/22 Service Manual page 59

Service Hints
C
Measuringpoint :
The first stage of the modulator is an integrator. An
integrated rectangle results in a triangle.
D
Measuringpoint :
The oscillator signal (squarewave) is fed to the second
integrator (7122-B).
E
Measuringpoint :
The integrated rectangle results in a triangle. 7122-C and
7122-F work as a comparator.
11-5
F
Measuringpoint : 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
Measuringpoint :
The low-side driver signal (Ch1) is the modulator output
level-shifted by transistor 7128. The high-side driver signal
(Ch2) is the inverted low-side driver signal level-shifted by
transistor 7119.