Panasonic KX-FT21RS - Manual (Page 63)

2.3.4.4. Analog Board Section
The analog parts check is actually different than the digital parts check. The signal route is determined by the purpose of the check.
For example, the handset TX route begins from the handset microphone and is output in the telephone line. In this route, it is mainly
an analog signal. Tracing the signal can be done easily using an oscilloscope. Each route is shown on the Check Sheet here. If
there is a problem with the unit (for example, you cannot communicate with the H/S, etc.), trace the signal in the area and determine
the cause.
Note:
{ }: Inside the digital board
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KX-FT21RS