Panasonic KX-FT71LA-B, Service Manual

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One of most difficult problems to deal with is when the system will not boot up.
The symptom: No response when the power is turned on. (No LCD display, keys are not
accepted.)
Note:
1. Electrical continuity may have existed at the factory check, but a
faulty contact occurred as a result of vibration, etc., during
transport.
2. Solder waste remaining on the board may get caught under the IC
during transport, causing a short circuit.
Before we begin mass production, several hundred trial units are produced at the plant, various
tests are applied and any malfunctions are analyzed. (In past experiences, digital IC (especially
SRAM and ROM) malfunctions are extremely rare after installation in the product.) / This may be
repaired by replacing the IC, (ASIC etc.). However, the real cause may not have been an IC
malfunction but a soldering fault instead. / Soldering faults which are difficult to detect with the
naked eye are common, particularly for an ASIC and RA (Resistor Array). But if you have an
oscilloscope, you can easily determine the problem site or IC malfunction by checking the main
signal lines. / Even if you don't have such a measuring instrument, by checking each main
signal line and resoldering it, in many cases the problem will be resolved. / An explanation of
the main signals (for booting up the unit) is below.
What are the main signals for booting up the unit?
Please refer to the
DIGITAL BLOCK DIAGRAM (). / The ASIC (including the CPU) (IC1) controls
all the other digital ICs. When the power is turned on, the ASIC (CPU) retrieves the operation
code stored in the ROM (IC2), then follows the instructions for controlling each IC. All ICs have
some inner registers that are assigned to a certain address. / It is the address bus by which the
ASIC (CPU) designates the location inside each IC. And the data bus reads or writes the data in
order to transmit the instructions from the ASIC (CPU) to the ICs. / These signal lines are all
controlled by voltages of 5V (H) / 3.3V (H) or 0V (L).
2.3.6.1. DIGITAL BLOCK DIAGRAM
The signal lines that must be normal for the system to boot up are listed here [List 1]. / For
signal lines other than these, even if they malfunction they do not directly affect booting up the
system.
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