Panasonic KX-MB772CX

KX-MB262CX/KX-MB772CX
15.6.
MAIN BOARD SECTION
When the unit fails
to boot up the
system, tak
e the troubleshooting procedures very carefully. It may have a serious problem.
The symptom: No response when the power is turned on.
(No LCD display
, and keys are not accepted.)
The first step is to check the power source. If there is no problem with the power supply unit, the problem may lie in the digital
unit (main board).
As there are many potential causes in this case (ASIC
, DRAM, etc.), it may be difficult to specify what you should chec
k first. If
a mistake is made in the order
of checks, a normal part may be determined faulty, wasting both time and money
.
Although the tendency
is to regard
the problem as
a serious one
(IC malfunction,
etc.), usually
most cas
es are cause
d by solder
faults (poor contact due to a tunnel
in the solder, signal short circuit due to solder waste).
Note:
1.
Electrical continuity may hav
e 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 circ
uit.
Before we begin mass production, several hundred trial units are produced at the plant, various tests are applied and any mal-
functions are analyzed. (In past experiences, digital IC (especially, DRAM and ROM) malfunctions are extremely rare after
installation in the product.)
This may be repaired by replacing the IC, (DRAM etc.). However
, the real cause may not have
been an IC malfunction but a sol-
dering fault instead.
Soldering faults difficult to detect with
the naked eye are
common, particularly for 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 pres
ented below.
What are the main signals for booting up the unit?
Please refer to
GENERAL BLOCK DIAGRAM
(P.11).
The ASIC (IC300) controls all the other digital ICs. When the po
wer is turned on, the ASIC retrieves the operation code stored in
the ROM (IC402), 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 designates the location inside each IC. And the data bus reads or writes the data in
order to transmit the instructions from the ASIC to
the ICs.
These signal lines are all
controlled by voltages of 3.3V (H) or 0V
(L).