Hitachi C1422R, Руководство по обслуживанию

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5
S
ERVICING
Microprocessor Control System
Microcontroller
Micro-controller,
I701
, is either a SAA5288 (on non-teletext sets) or a SAA5290 (on teletext sets). Both
devices have integrated on-screen display (OSD) generator; the SAA5290 also has an integrated teletext
decoder. The micro-controller has a single 12MHz crystal
X701
.
The television is controlled by serial (I²C) bus communication, digital switching inputs and outputs, analogue-
to-digital inputs and pulse-width-modulation (PWM) outputs.
Reset
At power on,
C709
is not charged so the voltage on pin 43 of the micro-controller rises with the 5V supply
and resets the micro-controller. An internal resistor to ground at this pin causes
C709
to charge up and the
voltage on the reset pin to drop to 0V.
In normal operation, transistor
Q702
is switched on and the reset pin is low, but if the 10V supply voltage
drops below about 8V the transistor will switch off and
R721
will pull the reset pin high and reset the device.
User Control
User input is via infra-red remote control (Philips' RC5 protocol), internally decoded from the receiver
I703
,
or from three (non-matrixed) local control keys on active low inputs,
I701
pins 18, 19 and 20. The LED
flashes each time a key is detected or remote command received.
Tuning
The tuning control voltage to the tuner is controlled via the PWM at pin 1 of
I701
and integrating circuit
around
Q001
. Minimum voltage is at maximum mark-space ratio (bottom of each band). Band-switching is
controlled by active low outputs on pins 14, 15 and 16 (high, mid, low) and transistors
Q002
to
Q004
. The
controller makes AFC corrections by reading on-tune information from
I501
via the I²C bus.
As the tuner is controlled by voltage synthesis, there is no direct correlation between the controller output
and the tuned frequency.
Non-volatile memory
The non-volatile memory,
I702
, holds configuration information, user settings, parameters as applicable for
I²C controlled ICs and the programme tuning records. It is itself accessed by I²C.
When a new memory IC is fitted, the microcontroller will automatically load default information, which takes a
few seconds during power-up. Regular re-loading of the data, or corruption of settings may indicate
I702
is
faulty.
AV switching
Inputs from pins 8 and 16 of SCART 1 are sampled by analogue to digital conversion on pins 9 and 10 of
I701
respectively. The input levels are adjusted such that the controller will automatically switch to AV1
when pin 8 is above 6V and RGB when pin 16 is above 1V. The time constant on pin 16 ensures the
controller will not detect real-time RGB insertion using pin 16.
OSD / Teletext
Line and field timings are obtained from V.sync on pin
37
and H.sync input on pin
36
. Teletext (on teletext
sets only) is obtained from the CVBS input on pin 23. The RGB outputs on pins 34, 33 and 32 respectively
are inserted into the TV output when gated by the OSD EN OUT signal on pin 35. OSD / teletext contrast is
controlled by the peak reference level on pin 31, generated from the microcontroller PWM output on pin 2 via
Q305
.
Error codes
Under the following fault conditions the television will switch to standby and flash the LED.