Central Control (Aio Part) - Philips VR120 Service Manual

9.2.1 Evaluation of the keyboard matrix
There are 12 different keys. Each key function is assigned a
fixed voltage value. This value is decoded using an
analogue/digital (A/D) port (7899-B, pin 56). Each
mechanical key position on the printed board can adopt any
key function via a coding resistor. Pressing keys
simultaneously may lead to undesired functions!
Schematic:
DC-KEY
[7899-B, pin56]
9.2.2 IR receiver and signal evaluation
The IR receiver [6170] includes a selective, controlled
amplifier in addition to a photo-diode. The photo-diode
changes the received transmission (approx. 940nm) in
electrical pulses, which are then amplified and demodulated.
On the output of the IR receiver [7220] a level lift 0V/5V pulse
sequence, which corresponds to the envelope curve of the
received IR remote control command, can be measured. This
pulse sequence is input into the controller for further signal
evaluation via input IRR [7899-B, pin 46].
9.2.3 Activation and function of the VFD display
In principle, the VFD display [7170] is a tube triode in which
the heating filaments in the tube serve as cathodes (F+,F-).
The 7 grids (G1 - G7) are activated via PC2 - PC7, PD0 on
the controller, and the 16 anodes (P1 - P16) are controlled via
ports PE0 - PE7, PF0 - PF7, PC0, PC1 on the controller,
each with a positive potential compared to the cathode.
The grids and anodes (digits and symbols to be displayed)
are activated in the time-multiplex procedure, voltage lift 5V/
-18V. A dimmer function is generated using pulse-width
modulation of the grid control signals. At maximum display
brightness, the pulse width for each grid is 2.16 ms. It can be
reduced, controlled using software, which reduces the visual
brightness of the VFD display accordingly.
A digit or symbol is only illuminated if the corresponding
anode and the surrounding grid are switched simultaneously
to 5V for a certain time within a scanning period. The
electrons emitted from the cathode are accelerated by the
positively charged grid and hit the luminous layer of the
anode which is also positively charged.
During the remainder of the scanning period, the
corresponding grid and parts of the anode are at -18V, due to
the internal pull-down resistors in the controller. This
potential is still lower than the average cathode potential of
approx. -15V, prevents the acceleration of electrons, thus
causing the relevant grid and anode segments to go dark.
The heating direct voltage of the display (U = 3.5V) is
supplied from the power supply via lines HELO or HEHI to
pins F+ and F- to the VFD display. Resistors [3070] and
[3071] restrict F- to approx. -15V.
Circuit descriptions and List of abbreviations VR120
9.3 Central Control AIO (AIO part)
The microcontroller (µC) TMP93CT76F [7899-B] includes
the following functions:
• PWM outputs
• A/D converters
• Composite sync input
• Special servo inputs for VCR functions
2
• I C-BUS interface
• Shuttle evaluation
9.3.1 Analogue interface to the C:
The following analogue levels are supplied to the µ C's
internal analogue/digital (A/D) converter:
• TAE/TAS
• TRIV
• TRIA
• AGC
• AFC
• 8SC1/2
voltage
• Key-in
9.3.2 Tape end - LED control :
The LED current is switched using transistor [7804]. The ON
time is approx. 1 msec and the OFF time approx. 12 msec
during playback and 1msec to 5.5msec during the winding
functions.
The LED current is typically 150 mA. In order to prevent
interference from the relatively high pulsed current
'spreading' through the entire unit, the LED is fed from the
14VM1, and filtered by 2 resistors [3800, 3805] with 10R
each and a 220µF electrolytic capacitor [2803].
9. GB 99
Tape End / Tape Start Detection
Tracking Information Video
Tracking Information Audio
Automatic Gain Control
Automatic Frequency Control
Pin 8 Scart1 or Scart2 switching
Keypad evaluation