Uhf Modulator - Pace MSS100 Series Service Manual

Output to the Modulator and Optional AV sockets
The video at the emitter of Q601 (labelled GRAPHICS_OUT in the
circuit diagram) also passes via a filter comprising C202, C213, C214
and R216 to the UHF modulator. Resistors R214 and R215 attenuate
the video to 500 mV pk to pk.
Note that because this video to the modulator is fed off from a point
by the TV SCART, it contains on-screen graphics which will be recorded
if a VCR is connected to the receiver by a UHF lead. (Video to the VCR
SCART does not contain on-screen graphics.)
RGB Disable Switch
0 Vthe TV is in PAL mode, when pin 16 of the TV SCART is at 5 V the
TV is in RGB mode. Pin 16 of the decoder SCART is disabled by the
microprocessor U700 switching on Q602. This shorts the base of Q603
to ground which in turn switches pin 16 of the TV SCART to ground
therefore disabling the TV's RGB function. RGB disable is performed
when in menus, because the menus are PAL only.
UHF MODULATOR
The RF output to the TV is generated by a frequency synthesisied UHF
modulator. "his allows the RF output to be precisely tuned to any
channel with: the UHF bands IV and V (channels 21 to 69).
The circuitry :onsists of two main parts, a loop-through amplifier
(U201) betwean the aerial input and the output to the TV/VCR, and
the UHF mos .:lator itself, which comprises U200 and associated
components.
The heart ot
xe modulator is the IC U200, a block diagram of which is
shown on ps. 18. It comprises the following:
* a pha: locked loop frequency synthesiser that covers the full
UHF Sand;
® anar stitude modulator to modulate the UHF carrier with the
video:
® an au:o oscillator for the sound subcarrier;
¢ avide> clamp to ensure the correct modulation index.
Tuning is er anied by a DC tuning voltage in the range 0 to 24 V being
applied to tne varicap diode D200.
The tuning voltage is derived from pulses on pin 7 of U200. They are
integrated by Q200, R207, R206, C208 and C209 and associated
circuitry and eventually fed to the cathode of D200.
The audio subcarrier oscillator circuit is connected across pins 2 and 3
of U200. This is set at the audio subcarrier frequency (6 MHz for PAL |;
5.5 MHz for PAL G). The audio signals from pins 6 and 10 of the audio
processor U500 are combined by R523 and R524, AC coupled by C200,
pre-emphasised by C201 and R200 and then input at pin 1 of U200.
The audio level at this point should be approximately 500 mV pk to pk
at full volume. Note that the outputs at pins 6 and 10 of U500 are also
fed to the TV SCART, so that the modulator always has the same audio
source as the TV SCART.
The video signal (labelled GRAPHICS_OUT in the circuit diagram)
passes via L202, which, together with C213 and C214, forms a low pass
filter. Resistor R214 and R215 attenuate the video to 500 mV pk to pk.
The video signal is fed into pin 19 of U200. This is used to amplitude
modulate the VCO output. The modulation index is set to be 80% by
resistor R213 on pin 17 of U200.
The RF output from pin 15 of U200 is fed via C217 and R217 into U201
an RF loopthrough amplifier chip which mixes the signal with the
terrestrial RF channels. When the satellite receiver is in standby the
UHF oscillator is switched off by U200 under control of the microproc-
essor U700 via the I?C bus.
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Circuit Description