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ST 03
Circuit Description
1. Power Supply
The power supply is a conventional off line, isolated switch mode
system. AC line voltage is applied to the circuit via switch fuse and the
main input RFI filter. The degauss coil supply is taken off after the filter
then the AC is rectified through a bridge circuit comprising D100 to
D103. After filtering by C103 and C104 the DC is now fed to T100
switch mode transformer. TR100 is the switching transistor and it, in
turn, is driven by the UC3844 IC100 which is a PWM (Pulse Width
Modulation) control IC. In the basic chassis, the UC3844 is run as a
fixed frequency PWM controller, the frequency being governed by
R105 and C107.
1.1 HT Adjustment
VR100 is used to set the +B, this varies the output voltage by controlling
the feedback voltage to the IC100. The +B rail is the line measured
when this control is used. This line is set in accordance with the CRT
that is fitted. When this rail is set then any variation in power demand
by the receiver will be reflected in the feedback winding which, in turn,
will appear as an error voltage on IC100-(2). This error voltage will, in
turn, vary the PWM output to the switching transistor which, in turn, will
maintain the output voltage constant.
1.2 Over Current Protection
The power supply is totally overcurrent protected by sensing the
current through the switching transistor. The combination of R114,
R113, D112, D113, R112 and C113 all act to provide conditioning of
the sensing voltage for the IC100 current sense input. As current flows
across TR100 collector-to-emitter junction (source drain for stereo) a
voltage proportional to the current is produced across R114 and R113.
This voltage is fed to Pin 3 current sense pin of IC100. When the
predetermined threshold voltage is reached because of excess current
drawn IC100 limits the current to protect the PSU and other circuits.
After a period of time it will try to restart, if the fault has not cleared then
it will shut down again. D112 and D113 are there to prevent the voltage
developed rising to an unacceptable level on the sense pin of the IC.
R112 and C113 provide a degree of filtering of the sense waveform.
T100 also provides the isolation from the AC line to the chassis of the
TV. All secondary supplies are isolated so that the main - and external
connections are at ground potential, therefore, safe to the user.
Warning! The heat sink on TR100 is live.
1.3 Supply Regulation
The 5V supply for some of the main ICs of the system is supplied from
the secondary winding of T100, the 9V output is fed into a fixed 5V
voltage regulator. The 33V tuner tuning volts is derived from the +B line
using a band gap close tolerance shunt regulator D120.
R115 and C114 are isolation components linking the non-isolated to
the isolated side of the chassis for RFI and electrostatic purposes.
2. Tuner and Demodulator Circuits
The tuner TUN600 can be one of two models depending on what
transmission system is being viewed. For the U.K. using PAL System I
the tuner is a Salcomp 1590R. For all other European PAL and SECAM
systems the tuner is a Salcomp 1490R. Both tuners are pin for pin
compatible, the only difference is that the 1590R is UHF only and the
1490R is a multi band tuner. Both tuners are voltage synthesis types
and tuning voltage is controlled by the system microcontroller IC500.
AGC control of the tuner is determined by the demodulator IC400.
2.1 SAW Filter Options
The IF output of both types of tuner is a balanced output and is fed into
a number of balanced input saws - SAW450, SAW600 and SAW601.
As to which or how many saws are fitted is determined by the options
fitted and the system to be received. In the stereo chassis SAW 600 is
fitted.
However, this can be a different type of SAW depending on the system
being received. A system table is presented on the circuit diagram
giving the specific type of SAW to be fitted in the 601 position for each
system.
2.1.1 SECAM L/L (option)
If the chassis is fitted with SECAM L/L the addition of SAW450 is
required for the sound.
GRUNDIG Service
2.1.2 Stereo SAW
Should the full stereo option be required then SAW601 is replaced with
SAW600 and SAW450 stays as it is if SECAM L/L is also required.
2.2 Vision IF Demodulator
Taking the basic case the outputs from whichever SAW600 or 60 is fed
to the IF input of the demodulator IC400-(19), -(20) which is an
STV8224. Under most applications there is only one adjustable
component in this circuit and that is L401 tank coil. However, when
SECAM L/L option is required an additional trim capacitor is required
along with R416, C412, D532, R417 and R425. The incoming signal is
demodulated in the STV8224. The demodulated video and intercarrier
sound are then fed out on pin 3 STV8224. The combined signals are
fed into the filter circuits for separation of sound and video. The
separation is achieved by the use of ceramic filters. CF400 and CF401
are the filters that perform this task.
2.3 Dual System Sound Options
It will be noted that there are two ceramic filters, CF400 and CF401.
Both sets are required if dual system operation is required. Details are
to be found in the system table on the diagram for each option.
The separated signals are then fed back into the IC400 where the
sound is demodulated and the audio processed. The video and audio
are then routed by the internal switching of IC400. The routing is
controlled by the microcontroller. The external video and audio from
the Peri connector are also fed into this IC.
2.4 Tuner AGC Control
The STV8224 also performs AGC control for the tuner. The operational
threshold for AGC action is controlled by the microcontroller, the
threshold level is set up on IC400-(24).
2.5 SECAM L/L' Vision IF (option)
In the event of SECAM L/L' option being required, then additional
IC450 needs to be fitted as does SAW450. This is in order to
demodulate the AM sound of SECAM L/L system. Audio switching is
then re-routed between IC450 and IC400, this is all done under the
system microcontroller software in IC500.
The switching network comprising TR450, TR451, TR452, D450 and
D451 is for the purpose of switching the SAW transducer with variation
in band switching. Because the sound and vision carriers are trans-
posed between Band 1 and Bands III, IV and V, then the balanced IF
output has to be switched to the right transducer on SAW450. The
switching is again controlled by the system microcontroller IC500. The
transducer that is not being used is short circuited by its respective
diode.
3. Peritel Output
The video out to the Peri connector is taken off before the filtered signal
is fed back into IC400. It is then fed to a buffer circuit TR700 and then
to pin 19 of the Peri connector.
4. Video Chroma Processing
IC800 is the video and chroma processing circuit. All controls are totally
under software control. Customer controls can be adjusted from the
front panel. The raster correction controls are only accessible in factory
set-up and troubleshooting mode set by the system microcontroller.
The customer controls are accessed in this mode, or by the conven-
tional remote control.
The composite video is fed into IC800-(2). Separation of luma and
chroma signals is carried out internally by the sampled data method
using clocks generated by the external crystal. This method is used for
all decoding processes in the chip. The sample clock is phase locked
to the subcarrier for correct colour ident.
5. Delay Line
The sample component chroma signals are now fed to a digital,
1 picture line length delay line. The same sample clock is fed to the
delay line IC801 as is fed to the IC800 so that phase relationship is
maintained within the whole system.
5.1 RGB Matrix and Sync Separator
After passing through the delay line it is returned to IC500-(26), -(27)
and then mixed with the luma and fed into the matrix circuits to produce
the RGB output. The luma signal is also fed through a short delay line
Circuit Description
2 - 1
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