Fault Insertion: Remove the shorting shunt from pin 1 & 2 and place it between 2 &
3 of jumper J39
Symptoms: Snow added picture is observed on the screen Fault Section: AV Section
Procedure:
• Select the Trainer in AV mode.
• Check the voltage of IC901 at pin no. 4. It should be 1.8V approximately If the present voltage observed is 5V approximately
• Proper voltage 0.650V approximately should be present on pin no. 5 & pin no.
21 of IC101. If not,
• Diode D702/703 may be faulty. If not,
• Check the continuity between D704 & IC901 (Pin no. 4). If continuity is not present then,
• Remove the shorting shunt from 2 & 3 and connect it between 1 & 2 of jumper J39.
Result: Plain, raster & mute sound is obtained. in AV mode
Experiment 21 Objective:
Study of detail circuit description of System Control Section
Scientech colour TV Trainer ST2651 uses IC901 FONDA4ST for system control purpose. It is dual in line package 40 pin IC which controls all the sections of the TV Trainer. Pin no. 1, 2 and 3 are band switching pins. 1st pin is used for switching the VL band. Second one is used to switch VH band and third pin is for UHF band Pin no. 4 provides TV/AV switching signals on pressing the AV switch from remote control, the RF signals coming from antenna or cable are disconnected with VIF section. Hence there is only raster on the screen without snow. Pin no. 5, 6, 7, 21, and 22 are the key matrix pins, which are connected with keyboard. Keyboard is used for selecting one function from front panel. Pins 8 & 9 are the on screen display oscillator pins. Horizontal and vertical sweep frequencies are obtained in these pins with the help of ON screen display oscillator. These frequencies (signals) play an important role for getting ON screen display (OSD) on the screen. The coil connected on pin no.
8 & 9 decides the frequency of Internal Oscillator. These horizontal and vertical sweep frequencies are synchronized with horizontal and vertical blanking pulses. Pin no. 11 & 12 are memory input and output pins. Pin no. 11 is connected with pin no. 5 of memory IC902 and pin no. 12 is connected with pin no. 6 of this memory IC.
Whenever we store any channel or put it into memory then these memory signals are stored in memory IC with the help of these pins. Therefore whenever the stored channel is required, memory IC feeds it to system control IC.
Pin no. 13 is horizontal blanking input pin. Pin no. 14 is vertical blanking input pin.
Pin no. 15 is AFT (Automatic Fine Tuning) input pin. IC101 (7680) pin no. 13 provides AFT voltage to pin no. 15 of system control IC through buffer transistor Q910A (BC547) .This AFT signal is important while channel locking. Pin no. 24 is reset input pin. It is an active low pin for resetting a transistor Q910 (BC558B) is used Pin no. 25 & 26 are VCO (Voltage Control Oscillator) pins. Here a 4 MHz crystal is connected.
A particular value of frequency is created here with the help of 4 MHz crystal of voltage controlled oscillators inside the IC This signal of particular frequency is converted in to function operating signal inside the IC and comes out from IC by key signal output pins.
Pin no. 30 is output pin of green OSD signals. The signals from this pin are fed to base of green OSD driver transistor Q906 (BC547B) and then to the base of green OSD output transistor Q1 (2SC1815). The output of this transistor is fed to the emitter of (G-drive) green colour output transistor Q508 (C2310).
Pin no. 31 is output pin of red OSD signal. These red OSD signals are fed to the base of red OSD driver transistor Q905 (BC547B). After amplification these signals are fed to red OSD output transistor Q2 (2SC1815) from emitter of previous transistor.
The amplified signals are obtained at collector of Q2 and are given to the emitter of red output transistor Q507 (C2310). Pin no. 33 is tuning pulse output pin the signal from this pin switches ON the tuning pulse switching transistor to get the 0-33V at TV terminal.
Pin no. 34 is Infrared remote input pin ON pressing the remote switches. The signals are obtained first at this pin for any function from remote switch; remote provides infra red waves, which falls on sensor EYE on front panel. This eye recognizes the signal and amplifies it and gives to pin no. 34 of this IC. So the signals work inside the IC for which remote key was pressed. Pin no. 36 is colour control pin. The colour control signals are fed to pin 7 of IC501 (7698). Pin no. 37 is contrast control pin. The contrast control signals are fed to pin 41 of IC7698. Pin no. 38 is brightness control pin. The signals from this pin are fed to pin no. 4 of IC7698.
Pin no. 39 is volume control pin. The volume control signals from this pin are supplied at the base of transistor Q901 (BC547 - Volume Control Transistor). The signals from the collector of this transistor are given to the signal input pin no. 1 of IC7680. Pin no. 40 is +5V supply pin. Output of IC102 (L7805) is directly connected on this pin. Pin 16, 19, 20 are ground pins. Pin no. 17, 18, 23 are Bias pins. The Biasing is provided from +5V supply. Pin no. 32 & 35 are no connection pins.
Experiment 22 Objective:
Study of system control section through following Test Points TP24 Blue + 18V (approximately)
TP25 Red + 5V (approximately)
TP42 Red System Control Section 9V approximately from system control IC to select VHF I band (VL) otherwise 0V.
TP43 Red System control Section 0V when VHF III (VH) band selected otherwise 9V approximately
TP44 Red System control Section 0V when UHF band selected otherwise 9V approximately
TP45 Blue System Control Section Horizontal blanking pulse input for system control IC.
Figure 23 TP46 Blue System Control Section Vertical blanking pulse.
Input for system control IC
Figure 24
TP47 Blue System Control Section Input for AFT 3.2 VDC (approximately)
TP48 Blue System Control Section Ident signal 0.2-5 VDC (approximately)
TP49 Red System Control Section Input pin for standby
Mode +5V when power is ON otherwise 0V
TP50 Red System Control Section Output of tuning signal 5Vpp approximately
TP51 Blue System Control Section +5V approximately Infrared signal from
Remote
TP52 Red System Control Section Colour control pin 0.2-8V approximately
(Middle position of colour control)
Figure 25
TP53 Red System Control Section Contrast control pin 6-10V approximately
TP54 Red System Control Section Brightness control pin 0.4 - 9V approximately
(Middle position of Brightness control)
Figure 27
TP55 Red System Control Section Volume control pin 0.2 - 4V approximately
(Middle position of Volume control)
Figure 28
Experiment 23 Objective:
Study of fault simulation and step by step fault finding procedure of system control section