Grundig TVR 3710 GB Betriebsanleitung Seite 26

Descriptions
If the load continues to increase, that is also the voltage at IC7310-(7),
the overload amplifier of the IC starts to reduce the pulse width of the
T7330 driving voltage at IC7310-(3). This point is the so-called reversal
point. The IC supply voltage V
cc
secondary voltages. This voltage decreases also along with the
increasing load.
With V < V , the IC7310 changes to the polling operation mode
cc dis1
(Burst Mode). The short-circuiting power is low because the interval
between the half-wave starts is large. The pulse width is reduced along
with the decreasing load. The switching frequency increases to the
oscillator frequency of IC7310 which is determined at Pin 10 by C2327.
If the load continues to decrease, IC7310 switches the frequency back
to approx. 30kHz (standby operation) from a certain threshold of the
voltage at Pin 7 (depending on the external circuit connected to
Pins 12 / 16). As a result, the switching losses at the transistor are low.
Overvoltage
At an operating voltage V > 17V at Pin 1 of IC7310 the output stage
cc
is switched off.
Excess Temperature
IC7310 is fitted with an excess-temperature sensor for blocking the
logic if the permissible chip temperature is exceeded (typ. 155
the temperature has fallen a new start-up is possible by re-connecting
the video recorder to the mains.
1.2 Power Chassis – High Voltage Section
The high voltage section is driven by the TV Signal Electronics on the
signal chassis. It contains the TV-IC7200 with the following circuit
stages for generating the drive signals:
– Sync pulse separation
– Horizontal oscillator
– Phase comparison between the horizontal oscillator and the line
flyback pulse
– Vertical oscillator
– Phase comparison between the vertical oscillator and the field
flyback pulse
Horizontal Deflection
The horizontal oscillator within IC7200 feeds out the "HDR" square-
wave signal on pin 37 which is then taken via plug contact 1922-(4) to
the horizontal deflection stage. This horizontal deflection consists of
the driver stage (T7587 / T7584 / transformer 5581), the horizontal
output stage (T7583), the horizontal deflection and the line transformer
(5550 or 5551).
The horizontal deflection is made up of the deflection coils, the forward
scan capacitor "S-correction capacitor" (C2584) and the flyback ca-
pacitors (C2585 / C2586). Current flows through the diode, integrated
in the line transistor, during the first half of scan (the electron beam
moves from the left edge to the middle of the screen), and the transistor
is switched on during the second half period of scan (from the middle
to the right edge of the screen). During the line flyback period, the line
transistor and the diode integrated in it are not conducting. The forward
scan and flyback capacitors are now in series. The resonant frequency
is increased and consequently the flyback speed as well (typ. t = 12µs).
The horizontal linearity (S-correction) is determined by the forward
scan capacitor C2584 and the linearity coil L5510 (optional). Line
tearing, a typical symptom occuring at the cross-over points in a grid
pattern test picture in high beam current condition, is eliminated by
R3587 / C2587 / D6582. Suppression of the ringing behaviour (anti-
ringing) is achieved by series connection of the line transformer and an
optional resonant circuit (L5590 / C2589 / optionally R3589 or R3590).
The horizontal flyback pulse (HFB/SC), 900V typical amplitude, is fed
for phase comparison through the voltage divider R3597 / R3594 /
R3595, T7585 and plug contact 1922-(5) to the signal chassis - TV
Signal Electronics IC7200-(38).
EHT Generation
When the line transistor T7583 is conducting, the line output trans-
former 5550 or 5551 is charged up. In this way, the high voltage for the
picture tube during the non-conducting period of T7583 is produced.
The line output transformer is also used to obtain the necessary
voltages for focussing, brightness, cathode heating, the RGB output
stage and the vertical deflection.
Vertical Deflection
The vertical oscillator within IC7200 feeds out at pin 43 the "VDR" saw-
tooth signal which is taken through plug contact 1922-(1) to the vertical
output stage IC7510-(1/3). The vertical deflection current flows from
2 - 10
behaves in the same way as do the
o C). After
TVR 3710 ..., TVR 5100 ...,TVR 5500 ...
the push-pull output stage in IC7510-(5) through the vertical deflection
coils, the coupling capacitor C2519 and the measuring resistors
R3525 / R3559. For attenuating and suppressing the horizontal deflec-
tion signals resulting from crosstalk from the horizontal to the vertical
deflection coils, C2516 and R3517 are connected in parallel with the
vertical deflection coils.
The picture height is adjustable with the control R3523 (v-amp.) which
is in parallel with the measuring resistors R3525 / R3559. The signal
(VFB) provided at the wiper is fed via plug contact 1922-(2) as a
negative feedback signal to the signal chassis - TV Signal Electronics
IC7200-(41). Adjustment of the correct vertical position is achieved by
applying a DC current to the vertical deflection coils via the preset
R3524 (v-shift). The picture linearity is determined by the setting of the
preset R3522. For this function, the parabolic voltage at C2519 is
integrated by C2520 / R3522 altering the current into a "S-"shaped
form which is used for correction.
Beam Current Limiting
The voltage drop (BCI) at the low-end capacitor C2551 is used for
determining the average beam current. For this function, the BCI
voltage is fed through plug contact 1923-(3) to the TV Signal Electron-
ics (signal chassis). Together with the diode D6200 located there the
BCI voltage is used to reduce the contrast setting when the beam
current is too high, and to control the vertical picture amplitude.
The Safety Circuit
If the picture tube develops a fault condition the safety circuit
(D6550...D6554 / D6556 / T7550) will be activated. In this case, a
LOW level (<1.5V) is present at plug contact 1923-(2), in normal
operating condition the level is HIGH (>3.5V). This control voltage,
called "PROT", is fed to the µC IC7801-(53) (signal chassis - keyboard
control unit) for evaluation. If the µC identifies a fault condition, the µC
uses the "MONI" status signal to stop the drive to the horizontal output
stage and thus switches the TVR to standby mode. For this purpose,
the "MONI" status signal is at LOW level thus applying via T7206 /
T7205 (TV Signal Electronics) a HIGH level to the horizontal output
stage so that the drive to this stage is stopped.
The safety circuit is triggered in the following operating conditions:
– beam current is too high (>1.5mA)
The average beam current is measured as a voltage drop on C2551.
If the beam current exceeds 1mA, the voltage will be negative going.
Form a level of approx. -18V, the diodes D6550...D6552 conduct
and the voltage at plug contact 1923-(2) decreases to <1.5V.
– EHT is too high
The voltage at the secondary winding 10 / 9 of the line output
transformer 5550 or 5551 rises linearly with the increasing EHT and
is used to trigger the safety circuit. The threshold at which T7550
starts to conduct thus causing the voltage at plug contact 1923-(2)
to decrease to <1.5V is mainly determined by the diodes D6553,
D6556, D6554 and R3554. The triggering threshold for the safety
circuit depends on the screen size:
• 14" typ. 29kV
• 20" / 21" typ. 30.5kV
– failure of the vertical deflection stage
A failure of the vertical deflection stage causes IC7510 to feed out
a HIGH level from pin 7 so that T7550 starts to conduct. Conse-
quently, the voltage at plug contact 1923-(2) rises to levels <1.5V so
activating the safety circuit.
The failure is caused by:
• short circuit or interruption in the vertical deflection coils
• short circuit in the coupling capacitor C2519
• voltage at IC7510-(8) <1V resulting from a defective component
in the vertical deflection.
2. Tube PCB
Drive to the CRT Cathodes
On the picture tube panel the RGB signals (RED / GREEN / BLUE)
coming from the signal chassis are inverted and amplified within
IC7900 (TDA6103Q/N2) to correspond to the CD level of the picture
tube. The gain of the red channel is fixed, the gain of the amplifiers for
the green channel and blue channel is adjusted by the presets R3919 /
R3921. As a result, the output amplitude can be adjusted in respect to
the red output amplitude to yield the desired colour temperature at
white level. The CUT-off presets R3917, R3918 and R3920 are
provided to adjust the DC level of the amplifier output signals making
it possible to compensate for the differences in the cut-off points (start
of beam emission) of the individual picture tube guns.
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