Hitachi CMT2195 Service Manual page 4

CMT2195
Horizontal Stage:
The composite video signal from pin 7 of I(201 is returned to
pin 13 via (302 as explained previously. This input is applied
to the internal sync, separator stages of the IC.
A internal phase detector stage is provided with a sawtooth
waveform, generated from the line pulse input to pin 38. The
phase detector will then compare this sawtooth waveform to the
sync. pulse. Any frequency drift will cause a corrective output
to be applied to the horizontal oscillator, thereby maintaining
the desired phase relationship.
The components connected to pin 40 form a filter network for
the phase detector, and VR701/connected to pin 39 provides
manual phase control. The horizontal output emerges at pin 37
and is then applied to the base of line drive transistor Q721,
1721 couples the output of Q721 to the line output transistor
Q781. Both these transistors are powered by the 95V supply.
A line pulse available at pin 6 of the flyback trasformer is
rectified by D751, smoothed by (756 and provides approximately
170V to drive the output transistors Q851, Q852, Q853,
Under certain fault conditions, i.e. increased H.T. supply,
low line osci!lator frequency, or reduced value of the tuning
capacitor C781, an excess of £.H.T. could be developed. To
prevent this happening, the rectified voltage of D751 is fed
via potential divider R757, R758, and applied to 20751. Should
the £.H.T. rise excessively, the threshold of the zener will
be exceeded, and a voltage will be applied to pin 35 of IC1101
via R1106, thereby shutt:ng down the power circuit, |
This effectively applies a "Low" to Q954 base, turning the
transistor off. Consequently, Q953 will be turned
off, and
the +8V supply to IC201 is then removed, thereby shutting
down the deflection stages of the IC, preventing further E.
H. 7. generation.
Excessive beam current can also occur under certain fault
conditions, so this is prevented in the following manner.
The H.7. current to the horizontal output stages is measured
by R781.
Should the current rise, the voltage drop across R781 wil!
increase, and a voltage will be applied to the gate of Q955.
This will then prevent further £.H.T. generation as descr ibed
earller.
A supply of +25V is required for 1(681. This is obtained from
pin 1 of the flyback transformer, and smoothed by C7954.
Vertical Stages :
The internal vertical sync, of 1C201 is fed toa triggered
vertical divider stage, which counts down the horizontal
frequency to obtain the vertical frequency, thereby eliminating
the need for a conventional oscillator circuit.
This also has the advantage that no external frequency control
is required,
C601 at pin 42 of the IC is used for ramp generation, and
produces the required sawtooth output.
The vertical output from pin 43 of {C201 is applied to pin
of (C681 via R604. The components D601 and C605 determine the
flyback generation time, and the vertical output to drive the
deflection coils is made available from pin 2.
The deflection current that occurs at the junction of R609,
is added to the feedback from R607/C608 etc. and the result
is applied to pin 4] of 1C201. The values of R607 and (608
determine the linearity, whilst VR601 sets the vertical height
Power Supply Circuit:
(1) Starting operation
e Power switch $901 turned ON — Rectified at
D901~D904 — Voltage at Q903 basevrises — First
Switching pluse generated at winding (P1—P2) of
T901 ~ Drive voltage -» Provide to Q905 of winding
(B1-B2) of T901— Q905 supply stable drive current
to Q903 base — come into normal operation.
(2) Switching constant voltage operation
¢ AC input voltage rises or +B load decreased (picture
gets dark) ~ +B (95V) rises — Ic increased at
Q951 — Id increased at IC901 — Ic increased at
IC901 -» Voltage decreased at pin G)of C901 +
ON period of Q901 increased — ON period of Q903
decreased -» Positive voltage of D951 anode
decreased — +B voltage falls -+> +B voltage stabilized.
(3) Standby operation
e Remote contro! power OFF — IC1101 @) and
% — Q954 OFF - Q953 OFF - IC201 H.
Start Pin 66) stopped — Horizontal deflection stopped.
* Also ~ Q952 OFF — R965 connect into R964 —
Voltage of Q951 base rises — ON period of Q903
is slightly decreased by IC901 -» +B voltage falls
(about 62V) — Power consumption decreased.
+5V power provided from T901 (S3-S4) winding
through D952 and C954 — 5V at pin @? of 1C1101
Stabilized by ZD1101,
Remote Control and Tuning Circuitry: :
This IC type M37210M4, performs channe! selection. UP/DOWN
analogue control, an screen display, search tuning, and
controls inputs and search tuning, and controls inputs and
outputs to and from the AV terminal.
101102 is the memory IC, which stores the data relating to the
above functions, then transfers that information to IC1101 when
required, Both these ICs are powered by the +5V supply.
X1101 supplies IC1101 with a basic clock frequency which
controls all operating mode requirements.
Hhen the TV is first switched on, IC1101 must be initially
reset, and this is achieved by 101101 stage. As the +5V
supply begins to rise from switch on, pin 3 of Q1105 is held
"Low". This is applied to pin 30 of IC1I01 thus resetting the
IC. Once pin 1 of Q1IO5 has almost reached its +5V potential,
the "Low" is removed from pin 3 thus releasing the reset
condition.
When the search routine has been initiated and a signal has
been located, pin 14 of IC201 will become "High". This is
applied to pin 34 of {C1101, and informs the IC that a signal
is present. The search routine then stops, and the IC will
monitor the AFC signal present at pin 15 to obtain the optimum
Signal,
Pins 46 and 47 contro! the signal system,
Contrast, colour, brightness, sharpness, tint, and volume are '
all controlled from the remote contro! handset (the volume can
also be adjusted by + and — buttons on the front of the TV),
and will produce 0C level changes from pins 3~8 of 101101,
which are then fed to the relevant pins of IC201.
Pins 31~32, 37~39, and 11 from the in and out matrix are
for the front control operations.
Pins 12 and 13 are the clock and data output pins. These
Signals are supplied to the memory |C1102,
| crzies
When the entered time has elapsed, the "Low" outputs from pins
21 and 22 of |C1101 are removed, and thé TV wil! return to
normal operation,