Hitachi I-28 Service Manual page 4

DESCRIPTION OF CIRCUIT
the preheat power and the starting time are shown in
Fig. 2.
When inserting the power source socket in a receptacle,
Preheat power is supplied to the heater through R901,
When endeavoring to -receive television images, if
switches $901 and 902 (interlocked) are thrown ON,
the heater is connected to the regulator output, rated
heater power (12V, 75mA) is supplied, and the normal Fig. 1
voice and picture are obtained in the short time of
within one second. Power consumption during preheat-
ing time is approximately 2W. It has been confirmed
that there is absolutely no bad influence on the picture
tube durability even though it is preheated continuous-
REGULATER
OUT PUT.
PRE HEAT
ly while the TV set is not in use. (Durability test b
completed). & =---V4W PRE HEAT -NO PRE HEAT
3 500
@ Constant voltage power source circuit 8
Ordinarily, composition of the constant voltage sta- 3 '/3W PRE HEAT
bilized power source circuit is as shown in Fig. 3.
(M Ib) 0
W:HEAT POWER
Should output voltage (Vo) fluctuate due to one cause 9 - 20 a
or another, the fluctuation of that output voltage is 5
ig.
compared with the standard voltage through an error
detection amplifier, it is amplified, and an error signal
is transmitted to the control section. In the control
section, according to an error signal, the output voltage
(Vo) is controlled so that it becomes a previously set
constant voltage. Through the foregoing operation, the
output voltage is always maintained constant regardless INPUT
of fluctuations in load and input voltage.
The constant voltage circuit of this television set is
shown in Fig. 4.
The control section is composed of constant voltage
control transistor TR901 and IC6 which has dual
capacity of error detection and amplification. R903 is
a variable resistor for adjusting +B voitage. The input
voltage (Vi) of this circuit is a pulsating voltage which
is rectified and smoothed by the rectifier diodes
(CR901~CR904) and the filter capacitor (C901), and
the voltage is stabilized in this circuit to constant
voltage of complete DC.
In general, when the input voltage (Vi) lowers under a
-certain voltage value, the constant voltage circuit
cannot perform constant voltage operation. In such a
case, to eliminate fluctuations of load and input
voltage from arising in the output voltage (Vo), an
active filter function is added to this television set.
Through an inner circuit consisting of R902, C902, and Fig. 4
IC6, an active filter circuit is composed. Consequently,
this televison set can perform stabilized operation for
lowered AC power source voltage and lowered battery
terminal voltage. The terminal @ of 1C6 is connected
to one end of P901, changing the active filter circuit
operation to either the AC side or the DC side.
CONTROL
SEC.
Fig. 3
R905
ACTIVE FILTER