Tandy CM-8 User Manual

6. High Voltage Shutdown System
The shutdown circuit prevents the high voltage from
rising above a preset level.
Under normal operating conditions, this circuit is in-
active.
Operation of the protector circuit depends upon a heat -
er
pulse which appears at pin@ of the horizontal out -
put transformer (T602) . It monitors a heater pulse
rectified by 0603 . If the incoming high voltage increa-
R615
0603 0605
ses and exceeds its limit , the heater pulse voltage also
increases. As a result, there is a larger voltage
produced to charge C617 so that its potential will
eventually be higher than the knee voltage ( + 22V) of
the Zener diode (0605) turning it ON. With D605
turned ON, the X -r ay protector (of IC601) operates to
stop the horizontal oscillator circuit, shutting down the
resultant high voltage.
R618
FROM PIN@
OF T602
~~-91~--41>----41>-----4--~-l\.IV'-~---~----'VVI
IC601
(
HEATER )
PULSE
R619
+
+
C617
C607
Figure 6. High Voltage Shutdown System Cirucit
7. Power Supply (Figs. 7-9)
The power supply circuit is a blocking oscillator type
switching power circuit and substantially consists of
the rectifier / smoother , blocking oscillator, control, and
output rectifier / smooth er circui t s.
The AC input voltage is full-wav e rectified by the rec -
tifier / smoother circuit and then placed on the smooth-
ing capacitor as a DC voltage.
The DC voltage is appli ed to the bloc ki ng oscillator
circuit .
The blocking oscillator circuit operates at an oscilla -
t i on frequency and duty ratio that depends upon the
action of the control circuit.
St abilization of the ou tput voltage is accomplished by
changing the conducting period of the ou tput tr ansis-
tor used in the blocking oscillato r circuit.
Operational Description
When the power is turned on , a small current flows
to the base of the output trans i stor (0 702 ) via the
startup resistor ( R712 ). As a result , the collecto r cur-
rent flows through the pr i mary windings@ and Q)
of the converter transformer , whi ch produ ces an elec-
tromotive force between those windings i rrespective
of the magnitude of the collector current, resulting in
a
voltage being indu ced between the driving w indings
@ and @. The induced voltage is positively feed-
back to the base of the output transistor (0 702 ) to in-
crease the base current of this transistor , resulting in
a
further increase in the collector current. The above
operation occurs instantaneously to impress sufficient
base current on 0 702 , keeping it on. The collector cur -
rent of 0 702 , determ i ned by the equation i = V /L ·t ,
increases rectilinearly with time.
9
While the control circuit is at rest , the collector cur -
rent increases with time , and the moment it reaches
h FE times that of the base current, 0 702 turns off.
. The control circuit always applies to the error amplifi -
er circuit a voltage induced in the detecting winding
situated at the primary side of the converter trans -
former .
This charges the capacitor (C7 14 ) while 0702 is off .
Also the capacito r (C708) is cha rged with a reverse vol-
ta ge produced between the driving windings @ and
@ while 0 702 is off . This voltage is a positive con-
stant voltage .
If the voltage induced in the detecting winding be-
comes greater than a specified value, it also exceeds
the voltage of th e Zener diode in the error amplifier,
resulting in a voltage being applied to the emitter of
0701 . This causes the base current of 0701 to flow ,
turning it on .
The flow of the collector curren t of 0701 produces a
voltage in the resistor connected between the collec -
tor of 0701 and the ground . When this voltage reaches
the gate trigger voltage , the thyristor whose gate is
connected to the above resistor turns on .
Th e instant the thyristor turns on , a current flows
through R710 , the thyristor , the emitter and base of
0 702 ; in this sequence , the charging voltage of C 708
is used as a power source , causing the base current .
of 0 702 to stop and thus 0 702 to turn off.
When Q 702 turns off, the energy accumulated be-
tween the primary windings@ and G) while 0702
is on is transmitted to the secondary winding and then
fed to the load via the rectifier circuit (D 71 0, C 711).

Tandy CM-8 Specifications

General

Brand	Tandy
Model	CM-8
Category	Monitor
Language	English