Omron 8025G Maintenance Manual page 76

THEORY OF OPERATION
SECTION 4
which is coupled through the focus control, R167, to the focusing grid (C4) of the
CRT (VI). Network CRl12, CRl13, C123, and C124 forms a voltage doubler that de-
livers approximately 1000 V dc to divider R163, R170. This divider provides approxi-
mately 800 V dc for the first anode (G2) of the CRT. The flyback pulse is also
transformer-coupled to the secondary of Tl03, and the stepped-up pulse is rectified
by CRlOO and CRl15 to produce approximately 17 kV dc and -100 V dc, respectively.
The 17 kV dc is the anode voltage for the CRT; the -100 V dc serves as the source
voltage for the brightness control.
Returning to the delay circuit (Ql09-Qlll), note that a separate and lower
supply voltage is used. A series dropping resistor, R15l, reduces the +55 V dc
supply voltage to approximately 25 V dc. This arrangement serves two purposes: (1)
it guards against the production of X-rays during an over-voltage condition, and
(2) it prevents triggering of Q123 by random drive pulses when the terminal is turned
on or off.
The circuitry that guards against X-ray production consists of SCR Q123,
zener diode VRl02, and associated components. Assume that the +55 V dc regulator
circuit fails and the output voltage exceeds approximately 60 V dc. The voltage
drop across divider network R147, R148, and R149 will also increase, causing current
to flow through VR102 and R150. The voltage developed across Rl50 then causes SCR
Ql13 to fire and to discharge Cl18. This effect drops the entire supply voltage
across R15l and disables Ql09-Qlll. Without drive, the horizontal output stage
and high-voltage supply are disabled also.
A separate, lower supply line also protects against triggering of the hori-
zontal output transistor (Q123) by random drive pulses during turn-on or turn-off.
Normally, several ac cycles are required after turn-on to bring the +55 V dc bus
up
to level. By virtue of the component values selected for Ql09, Qlll, and R15l, the
delay one-shot will not trigger until the regulator voltage exceeds approximately
+30 V dc. This dc voltage is adequate to provide stable operation of the horizon-
tal drive one-shot and to supply adequate base drive to Q123. In this way, random
drive pulses and' poor collector saturation of Q123 are avoided.
During turn-off this separate supply line also offers some degree of pro-
tection against CRT spot burn. After power is turned off, Cl13 is rapidly discharg-
ed by the load current. The +55 V dc regulator output consequently decays rapidly
to 30 V de and Ql09, Qlll will fail to trigger below this level. As a result, Ql23
and the high-voltage circuit are disabled, reducing the discharge current from C113
to approximately one-third its former rate. The energy thus retained by Cl13 is
used mainly by the vertical deflection circuit for a significantly longer discharge
period. CRT beam energy is consequently distributed along the vertical axis to pre-
vent spot burn
w~ile
the high voltage stored in the CRT aquadag is discharged.
The low voltage regulator circuit consists of Ql05-Q108, Q121, and re-
lated circuitry.
The +80 V dc at Pl02-l is dropped to +55 V dc by series regulator Q12l,
whose output is sampled by voltage divider R133, R134, and R13S. Approximately +7 V
dc, tapped from R134, is applied to the base of Ql08, while a reference voltage de-
veloped across VRIOI is applied to the emitter of Ql08.
Th~s
transistor develops
an error current that flows through Rl30 to the base of QI06 and the collector of
QI08 by QlOS, used as a dc current generator. 'The bias current tends to shift the
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