PV152
2-2
PRO1200 CIRCUIT DESCRIPTION
CIRCUIT DESCRIPTION
These pulses are rectified with D42 and the resulting
pulsating DC is filtered by C30, divided down with R70/
R35+R38 and applied to the base of Q10. A threshold
level of 4.7 volt is installed at the emitter with Z8. From
5.6 volt base voltage onwards Q10 starts conducting,
turning fully on Q18. In the LED of the opto-coupler
there is small current flow from the +300M through R9.
When Q18 gets forward biased, this current increases
heavily and the output pin 5 drops. Via the forward
biased Z5, the emitter of Q8 is lowered and Q9 is turned
on introducing the EHT HOLD DOWN .
Note that the same opto-coupler and transistor Q15 are
used to remove the influence of the hum on the +300M
supply via the feedback voltage. The +300M is, via a
filter C24+R51/R9, supplying current to the opto-cou-
pler (pin 1 of IC5). The voltage, containing this Main
hum, is taken from the collector of Q15 and capaci-
tively added to the FBHV input .
3. EHT Hold Down in the event of an excessive amount
of CRT beam currents
(Page 6-5 Sect. B-7)
Finally, in the event of an excessive amount of CRT beam
current, the negative HVL voltage can charge up C39 to
the -0.6V threshold and Q9 conducts to introduce the
EHT HOLD DOWN condition. The delay is determined by
the time constant R102/C39 and the amplitude of the
HVL voltage.
Note that a beam current proportional voltage (BCL) is
also sent to the RGB-Decoder drive board to reduce the
contrast and brightness from some level onwards.
As a summary, the EHT Hold Down is active for:
- too high EHT, information coming from the feedback
line FBHV
- too high EHT in “open loop” via Q10/Q18 and the
opto-coupler IC3.
- too high beam current lasting for some time (short in
a crt).
b) Switching off the EHT when a Horizontal or Verti-
cal scan failure occurs.
(Page 6-5 Sect. F-6)
In the event of a horizontal and/or vertical scan fail-
ure, the SF line is dropped to around zero volt. Q16 is
blocked turning on Q6 on the subunit. D3 becomes for-
ward biased and pulls pin 6 of the multivibrator at
ground level, stopping the EHT.
When the scan fail line goes high again prior to switch-
ing off the projector, C45 has to charge up first. This
prevent oscillations and further delays the action of
the slow start circuit (Q11-Q13).
c) Overcurrent protection of the Q7 switcher.
The drain-source current of Q7 is measured by the resis-
tor R21 in the source. This voltage developed across
the sensing resistor is applied to the base of Q14 via a
divider R40/R15.
The purpose of the circuit is to stop temporarily the
EHT when the beam current tends to go beyond a maxi-
mum to avoid damage of the phosphor coating of the
CRTs.
However, the pin 4 output of the opto-coupler may only
affect the EHT multivibrator ‘s duty cycle at the mo-
ment there is a positive drive pulse on the common
bases of the push-pull stage.
Therefore, when the drain voltage of Q6 is high, D5 is
reverse biased and Q14 can work. When on the other
hand the common bases are low, Q14 is inhibited as the
current through R69 is flowing away through D5 and Q6
to ground.
Slow start up of the EHT
(Page 6-5 Sect. F-4)
When the projector is switched on, the EHT voltage must
gradually be built up to prevent mechanical damage of
the gun.
The reference voltage grows slowly because of the delay
circuit around Q11.
The main delay at start-up is obtained by the circuit on
the subunit. As soon the feedback voltage would like
to rise beyond the reference voltage, Q6, gets forward
biased, drives Q7 on and the latter drives Q10 into satu-
ration. Thereby, D222 goes into conduction and the
multivibrator is stopped. The feedback voltage drops
and Q6 turns off. This phenomenon is repeated all the
time at start up. Consequently, the EHT rises to its maxi-
mum in steps.
When the unit is switched off, C40 is quickly discharged
via D9, taking the EHT reference voltage for the zener
IC4 quickly down, and therefore the EHT itself goes down.
The discharging of C40 turns quickly on Q11 because
its base is pulled in a negative direction.
Delay of the power switch drive
Fet Q30 is used to keep a drive pulse from driving Q7,
until the EHT pulse on the Drain of Q7 has not fully
dropped to its minimum. The EHT pulse is coupled to
the gate of Q30 and keeps the drain of Q6 at a low level
as long it is saturated. This prevents driving the power
switch Q7 the moment the drain has not yet dropped to
a minimum.
TECHNICAL DESCRIPTION SECOND RGB INPUT
Red, Green and Blue Inputs
(Page 6-46 Sect. A-2)
Three identical differential amplifiers are switched to
“active” with the ON voltage. This ON voltage is ob-
tained from the +PORT 3 voltage arriving at contact 28
