Date: 10/02/98 R762716
EHT Module
R762716
12
Technical description
Protections
a) EHT Hold down:
The EHT of the projector must be switched off in the event of a failure in the regulating
circuit or an absence of the feedback voltage. Moreover, when the current in one
or more tubes is excessively high (leaking of a tube), the projector must be switched
off via the EHT Hold Down rather than via the main switch. As the spot suppression
does not work, the CRTs could be damaged when switched off via the main switch.
1. EHT Hold Down due to fault in the regulating circuit (FBHV too high)
The slider voltage of P2 (hold down adjust potentiometer) is sent to the base
of Q5 and its emitter is set at a threshold of 5.6 volt by Z3. As soon the EHT
rises beyond 36.5kV, transistor Q5 starts conducting, turning on Q6. The
collector of Q6 starts to drive the base of Q7 and this in turn stops the EHT
multivibrator from oscillating, therefore halting the EHT. The collector of Q6 is
placing the base of Q5 high hence the combination is locked down until the
projector is powered off to reset the circuit.
2. EHT Hold Down due to a feedback loop fault condition
The EHT hold down protection must also operate when there is an open loop,
or no EHT feedback vFBHV from the splitter. If that were the condition,
there would be no way to monitor the EHT, and it could go higher than the
36.5kV, since there is no FBHV voltage available. Another reason is that the
maximum voltage on the MOSFET switcher is limited to 1150V as well. The
detection for open loop is built around the EHT flyback pulses taken from the
drain of Q12.
These pulses are rectified with D17 and the resulting pulsating DC is filtered
by C39, divided down with R53/R60+R75 and applied to the base of Q16. A
threshold level of 6.2 volt is installed at the emitter with Z8. From 6.8 volt base
voltage onwards Q16 starts conducting, turning fully on Q17. In the LED of the
opto-coupler there is small current flow from the +300M through R66. When Q17
gets forward biased, this current increases heavily and the output pin 5 drops.
Via the forward biased Z4, the collector of Q5 is lowered and Q6 is turned on
introducing the EHT HOLD DOWN .
Note that the same opto-coupler and transistor Q9 are used to remove the
influence of the hum on the +300M supply via the feedback voltage. The +300M
is, via a filter C45+R66/R67, supplying current to the opto-coupler (pin 1 of IC8.)
The voltage , containing this mains hum, is taken from the collector of Q9 and
capacitively added to the reference voltage.
3. EHT Hold Down in the event of an excessive amount of CRT beam
currents
Finally, in the event of an excessive amount of CRT beam current, the negative
HVL voltage can charge up C23 to the -0.6V threshold and Q8 conducts to
introduce the EHT HOLD DOWN condition. The delay is determined by the time
constant ((R40//R39+NTC1)/C23) 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.
Summary of Contents for GRAPHICS 1209S
Page 4: ...BARCO PROJECTION SYSTEMS GRAPHICS 1209S 90 00972 230V AC 90 00977 120V AC SAFETY NOTICE...
Page 9: ...BARCO PROJECTION SYSTEMS GRAPHICS 1209S 90 00972 230V AC 90 00977 120V AC GENERAL INFORMATION...
Page 28: ...BARCO PROJECTION SYSTEMS GRAPHICS 1209S 90 00972 230V AC 90 00977 120V AC SERVICE SHEETS...
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