Date: 10/02/98 R762120
G2+DIAGNOSTIC
R762120
10
Technical description
If, on the other hand, the HTHD voltage is too , the +CONV voltage drops also via D235/
R227.
C. Monitoring of the +210V and -170V.
A very stable Vref voltage is formed with IC2 (33V) and buffered with an OPAMP.
IC2 stabilizes a 33 volts which is then divided by R14/R1 to exactly 10 volt and buffered
to provide sufficient current.
If the +210V were absent, the Vref would disappear and the monitoring circuit cannot
work. The +9V takes over in this case and installs a reference voltage via Z1 / D11.
This Vref of 10 volt is used for the level detectors monitoring the supply voltages of the
video power stages : the +210V and -170V.
The absence of one or both of these voltages can damage the picture tubes. The Scan
Fail line switches low through Q1 as soon one of the voltages drops below some level.
D. G2 voltages.
1. Stabilisation.
The G2 voltages must be very stable to avoid thermal drift, and, they should be
adjustable between 400V and 1020 volt.
The voltage coming from the unit 76 2120S is already very stable since it is coming from
a stabilised source, the EHT.
The series output resistor in the unit forms together with P2, P3, P4 and R17 a divider.
If we can stabilize the voltage at the node mentioned Adjust for 1020V with P1 we
also have a stable G2 voltage at the sliders.
That 1020 volt point is sensed with the divider R15 / R13/ P1. Since the Vref is 10volt,
the output of the OPAMP will change until pin 13 is also at 10 volt.
Any voltage difference due to a change of the 1020 volt is adjusted by Q2 which will
feed a current from to the (negative) output of the OPAMP via the voltage dropping
Q2 transistor. Note that Q2 is needed because the 1020 volts is a too high voltage for
the OPAMP. A high voltage transistor such like the ON4046 (BU508A) is therefore
required.
2. G2 - Killer.
The G2 voltages must drop very quickly to zero in the event of a scan failure and at
switching off the projector. There is a risk that the spot suppression does not sufficiently
work when these voltage are too high.
The drop of the G2 voltage is realised with D100-D104 taking the heavy drop of the +17V
and +30V at information to cause a heavy conduction of Q2.
In the event of a scan failure, Q110 gets forward biased as its emitter Q2 becomes
negative.
E. LEDs for diagnosis.
The presence of the various voltages are all displayed with green LEDs. The variable
voltages like the +HTHD and +CONV drive a LED through a transistor to obtain a
constant light output of the LED.
The scan fail diode D16 is red and shows that there is a problem in the horizontal or
vertical deflection (see these boards for more explanations).
R762120 Rev. 18.06.96
