Date: 10/02/98 R762120
G2+DIAGNOSTIC
R762120
9
Technical description
Technical description G2 + DIAGNOSTIC (76 2120).
Introduction
This board comprises :
- the SMPS for the convergence end stages and the Focus power stages.
- the stabilisation and alignment circuits of the G2 voltages.
- monitoring circuits of the +210V / -170V to cause scan fail if absent.
- the LEDs for diagnosis of the supply voltages and scan fail.
A. SMPS for +/- CONV and +FOC voltages.
This power supply uses the same TDA4601 as the main SMPS. We refer to the
description of this board for more details. We just limit here to the differences and the
control loop to generate the correct amplitude of these voltages.
Because of the high scanning range of the projector, from 15khz to 135khz, we apply
a tracking of the power supply with the line frequency. This means the power supply
for the power convergence stages increases with the line frequency, but, not in a linear
way.
The available information on the line frequency is the +HTHD voltage. However, this
voltage is not linearly proportional for the full range of frequencies. Nevertheless, this
voltage is used for tracking, but an adaptation of the gain in the feedback will be needed
in the second range.
The TDA4601(IC179) totally relies on the +15V(derived from the +17M) delivered by the
first or main SMPS and the +300VM.
The transformer and associated rectifier circuits provide the +FOCUS, +/- CONV and
the +CONV for the feedback control loop discussed hereafter.
B. Control loop / Feedback / Adaptation.
To obtain a tracking with the line frequency, a portion of the +HTHD voltage is applied
to the base of Q208.
The main loop is realised by applying the +CONV voltage to the base of Q203, the error-
amplifier. The amplitude of the feedback depends on the status of the output of the level
detector LM393 (IC216) , pin 1. If that pin 1 output is low (=ground), R200 is added in
parallel on R201. In the other case R200 is hooked up to the +30V (output of level
detector).
Pin 2 of the LM393 is set at 5.6 volt with Z197. REL2 is connected to the collector of
Q8 on the HOR DEFL board. For the second range, this line is low or ground level and
pin 3 is then lower than the zener voltage. Pin 1 is then at ground level and consequently
R200 is put in parallel with R201. The feedback is changed to compensate for the
decrease of the +HTHD voltage.
Via an insulating opto-coupler, the collector current of Q203 is transferred to pin 3 of
the regulating TDA4601.
The +CONV voltage is also applied to another level detector in IC216, pin 5. The same
reference voltage of 5.6 volts is applied to pin 6. When an overvoltage situation occurs,
Q227 saturates and the collector of Q203 drops to ground level. The SMPS switches
into a safe loop, whereas the output voltage is dropped to a minimum.
