Sharpness Control; Tint Control; Crt Drive - Philips DPTV400 Series Training Manual

Sharpness Control (Figure 26)
The Sharpness controls for the YUV/RGB signals are located on the SSM. Since all three circuits
are basically the same, only the Blue one will be discussed.
The Blue signal is fed to Pin 1 of 7410. It is also fed to a Low Pass filter consisting of 5411 and
5410. This path is amplified by transistor 7411. The Low Pass filter blocks the Higher frequencies
as well as shifting the phase of the signal. The output of the Low Pass filter is also fed to Pin 4 of
7410. The mixing of these two phase shifted signals only allows the High frequency component to
be amplified and output on Pin 12. The gain of the High frequency component is controlled by the
Sharpness voltage, which is applied to Pins 8 and 10. The two signals are combined at Pin 12 of
the IC. If the input was a Blue signal it is amplified by 7412 and buffered by 7413. If the input was
a U signal, it is buffered by 7414.
Tint Control (Figure 27)
IC 7510 amplifies the U signal while 7520 amplifies the V signal. The Tint control voltage changes
the balance between the U (Pb) and V (Pr) signals to change causing the tint of the picture to
change.
CRT drive (Figure 28)
The output of 7600 is fed to the RGB amplifiers before being fed to the CRTs. Transistors 7720 and
7721 buffer the Blue output on Pin 42. The B-BIAS control voltage controls the gain of this circuit.
Transistor 7730 provides an additional voltage gain for the signal.
7710 and 7711 buffer the Green output from Pin 41 of 7600. The G-BIAS controls the gain of the
circuit.
The Red output from Pin 40 of 7600 is buffered by 7700 and 7701. The R-BIAS controls the gain of
the circuit.
The drive of the Red and Green outputs is compared with the Blue drive by 7900-A. The difference
signal is fed back to 7600 via the ABL line. If the Blue CRT is driven harder than the Green and
Red CRTs, the inverting input on Pin 2 will become greater than the non-inverting input on Pin 3,
resulting in the output on Pin 1 to go Low. The ABL line will go Low, causing 7600 to reduce the
drive to all of the CRTs. This circuit prevents the Blue tube from being over-driven.
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