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1.2 Horizontal deflection block

The operating principle of the horizontal deflection circuit is given below.

The Q502 operates as horizontal output, and the D503 as the dumper diode.

As shown in Fig. 5, the horizontal output transistor Q502 turns to ON/OFF by means of the drive pulse in

pin 25 of IC601 in substrate DEFL-SUB through the drive transformer T501, drive transistor Q501, or

Q560, Q561, Q562, etc.

The deflection current Idy during Q502 ON gets increased to the maximum level Ip according to the

equation shown below:

Idy = (Vcc/Ldy) x Ton

The maximum Ip is approximately 8A at full scan when fh = 106k.

Here;

Vcc: Output voltage of Q504

Ldy: Parallel value of the Lh value of DY (=62µH) and the horizontal output transformer

(=5mH)

TON: The ON time of Q502

When the drive pulse has negative polarity, Q502 turns OFF and Idy starts flowing to charge C506 until

the collector voltage reaches the maximum level Vcp.

Vcp = Vccx{1 + (π/2)x(Ts/Tr)}

With the maximum Vcp attained, the charges accumulated in C506 flow into DY as the discharge current.

This charge/discharge current is called retrace time, and is expressed by the equation given below.

Tr = π (Ldy•Cr) * Cr = C506 value

In the present model, the retrace time is set to approx. 1.8µs.

Ts is called trace time, and is expressed by the equation given below with the horizontal cycle as T.

T = Ts + Tr

With Vcp = 0, the dumper diode D503 turns ON and Idy gets decreased from –Ip to 0 ampere. Since

Q502 ON time and dumper diode ON time are set to overlap at 0 ampere point of Idy, the crossover

distortion is prevented from occurring at 0 ampere point of Idy.

The D503 causes the transient current to flow in the high-speed dumper diode.

The horizontal output transformer T502, connected in parallel to the deflection yoke, operates as a choke

coil. Figs. 6 and 7 show the image of circuit operation and the waveforms in actual machine.

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Compaq P1220 Wartungshandbuch Seite 17