315 Horizontal Amplifier

In the time positions of TIME/div switch SK2 the sawtooth voltage of the time-base generator is fed to the base of transistor TS603. The sawtooth is also applied, for external purposes, to TB OUT socket BU6 via emitter follower TS516.

Fig. 3.5. Horizontal amplifier

In position X via Y^ of TIME/div switch SK2, the external X deflection voltage applied to the Y^ channel is fed from transistor TS64 to the base of transistor TS603, via emitter follower TS75 and amplifier TS602. The external X deflection voltage is also applied to TB OUT socket BU6, via emitter follower TS516.

The horizontal output amplifier comprises two stages.

The first one with transistors TS603 and TS604 is provided with series-feedback.

The second one consists of two single-ended push-pull sections TS607-608 and TS609-611 with shunt-feedback. From the latter stage the deflection signal is fed to the c.r.t.

The gain of the output amplifier is determined by the shunt-feedback resistors R628 and R633, and the emitter resistance of transistors TS603 and TS604. X MAGN switch SK1 allows a gain control in two steps.

If switch SK1 is closed (X MAGN knob pullsed out), the series-feedback in the emitter circuit of transistors TS603 and TS604 is decreased in such a way that the gain of the output amplifier is increased by a factor of 5. In the latter mode, the magnified mode, the gain can be preset by means of potentiometer R617 and in the normal mode by means of potentiometer R618.

In position X via Y^ of TIME/div switch SK2, switch SK1 is bypassed and the output amplifier works in the magnified mode.

The horizontal shift voltage coming from X POSITION potentiometer R1 is supplied to the base of transistor TS604. Transistor TS606 acts as a constant emitter-current source for amplifier stage TS603-TS604.

3.1.6. CATHODE-RAY TUBE CIRCUITRY ,

3.1.6.1. General information

The CRT circuitry provides the voltage levels, timing pulses and comprises control circuits necessary for storage operation of the cathode-ray tube. ' ■

This circuitry consists of the following sections:

— Control circuits for correction and adjusting of astigmatism, trace rotation, orthogonality, flood gun voltages, barrel and pin-cushion distortion and focus.

— Automatic brightness control unit.

— Blanking circuit.

Astigmatic Circuits

Ht 9001

Fig. 3.6. Cathode-ray tube circuitry

Ht 9001

Fig. 3.6. Cathode-ray tube circuitry

3.1.6.2. Control circuits

. Astigmatism is corrected on a low-impedance level by means of potentiometer R809 via emitter follower TS801.

. Trace rotation is achieved by means of the trace rotation coil. This coil mounted inside the mu-metal screen, provides a magnetic field for rotational control of the entire scan.

The degree and direction of rotation is determined by setting of potentiometer R813 on the bases of the complementary transistors TS814 and TS816.

. Orthogonality correction is also achieved by means of a coil, which is mounted inside the mu-metal screen. The magnetic field of this coil provides adjustment of the X/Y deflection angle to 90°. The current through this coil is controlled by two complementary pairs of emitter followers (TS808, TS809 and TS811, TS812), the base voltages of which are controlled by potentiometers R834 and R838.

. Flood gun cathode and grid (g1) voltages are controlled on a low-impedance level by emitter follower TS813. By means of potentiometer R847 the balance of both cathode voltages is adjustable. The flood current is controlled by the voltage levels on the grids "g1" and adjustable with potentiometers R848 and R849. . Barrel and pin-cushion distortion is corrected with the aid of potentiometer R812.

. The display can be focussed by means of FOCUS potentiometer R9. In a spit-beam c.r.t. as used in this oscilloscope, focussing has certain consequences. These consequences and their remedies are discussed in section 3.1.6.2.1.

In the split-beam c.r.t. the electron beam is split into two parts which form the traces of the Y^ and Yg channel.

Both traces are controlled independently by the controls of both channels. The electron beam is focused, however, before it is split and, therefore, there is only one focus control. This mono-knob control of the focus has the following consequence.

Assume a focus setting as illustrated in Fig. 3.7.a.

Then the focusing point is exactly on the screen.

When the FOCUS potentiometer R9 is rotated, the focusing point is displaced to a point either side of the screen (Figs. 3.7.b. & 3.7.C.). This results in both traces moving away from each other. The more clockwise or anti-clockwise the FOCUS control is turned, the further the traces move apart.

Fig. 3.7. Effect of the focus control

To compensate for this trace shift, an extra shift voltage is fed to the vertical amplifier channels via transistor TS56. The magnitude of this voltage is controlled by potentiometer R9' which is part of FOCUS tandem potentiometer R9. The control is such, that the displacement caused by operation of the FOCUS control is compensated for.

The compensation voltage for channel Y^ is obtained from the emitter of transistor TS56 and that for channel Yg from the collector.

The compensation is so effective that the maximum displacement of each trace, caused by rotating FOCUS potentiometer R9 from fully anti-clockwise to fully clockwise, does not exceed 2 mm.

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