Inverter

The Inverter circuit changes the dc voltage from the Preregulator to ac for use by the supplies that are connected to the secondaries of T940.

The output of the Preregulator circuit is applied to the center tap of T940. Power-switching transistors Q940 and Q942 alternate conducting current through R941 from the primary circuit common to the Preregulator output line. The transistor switching action is controlled by T942, a saturating base-drive transformer.

When the instrument is first turned on, one of the switching transistors will start to conduct and the collector voltage will drop toward the common voltage level. This will induce a positive voltage from the lead of T942 which is connected to the base of the conducting transistor to reinforce conduction. Eventually T942 will saturate, and as the voltage across T942 (and T940) begins to reverse, the conducting transistor cuts off because of the drop in base drive. The other transistor will not start conduction until the voltage on the leads of T942 reverse enough to bias it on. This process will continue, and the saturation time of T942 plus the transistor-switching time will determine the frequency of Inverter operation (typically 20 kHz). After the initial Inverter start up, the switching transistors do not saturate; they remain in the active region during switching.

In instruments having the Current Limit board, diodes CR940 and CR942 serve as a negative-peak detector to generate a voltage controlling the outputs of both the Preregulator and the error amplifier. In instruments having the Preregulator board, diodes CR940 and CR942 serve as a negative-peak detector to generate a voltage for controlling the output of the error amplifier. Capacitor C951 will charge to the peak amplitude of the collector voltage of Q940 and Q942. This voltage level is applied to the divider composed of R945, R946, and R947. The error amplifier, composed of Q948 and Q954, is a differential amplifier that compares the reference voltage of VR951 with the voltage on the wiper of potentiometer R946. The current through Q954 will set the base drive of Q956 and thereby control the voltage on C957. This voltage will bias Q940 and Q942 to a level that will maintain the peak-to-peak input voltage of T940. The amplitude of the voltage across the transformer primary winding and thus, that of the secondary voltages of T940, is set by adjusting -8.6 V Adj potentiometer R946.

At turn on, Q948 is biased off and Q954 is biased on. All the current of the error amplifier will therefore go through Q954 to bias on Q956. Diode CR956 allows the base of Q956 to go positive enough to initially turn on Q940 or Q942. The current through Q956 controls the base drive for Q940 and Q942. Base current provided by base-drive transformer T942 will charge C957 negative with respect to the Inverter circuit floating ground (common) level.

NOTE

The following paragraph applies only to instruments having the Current Limit board.

Voltage from CR940 and CR942 also provides a measurement of the minimum collector voltage of Q940 and Q942 with respect to the Inverter circuit floating ground. This voltage is fed back to the Preregulator through optical isolator U931 to control the output voltage from the Preregulator circuit. As the negative peak voltage at the collectors of the switching transistors is regulated by the error amplifier with respect to the output of the Preregulator, control of the dc level from the preregulator will control the minimum voltage with respect to the floating ground. Potentiometer R952 (Head Room Voltage Adjust) is used to set this minimum voltage level to a point that prevents saturation and excessive power dissipation of the Inverter switching transistors.

CRT Supply

High-voltage multiplier U990 utilizes the 2-kV winding of T940 to generate 8 kV at one output to drive the crt anode. It also uses an internal half-wave rectifier diode to produce -2 kV for the crt cathode. The -2 kV supply is filtered by a three-stage low-pass filter composed of C990, R992, R990, C992, R994, and R995. Neon lamp DS870 protects against excessive voltage between the crt heater and crt cathode by conducting if the voltage exceeds approximately 75 V.

Auto Focus Circuit

Focus voltage is also developed from the -2 kV supply via a voltage divider composed of R884, R882, AUTO FOCUS potentiometer R883, R881, R880, R879, R878, R872, Auto Focus Adjust potentiometer R875, and Q877. The focus voltage tracks the intensity level through the action of Q877. The Intens Level signal from the Auto Intensity circuit (Diagram 6) is applied to the emitter of Q877 through R877. When the Intens Level signal changes due to a changing display intensity, the current through the divider resistors changes proportionally. Auto Focus Adjust potentiometer R875 is adjusted to produce the best focus tracking.

Low-Voltage Supplies

The low-voltage supplies utilize the secondary windings of T940 and are all full-wave, center-tapped bridges. The + 100 V supply uses CR961 and CR963 for rectification and uses C961 for filtering. Diodes CR965 and CR967 rectify ac from taps on the 100-V winding, and C965 filters the output to produce +30 V dc. The diode bridge consisting of CR971 through CR974 produces the +8.6 V and -8.6 V supplies. Filtering of the +8.6 V is accomplished by C971, 0975, and L971; while filtering of the -8.6 V is done by C972, C976, and L972. Voltage regulator U985 uses the rectified +8.6-V supply to produce the +5 V output. Diode CR985 protects the regulator by not allowing the output voltage to go more positive than the +8.6 V input voltage.

DC Restorer

The DC Restorer circuit produces the crt control-grid bias and couples both dc and low-frequency components of the Z-Axis Amplifier output to the crt control grid. Direct coupling of the Z-Axis Amplifier output to the crt control grid is not employed due to the high potential differences involved. Refer to Figure 3-8 during the following discussion.

The ac drive to the DC Restorer circuit is obtained from pin 16 of T940. The drive voltage has a peak amplitude of about 150 V and a frequency of about 20 kHz. The sinusoidal drive voltage is coupled through C863 and R863 into the DC Restorer circuit at the junction of CR860, CR863, and R864. The cathode end of CR860 is held at about +85 V by the voltage applied from the wiper of Grid Bias potentiometer R860. When the positive peaks of the ac-drive voltage reach a level that forward biases CR860, the voltage is clamped at that level.

The Z-Axis Amplifier output-signal voltage is applied to the DC Restorer at the anode end of CR863. The Z-Axis signal voltage level varies between +10 V and +75 V, depending on the setting of the AUTO INTENSITY control. The ac-drive voltage will hold CR863 reverse biased until the voltage falls below the Z-Axis Amplifier output voltage level. At that point, CR863 becomes forward biased and clamps the junction of CR860, CR863, and R864 to the Z-Axis output level. Thus, the ac-drive voltage is clamped at two levels on the positive swing of the cycle to produce an approximate square-wave signal with a positive dc-offset level.

The DC Restorer is referenced to the -2 kV crt cathode voltage through R867 and CR867. Initially, both C865 and C864 will charge up to a level determined by the difference between the Z-Axis output voltage and cathode voltage. Capacitor C865 charges from the crt cathode through R867, CR867, CR868, and R865 to the Z-Axis output. Capacitor C864 charges through R867, CR867, R864, and CR863 to the Z-Axis output.

When the ac-drive voltage starts its positive transition from the lower clamped level toward the higher clamped level, the charge on C864 increases due to the rising voltage. The increase in charge acquired by C864 is proportional to the amplitude of the positive transition. When the ac-drive voltage starts its negative transition from the upper clamped level to the lower clamped level, the negative transition is coupled through C864 to reverse bias CR867 and to forward bias CR868. The increased charge of C864 is then transferred to C865 as C864 discharges toward the Z-Axis output level. The amount of charge that is transferred is proportional to the setting of the AUTO INTENSITY control, since that control sets the lower clamping level of the ac-drive voltage.

The added charge on C865 also determines the control-grid bias voltage. If more charge is added to the charge already present on C865, the control grid becomes more negative, and less crt writing-beam current will flow. Conversely, if less charge is added, the control-grid voltage level will be closer to the cathode-voltage level, and more crt writing-beam current flows.

During periods that C865 is charging, the crt control-grid voltage is held constant by the long time-constant discharge path of C865 through R868.

Fast-rise and fast-fall transitions of the Z-Axis output signal are coupled to the crt control grid through C865. The fast transitions start the crt writing-beam current toward the new intensity level. The DC Restorer output level then follows the Z-Axis output-voltage level to set the new bias voltage for the crt control grid.

Neon lamps DS867 and DS868 protect the crt from excessive grid-to-cathode voltage if the potential on either the control grid or the cathode is lost for any reason.

Z-AXIS OUTPUT

CR863 SZ

C865

CR868V

C86i:

CR860

CR867 5Z

C863:

AC DRIVE VOLTAGE FROM T940, PIN 16

CATHODE VOLTAGE SUPPLY (*-2kV)

CONTROL GRID

CATHODE

3826-28

Figure 3-8. Simplified diagram of the DC Restorer circuit.

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