HEWLETT PACKARD (HP) 683C
Service Manual

Section 111 Paragraph 3-8

Model 683C C123 i s a coupling capacitor which couples ac ripple directly to the grid of the control tube which in turn acts to regulate it out of the output i n the same manner a s a dc change. This results i n very low values of ripple voltage i n the regulated output. +300 VOLT REGULATED SUPPLY. The300-volt supply circuit i s similar to that of the -150 volt supply except it is referenced to 150volts and includes a ripple compensating voltage divider (R137 and R138) from the unregulated output to ground. The voltage tap-off from the divider is applied to t h e screen grid of V107A (see figure 4-22). Compensating action of this arrangement is described below. B.

3-8. REGULATED P O W E R SUPPLIES.
In addition to the magnet regulated power supply, there a r e five regulated voltages generated in the 683C: 1) the regulated -150 volt supply which is not dependent upon any other supply but affects all other supplies, 2) the regulated, +300 volt supply which depends upon t h e -150 volt supply for a reference voltage, 3) the regulated helix supply which depends upon both the -150 volt and +300 volt supplies, 4) the collector supply which depends upon the regulated helix supply to determine its output voltage, and 5) a reference voltage supply (helix supply reference voltage generator) in the frequency modulator, which provides a reference for the regulated helix supply. The regulators a r e fed by half-wave silicon rectifiers powered from a single power transformer, T101. The sequence of operation upon applying l i n e power to the 683C and the operation of the helix overload relay a r e explained on the magnet power supply voltage and resistance diagram (figure 4-15). The operation of all the voltage regulators is similar, so only the operation of the -150 volt regulator is explained. -150 VOLT REGULATED POWER SUPPLY. In the -150 volt regulated supply (figure4-21), CR105 and CR106 are half-wave rectifiers which supply approximately 420 volts to the regulator series tube, V107B. Since the output voltage is negative with respect to ground the cathode of V107B is grounded. The grid-to-cathode voltage of V107B is adjusted to class A operating conditions, and the tube acts like a variable resistor which adjusts its resistance tomaintain a constant voltage on the -150 volt bus. Thus at normal l i n e voltage, the tube has approximately 270 volts drop from plate to cathode. V108, a glow discharge reference tube, maintains a constant voltage difference between 150 volts and the grid of the differential amplifier control tube, V109. The cathode of the triode section will maintain a constant 1-volt difference with the grid potential and thus keeps a constant 89-volt difference between the -150 volt bus and the cathode of the pentode section. Voltage divider R147, R148, and R149 provides theproper fraction of the -150 volts for the control grid of the pentode section. R148 adjusts the exact value of the bias so that the voltage at the grid of series tube V107B i s held at the correct value. If the -150 volt bus t r i e s to increase toward -151 volts, the increase will also make the control grid (pin 2) of V109 go in a positive direction with respect to the cathode. (The cathode is held at a constant voltage with respect to the -150 volt bus by the reference tube, V108, and the cathode follower triode section in V109.) The positive-going grid (pin 2) causes increased plate current to flow through the pentode section and R146. This causes increased voltage drop across R146 which lowers the grid voltage on V107B. The increased resistance of V107B brings the bus voltage back toward -150 volts. If the -150 volt level tends to decrease, the process is the same but in the reverse direction.
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REGULATED HELIX SUPPLY. Screen voltage for control tube V103 i n the regulated helix supply also is supplied from its unregulated source, through dividers R115-R119. With this arrangement of the control tube screen circuit, there is improved regulation for changes i n l i n e voltage. Circuit action is similar to that which occurs with a changing voltage on t h e grid of the control tube: a change in screen voltage results in a change i n conduction through the tube in such direction that a change in line voltage will be compensated for by the series regulator. The result is an extremely constant output voltage regulation over the range of 103 to 127 volts line voltage. In addition, ripple voltage in the unregulated source drives the screen i n the proper direction to help regulate ripple out of the output, further reducing the ripple level. Operation of the regulated helix supply is similar to that of the low voltage supplies. However, it is more elaborate due to the extreme range of voltage control necessary, and because modulating voltages must be introduced. The voltage control tubes, V l O l andV102 (series regulators), require the single-ended amplifier, V103, driven by a differential amplifier, V105, which serves two other purposes 1) the exponential helix waveform is taken at the cathode of V105 and applied to the rf leveler circuit 2) externally-supplied modulation voltages from the FREQ. MOD. jack a r e fed to the grid of V105 to vary the helix voltage, thus varying t h e frequency. Since two different helix modulating signals must also be applied to the voltage regulator, and since a greater order of regulation is necessary for helix operation, two stages of voltage amplification a r e used to obtain greater gain and bandwidth. The reference voltage for the regulated helix supply is obtained from the helix supply reference voltage generator in the frequency modulator, and is applied to one side of differential amplifier V105. A sample voltage from the regulated helix supply output and a calibrating voltage obtained from voltage divider stick R132, R133, R134, and R135 a r e fed to the other grid of the differential amplifier. The calibrating voltage divider stick contains potentiometers which set the upper and lower helix voltage limits and, in turn, the upper and lower rf output frequency limits. The sweep voltages, which produce the r swept output, a r e f controlled exponential decays in the regulated reference voltage obtained from the frequency modulator. 01 130- 1

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