Tektronix 1710J Series, Instruction Manual

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Theory of Operation
1710J-Series Waveform Monitors
5–19
The three output windings supply four output voltages. Each output is rectified
by a single diode and filtered by an LC pi filter.
The error amplifier regulates the +5 V output by feeding an error signal to the
pulse width modulator. VR1 is a 2.5 V shunt regulator containing an operational
amplifier and a voltage reference. The +5 V is divided by R69 and R70 to
provide 2.5 V to VR1, with fine adjustment provided by R99. C40 and R71
determine the gain and frequency response of VR1. VR4 controls overshoot of
the +5 V at power up. R98 and CR26 provide a minimum operating current for
VR1. R68 decouples C39 from VR1. Over-voltage protection for the +5V supply
is provided by a crowbar circuit formed by Q11, VR3, R13, and R14.
The 80 kHz sawtooth waveform at U3 pin 3 trips comparator U3. U3 pin 1 then
feeds a trigger pulse to one shot U4. U4 pin 13 outputs a 300 ns pulse to the
130 mA current source consisting of Q7 and Q8. When Q8 turns on, T2 pin 2 is
pulled down until CR15 (error amplifier) is forward biased. The negative-going
pulse at T2 pin 2 is peak detected by CR16 and C46. The dc voltage present at
the anode of CR16 feeds the pulse width modulator and the output under-voltage
shutdown circuit. CR29 resets T2 between pulses.
If the +5 V is below 4.9 V, the error amplifier will cause the peak detector output
to go below 2.9 V. The output of comparator U3B will pull low and shut down
pulse width modulator U5. C47 and R96 delay the operation of U3B long
enough for the power supply to power up. If the +5 V does not reach 4.9 V
within 50 ms of power up, U3B will shut down the switcher. The power supply
will then cycle on and off every couple of seconds.
High Voltage Power Supply, Diagram 8
The high voltage power supply generates the heater, cathode, control grid, focus
anode, and post accelerating potentials required to display the outputs of the
vertical and horizontal output amplifiers.
The high voltage power supply is generated by a sine-wave oscillator and step-up
transformer. Q6 and T1 are the principal elements of an Armstrong oscillator
running at about 22 kHz. Error amplifier U2 regulates the +100 V output and
keeps the high voltage power supply constant under varying load conditions by
controlling the base current to Q6. The +100 V output is regulated directly, while
the high voltage power supply is indirectly regulated through a current feedback
circuit.
Output Filters
Error Amplifier
Feedback Transformer
Driver and Peak Detector
Output Under-Voltage
Shutdown
HV Osc and Error Amp