Circuit Description—4 5 5 /A 2/B 2
Diode CR358 protects the Sweep Gate Am plifier in the
event that greater than +5 volts is applied to the + GATE
OUT connector.
CRT C IR C U IT
^
The CRT circuit provides the voltage levels and control
circuits to operate the crt. The circuitry consists o f the z
axis amplifier, high-voltage oscillator, high-voltage re
gulator, +95 volt low-voltage supply, high-voltage rectifier,
high-voltage m ultiplier, and the crt controls.
High-Voltage Oscillator
Transistors Q552, Q556 comprise a high-voltage (HV)
oscillator that produces drive fo r high-voltage transformer
T550. Transistor Q556 and inductor L554 store additional
energy from the +32 volt unregulated supply (at high power
line voltages) and supply that energy to the HV transformer.
In this way, the additional energy is used, not lost in heat.
The following describes the sequence o f events in the HV
oscillator circuits. See Figure 4-1 for waveform relationships.
When the instrument is turned on, R542 provides start-up
current to ensure that Q544 and Q548 turn on. Current
from Q548 charges C548 to a level determined by the —2
kV feedback (through R563 A and B) to the base o f Q544.
As the voltage induced into the feedback windings swings
positive, it adds to the voltage across C548, and Q552 is
biased into conduction. Capacitor C548 provides the
current for the base circuit, causing Q552 to turn on. Diodes
CR554 and CR553 prevent Q552 from saturating, and
provide temperature compensation for the base drive. The
amount of voltage on C548 determines when Q552 turns on
thereby controling the regulation. For example, Q552 turns
on when the output voltage (—2 kV) attempts to go more
negative.
Current then builds up through L554 and T550. Voltage is
induced into the feedback windings keeping Q552 on until
the rate of current change through L554 and T550 drops to
zero, causing feedback current to drop and turn o ff Q552.
When Q552 turns off, the polarity of the voltage across
L554 reverses, turning on Q556 by way of CR555. The
turning on of Q556 places L554 in parallel with the T550
primary winding, which discharges the energy stored in
L554 into T550. Transistor Q556 turns o ff when L554 has
discharged to a point where its base drive is not sustained
through CR555.
The feedback voltage again reaches a level which turns on
Q552. Capacitor C548, which has been charging since Q552
turned o ff, discharges through the base circuit, saturates
Q552 and the cycle repeats.
The oscillator frequency is determined by the resonant
frequency of T550, which is about 45 kHz.
Capacitor C558 and inductor L558 provide decoupling to
the +32 volt unregulated supply to prevent oscillator current
from disturbing the +32 volt supply.
High-Voltage Regulator
The high-voltage regulator circuit consists of Q544, Q548,
and their associated circuitry.
Regulation occurs as Q552 is turned on for more or less time.
Feedback from the —2 kV output at pin 9 of U550 is applied
to the base of Q544 through R563B. If the —2 kV starts to
go more negative, Q544 collector goes more positive. This
reduces the current in Q548 and allows C548 to maintain a
Fig. 4-1. HV-oscillator waveform relationships.
4-4
REV. A, O C T 1975
Summary of Contents for 455
Page 1: ...Tektronix 455 A2 B2 PORTABLE OSCILLOSCOPE WITH OPTIONS I N S T R U C T I O N M A N U A L ...
Page 9: ......
Page 13: ...455 A2 B2 x 455 A2 B2 Portable Oscilloscope ...
Page 61: ...Circuit Description 455 A2 B2 Fig 4 4 Channel switching gates REV A OCT 1975 4 9 ...
Page 66: ...Fig 4 6 Sweep generator 1C Function block diagram Circuit Description 455 A2 B2 ...
Page 80: ...Maintenance 455 A2 B2 BOTTOM FRAME Fig 5 7 VO LTS DIV switch disassembly 5 8 REV A OCT 1975 ...
Page 90: ...Maintenance 455 A2 B2 Fig 5 13 Troubleshooting chart 5 18 REV A O CT 1975 ...
Page 91: ...Maintenance 455 A2 B2 1907 1 A Fig 5 13 Troubleshooting chart cont REV A OCT 1975 5 19 ...
Page 132: ...Instrument Options 455 A2 B2 Option 7 Page 8 REV E AUG 1979 D C TO AC I NVERTER ...
Page 166: ...l 07 7 REV b NOV C M1 4 5 5 MAIN MODULE BLOCK DIAGRAM ...
Page 167: ...4 5 5 A 2 B 2 AS VERTICAL MODULE IR07 R8 RE V B M O ...
Page 168: ...I S 0 7 9 8 R E V B NOV IRT7 A2 VERTICAL MODULE BLOCK DIAGRAM ...
Page 169: ...REV B NOV 1977 Fig 9 2 A1 Partial Interface board compo ...
Page 170: ...455 A2 B2 Sea Parts List for serial number ranges tial Interface board component locations ...
Page 173: ...v I m B 2 HORIZONTAL MODULE BLOCK DIAGRAM ...
Page 174: ...455 A2 B2 REV B NOV 1977 Fig 9 3 A 1 Partial Interface board component locations ...
Page 182: ...455 A2 B2 Fig 9 4 A20 Vertical board component locations SN B0110 ...
Page 185: ...455 A2 S2 4 5 5 M A fN M O DULE ...
Page 187: ...455 A2 B2 Fig 9 5 A20 Vertical board component location SN B010100 B REV C NOV 1977 ...
Page 191: ...V O L T S 01 ...
Page 192: ...V O LT S Q IV S H O W N IN 57 H V ...
Page 198: ... U 1 X SlCjMAt TO P2 A4 h o p it MODULE SEE PARTS LIST FOR SEMICONDUCTOR TYPES f SWITCHING ...
Page 207: ...A 1 4 B T I M I N G S W I T C H B O A R D A13 A TIM ING SWITCH BOARD 1 9 0 7 3 ...
Page 209: ...4 5 5 A 2 B 2 B 2 h o r iz o n t a l m o d u l e R o i I 11 R E V C NOV IS77 ...
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Page 216: ...A B T IM IN G S W IT C H INTERFACE C O N N E C T O R ...
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Page 224: ...455 A 2 B 2 ...
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