Tektronix 485, Instruction Manual

Page: 19 / 197

Circuit Description—4 8 5 /R 4 8 5 Service
occur during holdoff.) Pin 5 of U1535A alternates from HI
to LO and the sweep switches from A to B and back again.
When B is called up and pin 5 is HI, the Z AXIS 1C will be
in the B mode via pin 15 signal and the B intensity control
will function and Autofocus control is also provided. Also,
the trace separation signal w ill be LO, causing the trace
separation control to function. This is via control of
U1530B. All inputs must be HI to get LO outputs, and this
only occurs in ALT when B is called up. The output of
U1530B also drives the vertical logic when the HORIZ
DISPLAY is in ALT. B disable is controlled by A, the X-Y
signal, and switch SWA2 (which is closed when A = 1, 2, or
5 ns).
POWER IN V E R T E R /R E G U L A T O R
General
The Power Inverter/Regulator circuit provides the opera­
ting power for this instrument from a line voltage source or
DC voltage source. This circuit also includes the Line
Voltage Selector switch to allow for selection of a 115 V or
230 V nominal operating voltage. Fig. 3-1 shows a block
diagram of the Power Inverter/Regulator circuit. A sche­
matic is shown on diagram 14 at the rear of this manual.
Line Filter
The purpose of the Line Filter is to prevent the
instrument from injecting power supply frequency inter­
ference into the power line, or power-line interference from
entering the instrument. L I 812 and LI 813 provide both
common-mode and differential filtering. R1812 and R1813
are damping resistors. C l812 and Cl 813 are common-mode
filters and C1814 is a differential filter. R1811 discharges
Cl 814 when the power switch is off.
Line Input Circuit
Thermal cutout S1802 provides thermal protection for
this instrument. If the temperature exceeds a safe operating
level, S1802 opens to interrupt the applied power. When
the temperature returns to a safe operating level, S I802
automatically closes to re-apply the power.
CR1821 contains the main power rectifiers for the Line
Input circuit. C l822 and C l823 are the line-storage
capacitors. With S1803 in the 115 V position, the circuit
acts as a full-wave voltage doubler so that the voltage across
the series combination of C l822 and C l823 will be the
peak-to-peak value of the line voltage. In the 230 V
position, CR1821 acts as a full-wave bridge rectifier.
Therefore the voltage developed across C l822 and C l823
will be approximately the same for either 115 V or 230 V
operation.
RT1821 and RT1822 are thermistors which lim it the
charging currents during turn on. When the POWER switch
is turned o ff, the Line Stop circuit stops the Inverter and
C1822 and C1823 discharge thru R1822and R1823.
W A R N I N G
Because the discharge is slow, dangerous potentials
w ill exist across capacitors C l822, C l823, and other
connected components fo r several m inutes a fte r the
POWER switch is turned o ff. The presence o f voltage
in the c irc u it is indicated b y relaxation oscillator
R1824, C l824, and DS1824. Neon bulb DS1824
blinks u n til the p o te n tia l drops to approxim ately
100 V.
3-6
REV. B, AUG. 1975
Fig. 3-1. Block diagram of power inverter/regulator circuit.