Tektronix 5441 Instruction Manual page 44

1 heory
of
Operation-5441 lR5441
When S610, STORE pushbutton, is pressed in, the
If S675, SAVE pushbutton, is not pushed in (out posi-
10 ms pulse generator feeds pulses to the storage mesh to
tion), the output of Q640 (Fig. 3-2) is supplied via CR640 to
increase the background level slightly for better storage and
the storage mesh.
to the Q625-Q640 monostable multivibrator for triggering.
When S675, SAVE pushbutton, is pushed in, the output
of (2640 (Fig. 3-2) is supplied to the flood-gun cathode.
Pushing S675 in forward-biases CR650 and holds pins 16A
and 17A of the time-base plug-in high, preventing a sweep.
Fig. 3-2. Variable Persistance and Save circuit output waveform.
r
Variable from 1 0 ps
t o 2 ms
by
varying
PERSIST
LOW VOLTAGE POWER SUPPLY
-
The low-voltage power supply circuit provides the oper-
Low-voltage Regulators
ating power for the oscilloscope system. Electronic regula-
tion is used, where necessary, to provide stable, low-ripple
-30 V Supply. The -30 V supply, besides providing
output voltages. The circuit also includes the calibrator cir-
power to circuitry throughout the instrument, provides a ref-
cuit to produce an accurate square-wave output.
erence-voltage source to establish operating levels for the
feedback regulators in the
-
15 V,
+
15 V, +30 V and
I
control
Power Input
I
I
I I
1 -
1 0 m s - 1
Power is applied to the primary of transformer T8OOI
F300lS300 through fuse F300, thermal cutout S300, and
POWER switch S302, and line-selector block P8OO or P801.
The line-selector blocks allow changing the primary-winding
taps of T800 to fit different line requirements.
Low-voltage Rectifiers and Unregulated Outputs
The full-wave bridge rectifiers and associated filter com-
ponents in the secondaries of T800 provide filtered dc volt-
ages. The unregulated outputs are +200 V,
+
18 V, +38 V,
-
18 V, and -38 V. The +200 V outputs to the display unit
are protected by F8OO.
+5 V supplies. The regulator for the -30 V supply is a
feedback amplifier system that operates between ground
and the unregulated -38 V. Current to the load is delivered
by the series-pass transistor, Q940. The supply voltage is
established by the drop across R948, R950, and R952,
which is compared to the voltage drop across VR950 and
the emitter-base junction of Q950. The feedback path is
through R949, Q955, and (2958 to the base of Q940. Any
variation in output voltage due to ripple, change of current
through the load, etc., is immediately transmitted to the
base of Q940 and nullified by a change in Q940 conduction,
thus maintaining a steady output. The output of the supply
is set to exactly -30 V by adjustment of R950, -30 V adj.
This control sets the conduction of (2950, which controls the
bias levels of Q958 and Q940. CR955 and Q958 provide
short-circuit protection by limiting the current through (2940
when the voltage drop across R940 exceeds 1.1 V.
REV MAY 1982