Tektronix 502A, Instruction Manual

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Maintenance— Type 502A
the short is removed, the vertical amplifier is in a state of dc
unbalance. To determine the cause of this condition, short
the plates of the output stage (V474B and V484B) together.
If the trace does not appear, one side of the circuit, between
the output stage and the crt, is open. A continuity check
with an ohmmeter is perhaps the best way to determine
which side is open. On the upper beam vertical amplifier,
check particularly the connections at the HORIZ DEF PLATE
SELECTOR switch, SW489.
Fig. 4-10. Checking for unbalance in the upper beam vertical am­
plifier by shorting the vertical deflection plates together.
If the trace does appear when the plates of the output
stage are shorted together, the circuit between this point
and the crt pin connections is normal. The trouble then
lies somewhere in the vertical amplifier ahead of the plate
circuit of the output stage. To locate the defective stage,
move the shorting strap back, point by point, between corres­
ponding points on opposite sides of the circuit. As you
short between the points, in turn, the spot should return
on the screen as each connection is made. You may have to
readjust the DC BAL controls when shorting between grids of
the input stage. When you reach a point where the spot
does not return to the screen, the stage immediately following
this point is at fault. The trouble may be caused by a de­
fective tube, resistor, or capacitor.
For insufficient or no vertical deflection.
Insufficient vertical deflection indicates a change in the
gain characteristics of the Vertical Amplifier. If the change
is small, the Vertical Amplifier can usually be recalibrated
for gain. Refer to the Calibration Procedure for this.
If the change in gain is more pronounced, or if there is
no vertical deflection at all, the tubes should be checked
first. Then check for components which will affect the gain
of both sides of the amplifier without unbalancing the am­
plifier. Such parts are common cathode resistors in the
amplifier stages, or plate dropping resistors which are com­
mon to both sides of the amplifier.
Insufficient vertical deflection will also be caused if the
upper or lower frequency limit of the amplifier is exceeded.
For waveform distortion
Waveform distortion can be divided into two cate­
gories
. . . low frequency and high frequency. If a square
wave is applied to an input of the oscilloscope, the type of
distortion can be determined by the shape of the displayed
waveform. High frequency distortion will primarily affect
the leading edge and trailing edge of the applied square
wave while low frequency distortion will primarily affect the
mid-portion of the waveform.
Waveforms showing low frequency distortion and two
types of high frequency distortion are shown in figure 4-11.
The shape of these waveforms will vary widely however, with
the cause of the distortion and the frequency of the applied
wave. A nominal amount of low frequency distortion is nor­
mal for very low input frequencies when ac coupling is used,
and a nominal amount of high frequency distortion is normal
at the upper frequency limits of the instrument. It is only
when this distortion is excessive in the normal frequency
range of the instrument that it constitutes a trouble.
Fig. 4-11. Frequency distortion, (a) Low frequency distortion of a 20 cycle square wave due to attenuation of the low frequency components
of the waveform, (b) High frequency distortion of a 1200 cycle square wave due to excessive boost of the high frequency components of
the waveform, (c) High frequency distortion of a 1200 cycle square wave due to attenuation of the high frequency components of the wave-
form.
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