pear, 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 de
termine which side is open. On the upper beam
vertical amplifier check particularly the connec
tions at the HORIZ DEF PLATE SELECTOR switch,
SW489.
If the trace does appear, however, 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 corresponding points on
opposite sides of the circuit. As you short be
tween the points, in turn, the spot should return
on the screen as each connection is made. You
may have to readjust the POSITION control
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 fol
lowing this point is at fault. The trouble may be
caused by a defective tube, resistor, or capacitor.
f
I
v.
\
" T "
r
mlm
t
f
Fig. 5-6. 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 waveform.
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 Recalibration Procedure for this.
If the change in gain is more pronounced, or
if there is no vertical deflection at all, the tubes
should be first checked. Then check for com
ponents which will affect the gain of both sides
of the amplifier without unbalancing the ampli
fier. Such parts are common cathode resistors in
the difference amplifier stages, or plate dropping
resistors which are common 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
categories. .. low frequency and high frequency.
If a square wave is applied to an input of the
oscilloscope, the type of distortion can be de
termined by the shape of the displayed wave
form. High frequency distortion will primarily
affect the leading edge and trailing edge of the
applied square wave while low frequency dis
tortion 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 5-6. The shape of these wave
forms will vary widely however, with the cause
of the distortion and the frequency of the applied
wave. A nominal amount of low frequency dis
tortion is normal 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 nor
mal frequency range of the instrument that it
constitutes a trouble.
Low frequency distortion is usually caused by
a change in the time constant of the input coupl
ing circuit. If tubes become gassy however, their
resultant grid current will establish a time con-
©
5-10
MAINTENANCE — TYPE 502
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