84
To locate voltage reference points within
the unit proper, several methods may be
used. First, the bottom cover of the unit may
be removed, exposing the under portion of
the printed circuit boards. All voltage test
points are indicated in the copper foil of
the underside of the boards. Points which
are called out on the schematic are identi-
fied accordingly on the underside of the
P.C. boards. All reference points may also
be located, but not as readily, from the top
side of each printed circuit board. In this
way, it is not necessary to remove the
bottom cover of the unit for voltage checks.
Reference to the parts location diagrams of
Figures 12.2, 12.3, 12.4 and 12.5 identify
various test points as they would appear
from the top. The corresponding identifica-
tion lettering is not visible from the top.
Because the drawings are to scale, how-
ever, voltage points may be located by
reference to the drawings and to the particu-
lar board under investigation. A similar ap-
proach may be used in locating signal
points as well as voltage measurement
points. All interconnecting cables and wires
in the Model 415 are also terminated at tie
points which are identified on the underside
of the P .C. board. By referring again to the
parts layout diagrams, the points can be
located from the component side of the
boards for signal tracing purposes. The
parts layout drawings indicate the orienta-
tion of each board when viewed from the
front of the unit.
The test point identification procedure will
be outlined by taking each of the parts
location diagrams in order. Referring to
Figure 12.2 notice that test points have been
called out, and that each refers to a specific
point on the P .C. boards. These points are
identified on the underside of the P.C. board
but, because of the scaled layout of the
parts drawing these points can be located
from the component side of these P.C.
boards. For example in Figure 12.2 TP-1
indicates a point where + 14 volts is always
present when the unit is turned on.
TP-2
indicates that terminal where +25 volts
should be present at all times. TP-3 indi-
cates the point where
-25 volts should be
present at all times. TP-4 indicates the point
where supply voltage to Q2 (ADJ PIX os-
cillator) would appear when the ADJ PIX
marker switch is energized at the front
panel. A similar procedure can be used to
verify that A+ appears at each marker
oscillator by turning on the appropriate
switch and checking for the appearance of
voltage at points 3,
4, 5, etc., across the
marker board. The
"TP" numbers do not
appear on the P.C. board foil. They are
used here for convenience in discussing
troubleshooting and adjustment procedures.
Referring to Figure 12.3, a similar identifi-
cation of certain voltage test points is made.
As indicated, TP-301 indicates a point at
which + 14 volts is present when the Model
415 is turned on. TP-302 indicates a
-25
volt point. TP-303, TP-305, and TP-306 indi-
cate points at which + 14 volts, +25 volts
DC and -25 volts, respectively, appear in
the power supply portion of the Model 415
when the unit is turned on.
Referring to Figure 12.4, TP-202 indicates a
point at which + 14 volts should appear
when the 10.7 MHz sweep oscillator is ener-
gized. TP-204 indicates the point at which
+ 14 volts should appear when the 44 MHz
sweep oscillator is energized. TP-206 indi-
cates a point at which
-25 volts is always
present when the Model 415 is turned on.
To check for proper sweep voltage applica-
tion to the sweep oscillators, connect the os-
cilloscope to TP-203 and adjust the SWEEP
WIDTH control from min to max. The ob-
served waveform should vary as indicated
in Figure 12.6F, with maximum amplitude
occurring at the maximum clockwise posi-
tion of the SWEEP WIDTH control.
Referring to Figure 12.5, TP-401 indicates a
point at which + 14 volts appears when
Video Sweep Amplifier Q401 is energized,
and TP-402 indicates the point at which+ 14
volts appears when Pix Oscillator Q402 is
energized.
12.1.3
HORIZONTAL SWEEP CIRCUITS
All internal sweep voltages are generated
by the waveform generator consisting of
QI2 and Ql3. The horizontal sweep voltage
is taken from the base circuit of Ql3 and
supplied to QI4 from which sweep voltages
of opposite polarities may be selected.
These outputs are supplied to the sweep
reversing terminals of CHROMA switch
S522 and then to the HORIZ SWEEP switch
S514 and from there to SCOPE HORIZ IN-
PUT jack J505. To aid in evaluating circuit
performance the waveforms of Figure 12.6
are provided for reference. These circuits
are operational in all positions of the
FUNCTION switch; therefore, loss of oscillo-
scope sweep when the oscilloscope sweep
selector is in the external sweep position
indicates a defect in the signal path de-
scribed. The synchronizing trigger for the
waveform generator is developed by neon
lamp P513, which is also the POWER ON/
OFF indicator lamp (See Schematic 488-
084-9-001). Failure of this lamp results in
loss of sync, which causes the basic sweep
rate to be slightly less than 60 Hz. The
Model 415 can still be used satisfactorily;
the only noticeable effect is that there will
be a slight low-frequency "breathing" of
response curves and waveforms observed
on the oscilloscope.
12.1.4
PATTERN AMPLIFIER CIRCUITS
(019 and
Q20)
Input signals from receiver under test are
connected to either the DIRECT jack (J502)
I
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