12.4
12.4.l
86
from the front panel. Sufficient lead length
is provided so that the board can now be
turned over for access to the foil side. After
replacing the board, align the pointer on
the line previously scribed and tighten the
retaining screw. Make sure the pointer is
not binding against the mounting bracket
or the front panel.
INTERNAL ADJUSTMENT PROCEDURES
With only a few exceptions the internal
adjustments of the Model 415 that may be
required consist primarily of tuned circuit
adjustments. For example, referring to the
trouble shooting chart of Table 12.I. the
absence of any one of the markers may be
the result of a mistuned coil in each oscil-
lator stage.
MARKER OSCILLATOR ADJUSTMENTS
One of the most straightforward methods
of performing marker oscillator tuning is to
first connect the Model 415 to an oscillo-
scope as shown in Figure 5.2 (fold-out at end
of manual). Place the FUNCTION switch in
the IF position. Turn on several of the IF
markers and adjust the
MARKER AMPLI-
TUDE control for suitable marker amplitude
as observed on the oscilloscope. Adjust the
SWEEP WIDTH control as required, to-
gether with the CENTER FREQUENCY con-
trol so that the frequency region which
includes the missing marker is displayed
on the oscilloscope. Now adjust the appli-
cable coil in a counterclockwise direction
until the marker appears on the oscillo-
scope.
The parts layout drawing of Figure 12.2 is
useful in locating the applicable marker,
when used in conjunction with the trouble-
shooting chart of Table 12.1. For example,
if the CHROMA (42.67) marker is missing,
then reference to the troubleshooting chart
indicates that the possible defect is
QS, X6
or mistuning of 16. The parts location dia-
gram makes it possible to locate the ques-
tionable stage immediately. With the 42.67
(3.08) CHROMA marker switch in the ON
position, adjust LS in a counterclockwise
direction as required to restore the marker.
If the marker is not restored, perform the
voltage measurements on the oscillator
stage.
Once the marker is restored this indicates
that the oscillator as well as the post injec-
tion system of the Model 415 are function-
ing properly.
Next, it must be verified that
the marker output is available at the output
jack. The PR-151 Demodulator Probe can be
used effectively in conjunction with a vacu-
um tube volt meter to determine the pres-
ence of marker output at the output jack.
The PR-151 can be connected to any
VTVM
having a microphone type connector such
as that employed on the
B&K Model 177
VTVM. Turn the ATTENUATOR control to
max.
Using the lowest d-c voltage range avail-
able (0.5 volts on the
B&K Model 177) with
negative polarity, turn on only the marker
which is to be tuned. A
d-c voltage reading
will be indicated on the
VTVM if the marker
oscillator is functioning. Adjust the marker
coil tuning slug for a maximum indication
on the VTVM. The slug is then adjusted
one-half turn counterclockwise from the
maximum output point. Return the FUNC-
TION switch to the IF position and observe
that the marker is visible on the oscillo-
scope. The method outlined here can be
used for peaking all marker oscillators.
12.4.2
CH 4 OSCILLATOR ADJUSTMENT
The method of using the PR-151 for peaking
oscillators can be used for the Channel 4
oscillator, Channel 10 oscillator and the pix
oscillator as well. To perform alignment of
the Channel 4 oscillator, place the FUNC-
TION switch in the Ch 4 position. Next, re-
ferring to Figure 12.4, ground the end of
resistor R207 indicated as TP-205. This dis-
ables the
IF sweep so that only the Channel
4 oscillator component is supplied to the
output jack of the Model 415. With the PR-
151 connected to the output jack of the
Model 415 adjust coils 1301 and 1302 for
maximum indicated output, using the lowest
d-c voltage range of the VTVM.
12.4.3
CHANNEL 10 OSCILLATOR ADJUSTMENT
The Channel 10 oscillator tuning adjust-
ments can be performed in a manner sim-
ilar to that oulined for Channel 4. Place the
FUNCTION switch in the CH
10 position
and ground TP-205 of Figure 12.4 as was
done for Channel 4. With the PR-151 con-
nected to the output jack of the Model 415
and the ATTENUATOR at maximum, adjust
the coils 1305 and 1307 for maximum indi-
cated de on the VTVM.
12.4.4
PIX OSCILLATOR ADJUSTMENT (BOTTOM
COVER OF MODEL 415 REMOVED)
The pix oscillator peaking can be performed
in
a similar manner. Place the FUNCTION
switch in the
IF position. Ground TP-205 of
Figure 12.4. Place the CHROMA switch in
the ON position and connect the PR-151 to
the output jack on the Model 415. Adjust
the shielded coil L-404 of the pix oscillator
(refer to Figure 12.4) for maximum output as
indicated on the VTVM. REMOVE THE
SHORTING LEAD UPON COMPLETION OF
THE ABOVE OSCILLATOR ADJUSTMENTS.
12.4.5
USING THE SPARE MKR OSCILLATOR
This oscillator stage is energized by the
SP ARE MKR switch. The primary purpose
of this oscillator stage is to provide an ad-
ditional crystal-controlled marker oscillator
in the event that a user does extensive align-
ment work in sets which require a special
alignment frequency. This is true in trap
frequency alignments; for example, Motor-
ola and Philco sets have trap frequencies
of 35.25 MHz and 38.75 MHz respectively.
I
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