Radio Shack TRS-80 Service Manual page 24

ADJUSTMENT OF VSB (vestigial side band)
FILTER
Make Connection according to Figure 7
IC1
C117
12
0.01
C116
L102 L103
L101
TRACKING
GENERATOR
SPECTRUM
OUT
ANALYZER
Figure 7
1. Adjust marker of tracking generator at
61.25 MHz (marker - 2)
2. Adjust L102 so lower side band peak point
meets 61.25 MHz (marker - 2)
3. Adjust marker of tracking generator at
72.15 MHz (marker - 3)
4. Adjust L103 so higher side peak point
meets 72.15 MHz (marker - 3)
5. Repeat steps 1 to 4 until lower and higher
side band peaks agree with 61.25 MHz
(marker - 2) and 72.15 MHz (marker - 3)
Refer to Figure 8
6. Adjust L101 so that filter characteristic of -1
dB stays about within 12 MHz.
7. Adjust marker point of tracking generator
at 57 MHz (marker - 1) and check the level
difference between 61.25 MHz (lower side
peak point - marker 2) and 57 MHz (marker -
1) is more than 20 dB.
12 MHz
2 dB
2
1
57 61.25
Figure 8
ADJUSTMENT OF VIDEO
QUENCY
Prepare switch, 0.01 µF capacitor and 2.2K ohm
resistor and 75 ohm coaxial cable with RCA
and 50/75 ohm converter.
Make connection according to Figure 9.
1. Set switch to CH 4.
2. Adjust T102 core so that frequency counter
of spectrum analyzer shows 67.65 MHz
3. Set switch to CH 3.
4. Adjust trimmer capacitor of C113 so that
frequency counter of spectrum analyzer
shows 61.25 MHz.
RF OUT
RCA
JACK
75Ω COAXIAL
CABLE
INPUT
50/75Ω Impedance
3
1 dB
Freq.
72.15 (MHz)
CARRIER FRE-
Luminance In
Sound In
5 ± 0.1V
DC Power
Supply
ADJUSTMENT OF SOUND CARRIER
FREQUENCY
Q1
Sound
In
Prepare capacitor of 470 µF [pF] and make con-
nection according to Figure 10.
Adjust T101 core so that freq. counter shows
4.500 MHz.
ADJUSTMENT OF CHROMATIC FREQUENCY
Make connection according to Figure 11.
Adjust trimmer capacitor C102 so that freq.
counter shows 3.579545 MHz.
5 ± 0.1V
Regulated
D.C. Power
Supply
Note: On actual board, unit of hundred of the
reference numbers is omitted. Ex. L102 is
marked "L2."
– 24 –
CH. 4
CH. Select
0.01 CH. 3
MDV - 1
RF Out
75Ω Coaxial
Cable
SPECTRUM
50/75Ω
ANALYZER
Impedance
Figure 9
Video Carrier
Sound Carrier
Freq. (MHz)
4.5 4.5
T101
IC1
R103
10
470 pF
Frequency
Counter
Figure 10
0.001
CLK
MDV-1
OUT
Freq.
Counter
Figure 11