Radio Shack TRS-80, Руководство по обслуживанию, Страница: 24 / 44

– 24 –
ADJUSTMENT OF VSB (vestigial side band)
FILTER
Make Connection according to Figure 7
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.
Figure 8
ADJUSTMENT OF VIDEO CARRIER FRE-
QUENCY
Prepare switch, 0.01
µ
F 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.
12
IC1
0.01
C117
C116
L102 L103
TRACKING
GENERATOR
INPUT
RF OUT
RCA
JACK
75
Ω
COAXIAL
CABLE
L101
50/75
Ω
Impedance
OUT
SPECTRUM
ANALYZER
1 dB
2 dB
20 d
B
12 MHz
57 61.25 72.15 (MHz)
Freq.
3
1
2
Figure 9
ADJUSTMENT OF SOUND CARRIER
FREQUENCY
Figure 10
Prepare capacitor of 470
µ
F [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.
Figure 11
Note: On actual board, unit of hundred of the
reference numbers is omitted. Ex. L102 is
marked “L2.”
MDV - 1
CH. Select
RF Out
Luminance In
Sound In
DC Power
Supply
CH. 4
0.01
CH. 3
75
Ω
Coaxial
Cable
50/75
Ω
Impedance
5 ± 0.1V
SPECTRUM
ANALYZER
Video Carrier
Sound Carrier
Freq. (MHz)
4.5 4.5
T101
R103
IC1
470 pF
Q1
Sound
In
Frequency
Counter
10
0.001
5 ± 0.1V
MDV-1
CLK
OUT
Freq.
Counter
Regulated
D.C. Power
Supply