February 14, 2012 6816985H01-F
6.2-16 UHF1 Troubleshooting Charts: UHF Range 1 Receive RF Troubleshooting Chart
6.2.17UHF Range 1 Receive RF Troubleshooting Chart
NOTE: INJECT MODULATED ON CARRIER
NOTE: The presence or absense of RF signals is generally not enough
FREQUENCY SIGNAL AS REQUIRED.
Bad SINAD
No Recovered
Audio
Spray or Inject 1st IF
VHF2 1st IF:73.35MHz
VHF2 2nd IF:2.25MHz
VHF2 2nd LO:71.1MHz
into XTAL Filter FL51
to diagnose sensitivity degradations. It is best to compare signal levels
at each decision node, to the same circuit in working radio.
If present
at U401,
pin 47?
Is
2nd IF on
U401, pins 4
and 5?
Before replacing U401,
check U401 voltages.
Check for no RX audio
VOCON flowchart
Check tank
L412, C476.
Check for 3Vdc
at pin 48
Check
FL51
No
No
components L411,
Yes
Yes
MAEPF-27342-O
Check
FGU Chart
Check Mixer
(U1) circuit
VCO locked?
16.8MHz
check at pin 35
on U401?
18MHz
at ABACUS
(U401) pin
20?
Before replacing U401, check 2nd
Disconnect R418 and verify that
a 0.5V to 5V control voltage VCO
will sweep about the IF frequency
Remove SH401.
Check 2nd LO
U401, pin 38
Inject RF
into J101
Is it
symmetric
centered?
No
No
Does
the pulse
frequency
vary?
See VOCON
chart, RX
audio problems
No
1st LO
Locked?
Inject -47 dBm into J101 and
toggle signal continuously
TP403 data pulse
RF Signal
at C51?
Check
voltage at C46
RF
Signal postselector
input R37?
RF Signal
at C20?
RF
Signal at preselector
input TP1?
Check harmonic
filter (Components placed
between C192 - C193)
and antenna switch
Program filter to test
freqs. and check
varactor voltages
for both preselector
& postselector
No
No
No
No
No or
No
Check bias
voltages on
Q29
IF Signal
at L54?
Audio
Heard?
from L10 to
U401, pin 47
Check IF
spectrum at
U401, pin 47
No
Check
FGU
Chart
No
Check U401 voltages,
programming, and
10MHz clock VCO
components
No
Check IF filter
FL51. Make sure
LO is locked and
on frequency
VCO Q401. Check VCO O/P level;
Control Voltage at
Yes
Yes
Yes
Yes
Yes
Yes
Yes
O/P OK?
(on/off) while monitoring
postselector
Check preselector filter.
Weak RF
Yes
Yes
Yes
Yes
Yes
Trace IF signal
VHF Band :
0.8 - 4.5V
If voltages are not OK,
check U102, (PCIC)
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