February 14, 2012 6816985H01-F
6.1-16 VHF Troubleshooting Charts: VHF Receive RF Troubleshooting Chart
6.1.17VHF Receive RF Troubleshooting Chart
MAEPF-27342-O
NOTE: INJECT MODULATED ON CARRIER
FREQUENCY SIGNAL AS REQUIRED.
NOTE: The presence or absense of RF signals is generally not enough
to diagnose sensitivity degradations. It is best to compare signal levels
at each decision node, to the same circuit in working radio.
Bad SINAD
No Recovered
Audio
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
VCO Q401. Check VCO O/P level;
Disconnect R418 and verify that
a 0.5V to 5V control voltage VCO
will sweep about the IF frequency
Remove SH401.
Check 2nd LO
Control Voltage at
U401, pin 38
Inject RF
into J101
Is it
symmetric
centered?
Yes
No
Yes
Yes
Yes
Yes
No
Does
the pulse
frequency
vary?
See VOCON
chart, RX
audio problems
Yes
Yes
No
1st LO
O/P OK?
Locked?
Inject -47 dBm into J101 and
toggle signal continuously
(on/off) while monitoring
SSI_D data pulse
RF Signal
at C51?
Check
postselector.
Check voltage
at C46
RF
Signal postselector
input C41?
RF Signal
at C20?
RF
Signal at preselector
input C1?
Check harmonic
filter C177 - C180
and antenna switch
Check preselector filter.
Program filter to test
freqs. and check
varactor voltages
for both preselector
& postselector
Spray or Inject 1st IF
into XTAL Filter FL51
No
No
No
No
No or
Weak RF
No
Yes
Yes
Check bias
voltages on
Q29
Yes
Yes
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
components L411,
L412, C476.
Check for 3Vdc
at pin 48
Yes
Yes
Check
FL51
No
No
VHF Band :
0.8 - 4.5V
If voltages are not OK,
check U102, (PCIC)
IF Signal
at L54?
Yes
Audio
Heard?
Trace IF signal
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
VHF2 1st IF:44.85MHz
VHF2 2nd IF:2.25MHz
VHF2 2nd LO:42.6MHz
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