©1994 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 10/2/07
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cold
solder
joints,
and
solder
splashes. Another common trouble is
blown transistors and ic's due to re-
verse polarity or power line transients.
Remember if you encounter problems
during initial testing that it is easy to
install parts in the wrong place. Don't
take anything for granted. Double
check everything in the event of trou-
ble.
If the receiver is completely dead,
try a 10.700 MHz signal applied to
TP5 (left side of coil L11) with a coax
cable clip lead. You should be able to
hear the quieting effect of a 20-30 uV
carrier at 10.700 MHz. You can also
connect the 10.700 MHz clip lead
through a blocking capacitor to vari-
ous sections of the crystal filter to see
if there is a large loss of signal across
one of the filter sections. Also, check
the 10.245 MHz oscillator with a
scope or by listening with an hf re-
ceiver or service monitor.
A signal generator on the channel
frequency can be injected at various
points in the front end. If the mixer is
more sensitive than the rf amplifier,
the rf stage is suspect. Check the dc
voltages looking for a damaged fet.
If audio is present at the VOLUME
control but not at the speaker, the
audio ic may have been damaged by
reverse polarity or a transient on the
B+ line. If no audio is present on the
volume control, the squelch circuit
may not be operating properly. Check
the dc voltages, and look for noise in
the 10 kHz region, which should be
applied to noise detector CR2/CR3
with no input signal. (Between pins
12 and 13 of U2 is an op amp active
filter tuned to 10 kHz.)
Typical Dc Voltages.
The dc levels in Table 1 were mea-
sured with an 11 megohm fet vm on
a sample unit with 13.6 Vdc B+ ap-
plied. All voltages may vary con-
siderably
without
necessarily
indicating trouble. The chart should
be used with a logical troubleshooting
plan. All voltages are positive with
respect to ground except as indi-
cated. Voltages are measured with
no signal applied but oscillator run-
ning properly and with squelch open
unless otherwise specified.
Typical Audio Levels.
Following are rough measure-
ments of audio circuits, using an os-
cilloscope. Measurements were taken
under two conditions. The first is
with no input signal, just white noise
so conditions can be reproduced eas-
ily. The second is with an input sig-
nal having a 1000 Hz tone modulated
±3kHz.
TABLE 1. TYPICAL DC VOLTAGES.
Xstr
E(S)B(G1)C(D) G2
Q1
0
0
8
4
Q2
1.5
0
8
4
Q3
4
4
8
-
Q4
1.5
0
8
-
Q5 Squelched 0
0
0.7
-
Q5 Unsq
0
0.7
0
-
Q6
1.5
0
8
-
U1 1
6
8
14
7
0
7 13.6
U2 1
2
3
4
5
6
7
8
7.9 7.2
7.2
8
1.1 1.1 1.1
8
U2 9 10 11 12
13
14
4.4
3.5 1 To 8 2.5
2.5 .3 To .7
U2
.
15
16 17 18
0(Sq), 7.2(Unsq)
0
0
2.3
TABLE 2. TYPICAL AUDIO LEVELS (V p-p)
Measured At
On Noise
On Tone
U2 pin 10 (discrim.):
4
2
E5 (discriminator out):
3
1.5
U2 pin 13 (noise ampl):
4
0.4
Top of volume control:
0.8
0.2
Across 8
Ω
spkr term:
12
10
