©2009 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 7/28/11
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+3.3Vdc at test point TP-3.
➠
If you can't find the signal at all, tune
your signal generator up and down the band
slightly. Also check that oscillator is peaked as
per step c. If your crystal has the wrong load
correlation or is slightly out of tolerance, you
may be able to compensate by changing the
value of C16 so C15 can net the crystal on fre-
quency. The piston capacitor tuning range is
restricted to provide best frequency stability;
so sometimes it may be necessary to change
the fixed capacitor. The proper adjustment
results in +3.3Vdc, the same as preset for the
exact 10.700 MHz i-f frequency earlier.
Maximum capacitance (lowest frequency)
occurs with the piston screwed in all the way,
and minimum capacitance (highest frequency)
is with the piston all the way up. Be careful
not to completely remove the piston. If the
piston screw becomes a little tight (squeaky),
you can apply a small amount of silicone oil to
the threads.
Note:
There are two methods of adjusting
the mixer and front end. One is to use an fet
voltmeter with test point TP-2, which is the
top lead of R13. The voltage at this point is
proportional to the amount of noise detected
in the squelch circuit; so it gives an indication
of the quieting of the receiver. With SQUELCH
control fully ccw, the dc voltage at TP-2 varies
from -0.5 Vdc with no signal (full noise) to
+0.8 Vdc with full quieting signal.
The other method is to use a regular pro-
fessional SINAD meter and a tone modulated
signal.
In either case, a weak to moderate signal
is required to observe any change in noise. If
the signal is too strong, there will be no
change in the reading as tuning progresses; so
keep the signal generator turned down as re-
ceiver sensitivity increases during tuning.
If you use TP-2 with a voltmeter, the signal
can be modulated or unmodulated. If you use
a SINAD meter, the standard method is a 1000
Hz tone with 3 kHz deviation.
g. Connect fet dc voltmeter to TP-2 (top
lead of R13). Set signal generator for rela-
tively weak signal, one which shows some
change in the dc voltage indication at TP2. Al-
ternately peak multiplier coils L8 and L9 until
no further improvement can be made.
h. Likewise, alternately adjust rf amplifier
coils L1-L4 until no further improvement can
be made.
When properly tuned, sensitivity should
be about 0.15 to 0.2 µV for 12 dB SINAD.
Mixer output transformer L5 normally
should not be adjusted. It is usually set ex-
actly where it should be right from the fac-
tory. The purpose of the adjustment is to
provide proper loading for the crystal filter,
and if misadjusted, ripple in the filter response
will result in a little distortion of the detected
audio. If it becomes necessary to adjust L5,
tune the signal generator accurately on fre-
quency with about 3kHz fm deviation using a
1000 Hz tone. In order of preference, use ei-
ther a SINAD meter, an oscilloscope, or just
your ears, and fine tune L5 for minimum dis-
tortion of the detected audio.
TROUBLESHOOTING.
The usual troubleshooting techniques of
checking dc voltages and signal tracing work
well in troubleshooting the receiver. DC volt-
age charts and a list of typical audio levels are
given to act as a guide to troubleshooting. Al-
though voltages may vary widely from set to
set and under various operating and mea-
surement conditions, the indications may be
helpful when used in a logical troubleshooting
procedure.
The most common troubles in all kits are
interchanged components, cold solder joints,
and solder splashes. Another common trou-
ble is blown transistors and ic's due to reverse
polarity or power line transients. Remember
if you encounter problems during initial test-
ing that it is easy to install parts in the wrong
place. Don't take anything for granted. Dou-
ble check everything in the event of trouble.
Signal Tracing.
If the receiver is completely dead, try a
10.700 MHz signal applied to TP-4 (the top
lead of R3), using coax clip lead. Connect coax
shield to pcb ground. Set level just high
enough for full quieting. At 2 uV, you should
notice some quieting, but you need something
near full quieting for the test.
You can also connect the 10.700 MHz clip
lead through a blocking capacitor to various
sections of the crystal filter to see if there is a
large loss of signal across one of the filter sec-
tions. Also, check the 10.245 MHz oscillator
with a scope or by listening with an hf receiver
or service monitor.
A signal generator on the channel fre-
quency 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,
which can occur due to transients or reverse
polarity on the dc power line. Also, it is possi-
ble to have the input gate (gate 1) of the rf
amplifier fet damaged by high static charges
or high levels of rf on the antenna line, with
no apparent change in dc voltages, since the
input gate is normally at dc ground.
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 tran-
sient on the B+ line. This is fairly common
with lightning damage.
If no audio is present on the volume con-
trol, the squelch circuit may not be operating
properly. Check the dc voltages, and look for
noise in the 10 kHz region, which should be
present at the top lead of R14 (U1-pin 11)
with no input signal. (Between pins 10 and 11
of U1 is an op-amp active filter tuned to 10
kHz.)
Current Drain.
Power line current drain normally is about
45mA with volume turned down or squelched
and up to 120 mA with full audio output.
If the current drain is approximately 100
mA with no audio output, check to see if volt-
age regulator U3 is hot. If so, and the voltage
on the 8V line is low, there is a short circuit on
the +8Vdc line somewhere and U3 is limiting
the short circuit current to 100mA to protect
the receiver from further damage. If you clear
the short circuit, the voltage should rise again.
U3 should not be damaged by short circuits on
its output line; however, it may be damaged
by reverse voltage or high transient voltages.
Test Point Indications.
The following measurements are typical of
those found at the three built-in test points
used for alignment. They can vary considera-
bly without necessarily indicating a problem,
however; so use with other findings to analyze
problems, don't jump to conclusions.
Oscillator Test Point 1
Approx. +1 to 2 Vdc with osc running and
output tuned circuits aligned. Varies as
L6 and L7 are aligned. 0Vdc with oscilla-
tor not running or coils not properly
aligned.
Not used on 50 MHz model.
Signal Strength Test Point 2
With SQUELCH control fully ccw, the dc
voltage at TP-2 varies from -0.5 Vdc with
no signal (full noise) to +0.8 Vdc with full
quieting signal.
With the SQUELCH control set to where
the squelch just closes, the dc voltage at
TP-2 varies from about -0.55V with no sig-
nal to +0.75 with full quieting signal
Discriminator Test Point 3
Varies with frequency of input signal. Volt-
age at this point normally is adjusted for
+3.3Vdc with a signal exactly on frequency.
Can vary a little without being a problem.
Typical Dc Voltages.
The following dc levels were measured
with an fet voltmeter on a sample unit with
+13.6 Vdc power applied. All voltages may
vary considerably without necessarily indicat-
ing trouble. The chart should be used with a
logical troubleshooting plan.
All voltages are positive with respect to
ground except as indicated. Voltages are
measured with no signal applied but with crys-
tal(s) installed and oscillator(s) running prop-
erly and with squelch open unless otherwise
specified.
