©1998 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 4/2/03
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channel frequency and other opera-
tional characteristics of the synthe-
sizer are applied to synthesizer U2 by
microcontroller U1. Everything the
synthesizer needs to know about the
band, division schemes, reference fre-
quency, and oscillator options is gen-
erated by the controller. Information
about the base frequency of the band
the Receiver is to operate on and the
channel within that band is calculated
in the controller based on information
programmed in the eprom on the con-
troller and on channel settings done
on dip switch S1 and jumper E6-E7.
Whenever the microcontroller boots at
power up, the microcontroller sends
several bytes of serial data to the syn-
thesizer, using the data, clock, and
/enable lines running between the two
ic’s.
+13.6Vdc power for the Receiver is
applied at E1. Audio output amplifier
U5 is powered directly by the
+13.6Vdc. All the other stages are
powered through voltage regulators
for stability and to eliminate noise.
U6 is an 8Vdc regulator to power IF
amplifier U4, RF amplifier Q6, mixer
Q7, and the vco, buffer, and phase de-
tector in the synthesizer. Additional
filtering for the vco and buffer stages
is provided by capacitance amplifier
Q4, which uses the characteristics of
an emitter follower to provide a very
stiff supply, eliminating any possible
noise on the power supply line. Q8
provides a stiff +5Vdc supply for the
frequency synthesizer and microcon-
troller, which are both low current
consumption CMOS devices.
TROUBLESHOOTING.
General.
The usual troubleshooting tech-
niques of checking dc voltages and
signal tracing with an RF voltmeter
probe and oscilloscope will work well
in troubleshooting the R304. DC volt-
age charts and a list of typical audio
levels are given to act as a guide to
troubleshooting. Although voltages
may vary widely from set to set and
under various operating and measure-
ment 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 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.
Current Drain.
Power line current drain normally
is about 60 mA with volume turned
down or squelched and up to 200 mA
with full audio output.
If the current drain is approxi-
mately 100 mA with no audio output,
check to see if voltage regulator U6 is
hot. If so, and the voltage on the 8V
line is low, there is a short circuit on
the +8Vdc line somewhere and U6 is
limiting the short circuit current to
100mA to protect the receiver from
damage. If you clear the short circuit,
the voltage should rise again. U6
should not be damaged by short cir-
cuits on its output line; however, it
may be damaged by reverse voltage or
high transient voltages.
Audio Output Stage.
Note that audio output ic U5 is de-
signed to be heatsunk to the pc board
through the many ground pins on the
ic. When running moderately low au-
dio levels as most applications re-
quire, it is no problem to use an ic
socket; so we have provided one for
your convenience. If you will be run-
ning high audio levels, check to see if
the ic is getting hot. If so, you should
remove the ic socket, and solder the
LM-380N-8 ic directly to the board for
better heatsinking.
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. This is fairly common with
lightning damage.
If no audio is present on the vol-
ume 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 pre-
sent at U1-pin 11 with no input sig-
nal. (Between pins 10 and 11 of U1 is
an op-amp active filter tuned to 10
kHz.)
RF Signal Tracing.
If the receiver is completely dead,
try a 10.700 MHz signal applied to
TP-5 (the left side of coil L9), using
coax clip lead. Connect coax shield to
pcb ground. Set level just high enough
for full quieting. At 20µV, you should
notice some quieting, but you need
something near full quieting for the
test, which requires about 200µV.
You can also connect the 10.700
MHz clip lead through a .01µf block-
ing capacitor to various 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 receiver 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,
which can occur due to transients or
reverse polarity on the dc power line.
Also, it is possible 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 volt-
ages, since the input gate is normally
at dc ground.
Synthesizer Circuits.
Following is a checklist of things to
look for if the synthesizer is suspected
of not performing properly.
a. Check the output frequency of
the vco buffer with a frequency coun-
ter.
b. Check tuning voltage at TP1.
It should be about +4Vdc. Actual
range over which the unit will operate
is about +1Vdc to just under +8Vdc.
However, for optimum results, the vco
should be tuned to allow operation at
about +4Vdc center voltage.
c. Check the operating voltage
and bias on the vco, doubler, and
buffer.
d. Check the 10.240 MHz TCXO
signal at pin 1 of the synthesizer ic
(actually best to check at pad next to
pin 1; avoid trying to probe surface
mount ic leads which are close to-
gether). A scope should show strong
signal (several volts p-p) at 10.240
MHz.
e. Check the oscillator at pin 1 of
microcontroller ic U1 with a scope.
There should be a strong ac signal
(several volts p-p) at the oscillator fre-
quency.
f. The data, clock, and /enable
lines between the microcontroller and
synthesizer ic’s should show very brief
and very fast activity, sending data to
the synthesizer ic shortly after the
power is first applied or a dip switch
setting is changed. Because this hap-
pens very fast, it can be difficult to see
on a scope. Use 100µSec/div,
5Vdc/div, and normal trigger on ris-
ing pulse.