©1998 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 12/11/02
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+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 Q4, mixer
Q5, and the vco, buffer, and phase de-
tector in the synthesizer. Additional
filtering for the vco and buffer stages
is provided by capacitance amplifier
Q3, which uses the characteristics of
an emitter follower to provide a very
stiff supply, eliminating any possible
noise on the power supply line. Q7
provides a stiff +5Vdc supply for the
frequency synthesizer and microcon-
troller, which are both low current
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 R301. 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 55 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 the top lead of R27 (U1-pin 11)
with no input signal. (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-4 (the top lead of R19), using coax
clip lead. Connect coax shield to pcb
ground. Set level just high enough for
full quieting. At 1 µV, you should no-
tice 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 ca-
pacitor to various sections of the crys-
tal 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.
c. Check tuning voltage at TP2.
It should be about +4.0Vdc. 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 +4.0Vdc center voltage.
d. Check the operating voltage
and bias on the vco and buffer.
e. Check the 10.240 MHz oscilla-
tor or TCXO at pin 1 of the synthe-
sizer ic (actually best to check at top
lead of R3 or the pad which it would
be connected to; avoid trying to probe
surface mount ic leads which are
close together). A scope should show
strong signal (several volts p-p) at
10.240 MHz.
f. 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.
g. 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.
h. Check the microcontroller to
see that its /reset line is held low
momentarily when the power is first
applied. C1 works in conjunction
with an internal resistor and diode in
the ic to make C1 charge relatively
slowly when the power is applied. It
should take about a second to charge
up.
i. Check the switch and E6-E7
jumper settings to be sure you have
the correct frequency information go-
ing to the microcontroller.
j. If you have a scope or spec-
trum analyzer, you can check the
output pin of the divide by 64 presca-
ler at pin 13 of U2. There should be a
strong signal (several volts p-p) at
about 2.25 MHz. If this signal is ab-
sent, there may not be sufficient level
of sample signal from the buffer at U2
pin 11.
Be careful not to short adja-