©2009 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 8/10/11
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limits the tuning range of D1. The tuning volt-
age is applied to D1 through a third-order low-
pass loop filter, which removes the 5kHz ref-
erence frequency from the tuning voltage to
avoid whine.
Serial data to indicate the desired channel
frequency and other operational characteris-
tics of the synthesizer are applied to synthe-
sizer U2 by microcontroller U1. Everything the
synthesizer needs to know about the band, di-
vision schemes, reference frequency, and os-
cillator options is generated by the controller.
+13.6Vdc power for the Receiver is ap-
plied at E1. Audio output amplifier U5 is pow-
ered directly by the +13.6Vdc. All the other
stages are powered t5V regulator U6
for stability and to eliminate noise. Additional
filtering for the vco and buffer stages is pro-
vided by capacitance amplifier Q1, which uses
the characteristics of an emitter follower to
provide a very stiff supply, eliminating any
possible noise on the power supply line.
TROUBLESHOOTING.
General.
The usual troubleshooting techniques of
checking dc voltages and signal tracing with an
RF voltmeter probe and oscilloscope will work
well in troubleshooting the R306. DC voltage
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 measure-
ment conditions, the indications may be help-
ful when used in a logical troubleshooting
procedure.
Current Drain.
Power line current drain normally is about
38 mA with volume turned down or squelched
and up to 100 mA with full audio output.
If the current drain is approximately 100
mA with no audio output, check to see if volt-
age regulator U6 is hot. If so, and the voltage
on the 5V line is low, there is a short circuit on
the +5Vdc 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 circuits 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 designed
to be heatsunk to the pc board through the
many ground pins on the ic.
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 U4-pin 11 with no input signal. (Be-
tween pins 10 and 11 of U4 is an op-amp ac-
tive filter tuned to 10 kHz.)
RF Signal Tracing.
If the receiver is completely dead, try a
10.700 MHz signal applied to TP-2. 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 blocking 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 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.
Synthesizer Circuits.
Following is a checklist of things to look
for if the synthesizer is suspected of not per-
forming properly.
a.
Check the output frequency of the
vco buffer with a frequency counter or spec-
trum analyzer.
b.
Check tuning voltage at TP1. It
should be about +2Vdc. Actual range over
which the unit will operate is about +1Vdc to
just under +5Vdc. However, for optimum re-
sults, the vco should be tuned to allow opera-
tion at about +2Vdc 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
together). A scope should show strong signal
(several volts p-p) at 10.240 MHz.
e.
The data, clock, and latch enable lines
between the microcontroller and synthesizer
ic’s should show very brief and very fast activ-
ity, sending data to the synthesizer ic shortly
after the power is first applied or a dip switch
setting is changed. Because this happens very
fast, it can be difficult to see on a scope. Use
1mSec/div, 5Vdc/div, and normal trigger on
rising pulse.
Microphonics, Hum, and Noise.
The vco and loop filter are very sensitive
to hum and noise pickup from magnetic and
electrical sources. Some designs use a
shielded compartment for vco’s. We assume
the whole board will be installed in a shielded
enclosure; so we elected to keep the size
small by not using a separate shield on the
vco. However, this means that you must use
care to keep wiring away from the vco circuit.
Having the board in a metal enclosure will
shield these sensitive circuits from florescent
lights and other strong sources of noise.
Because the frequency of a synthesizer
basically results from a free running L-C oscil-
lator, the tank circuit, especially L1, is very
sensitive to microphonics from mechanical
noise coupled to the coil. You should mini-
mize any sources of vibration which might be
coupled to the Receiver, such as motors.
Excessive noise on the dc power supply
which operates the Receiver can cause noise
to modulate the synthesizer output. Various
regulators and filters in the Receiver are de-
signed to minimize sensitivity to wiring noise.
However, in extreme cases, such as in mobile
installations with alternator whine, you may
need to add extra filtering in the power line to
prevent the noise from reaching the Receiver.
Typical Dc Voltages and Signal
Levels.
Tables which follow give dc levels mea-
sured with a sensitive dc voltmeter on a sam-
ple unit with 13.6 Vdc B+ applied. All voltages
may vary considerably without necessarily in-
dicating trouble. Signal levels at significant
points are also given. The charts should be
used with a logical troubleshooting plan. All
voltages are positive with respect to ground
except as indicated.
Use caution when measuring voltages on
the surface mount ic. The pins are close to-
gether, and it is easy to short pins together
and damage the ic. We recommend trying to
connect meter to a nearby component con-
nected to the pin under question.
Table 2. Typical Audio Voltages
Audio Test Point
Normal Level*
U4-9 (Discriminator)
1.5V p-p audio
E4 (Disc Output)
1V p-p audio
E1 (Repeater Output)
600mV p-p audio
U4-11
1.5V p-p noise
(noise ampl output)
CW lug Vol Cont R32
300mV p-p audio
U5-3 (af ampl input)
0 to 100mV p-p
(depends on
volume control)
U5-5 or E2
0 to 5V p-p audio
(speaker ampl output)
* Readings taken with strong input signal
with 1000Hz modulation at ±3kHz deviation.
