©2008 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 6/3/10
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for if the synthesizer is suspected of not per-
forming properly.
a.
Check the output frequency of the
vco buffer with a frequency counter.
b.
Check the lock detector at either pad
of R25 with a dc voltmeter. (7Vdc locked,
0Vdc unlocked).
c.
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.
d.
Check the operating voltage and bias
on the vco and buffer.
e.
Check the amplified 10.240 MHz
TCXO signal at pin 1 of the synthesizer ic. Be
very careful not to short adjacent pins of the
ic. A scope should show several volts p-p at
10.240 MHz.
f.
Check the oscillator at pin 1 of micro-
controller ic U1 with a scope. There should be
a strong ac signal (several volts p-p) at the os-
cillator frequency.
g.
The data, clock, and /enable lines be-
tween the microcontroller and synthesizer ic’s
should show very brief and fast activity, send-
ing 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
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 con-
junction 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 dipswitch and E6-E7 jumper
settings to be sure you have the correct fre-
quency information going to the microcontrol-
ler.
Audio.
You can check the following levels with a
scope.
a.
The audio input must be 40mV p-p or
greater at input E2 for full 5kHz deviation.
b.
Gain control R26 sets the gain of am-
plifier U5a. Provided enough gain and audio
input, the limiter output will provide about 1V
p-p at the input of deviation pot R35.
c.
The output of active filter U5b is a
maximum of about 2V p-p if the limiter is
driven into limiting. This assumes a test signal
at about 1000 Hz. This ac signal should be rid-
ing on a dc center voltage of about +4.4Vdc.
That is what should be applied to the modula-
tor diode through R42.
d.
You can also check for the presence
of the proper dc voltages on the op amps,
which use bias voltages of +4Vdc and +2.2Vdc.
Refer to the power supply circuits on the sche-
matic diagram.
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
at the right side of the board. 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 exciter, such as motors.
Excessive noise on the dc power supply
which operates the exciter can cause noise to
modulate the signal. Various regulators and
filters in the exciter are designed to minimize
sensitivity to wiring noise. However, in ex-
treme 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 exciter.
Other usual practices for mobile installa-
tions are recommended, such as tying the +
power and ground return lines directly to the
battery instead of using cigarette lighter sock-
ets or dash board wiring.
To varying degrees, whine from the 5kHz
reference frequency can be heard on the sig-
nal under various circumstances. If the tuning
voltage required to tune the vco on frequency
is very high or low, near one extreme, the
whine may be heard. This can also happen
even when the tuning voltage is properly near
the 2Vdc center if there is dc loading on the
loop filter. Any current loading, no matter
how small, on the loop filter causes the phase
detector to pump harder to maintain the tun-
ing voltage. The result is whine on the signal.
Such loading can be caused by connecting a
voltmeter to TP1 for testing, and it can also be
caused by moisture on the loop filter compo-
nents.
Typical Dc Voltages.
The following dc levels were measured
with a sensitive dc voltmeter on a sample unit
with 13.6 Vdc B+ 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 in-
dicated. Voltages are measured with the ex-
citer operating and fully tuned to provide
normal output. Note that meter probe must
have a 10 meg
Ω
or similar resistor in probe to
isolate from RF signals. Even then, the type of
meter and probe has an effect on the readings
taken on points where RF is present.
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. Also, some
pins are not used in this design, and you can
generally not be concerned with making
measurements on them.
Typical Audio Voltages.
Table 6 gives rough measurements of au-
dio voltages which may be measured with a
sensitive voltmeter or an oscilloscope when
an audio source with a tone about 1000 Hz is
connected and modulating to full 5 kHz devia-
tion. Measurements given were taken with an
oscilloscope with audio gain and deviation
controls fully cw and sufficient audio input
applied for full deviation of the RF signal.
Measurements are typical of what might be
indicated during a sustained whistle or with an
audio signal generator. Of course, readings
Table 3. Typical Test Point Voltages
TP1
Normally set at 2V
TP2
Roughly 0.3V
Note: These can vary considerably without
necessarily indicating a problem.
Table 4. Typical Xstr DC Voltages
STAGE
E
B
C
Q1 vco
1.5
2.2
7.2
Q2 buffer
0
0.75
5
Q3 dc filter
7.2
7.8
8
Q4 pre-driver
0.3
0.3
13.6
Q5 driver
0
0.2
13.6
Q6 pwr ampl
0
0
13.6
Limiter R33
0.43
Limiter R34/diodes
1
Limiter R35
0.43
Figure 5. Typical IC DC Voltages
U1-1
4
U1-2
4
U2-1
2.7
U2-10
2.7
U2-2
5v locked
U2-11
2.7
(pulses unlocked)
U2-12
5
U2-3
5 *
U2-13
3.3 *
U2-4
5 *
U2-14
5
U2-5
5
U2-15
*
U2-6
0-5 (2V tuned) U2-16
*
U2-7
0
U2-17
5
U2-8
4.8
U2-18
0
U2-9
5 *
U2-19
0
* = pin not used
U2-20
2.7
U5-1
4
U5-8
7
U5-2
4.5
U5-9
4
U5-3
4
U5-10
4.7
U5-4
8
U5-11
0
U5-5
2.2
U5-12
5
U5-6
2.4
U5-13
5
U5-7
4.5
U5-14
5
Table 6. Typical Audio Voltages
Test Point
mV p-p
E2 AF input
40(min)
U5-1
1000
D5 cathode
1000
U5-7
2000
