©1994 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 1/31/00
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feedthrough capacitors. Just run a
short length of wire directly to the ex-
citer at the connection point given in
step c.
e. If you need to connect the tone
to another type of transmitter, use a
similar approach. It is important to
inject the tone into the modulator of
the transmitter in a way which by-
passes the speech processing circuits
for the microphone, but at the same
time does not load down the audio
coming from those circuits. Normally,
there will be a low-pass filter which
allows the audio to be injected into the
modulator while keeping the oscillator
rf signal from getting out into the
audio circuits.
Once the mic gain control in the
exciter is adjusted for proper deviation
of the repeater, XMIT LEVEL control
R11 on the TD-3 is adjusted for the
desired deviation of the subaudible
tone. The TD-3 has a resistor (R13) in
series with its output to avoid loading
down the exciter audio circuits. If
XMIT LEVEL control R11 provides
too high a level, even with it adjusted
to a low setting, additional resistance
can be added in series with output ter-
minal E5 or R13 can be made larger.
Normally, the level of the tone
should be set for about 300 Hz de-
viation. That is sufficient for the de-
coder to detect. Sometimes people
want to use much higher levels on
tones, and not only is this unnecessary
but it causes the tone to be heard as a
buzz on the voice signal, and if set
high enough, may even have a detri-
mental effect on a touch-tone com-
mand system.
ALIGNMENT.
The following assumes all of the
installation procedures have been done
prior to alignment.
The tone frequency can be set in
either of two ways. If a frequency
counter is available, connect it through
a resistor to Test Point 1, which is a
loop formed in the right side of R8. A
resistor of 10K to 100K normally must
be connected in the pick up line at the
TD-3 to minimize loading and noise
pickup to prevent erratic readings.
For an accurate adjustment, it is nec-
essary to remove any possible audio
input to the decoder ic by turning R4
fully ccw. With the lowest frequency
range, adjust FREQ pot R6 for the de-
sired tone frequency. If R6 cannot be
adjusted to the desired frequency, it
may be necessary to change the value
of R7 as explained in the section after
ASSEMBLY on page 1.
The alternate method of adjusting
frequency is to simply apply a re-
ceived signal with the proper subaudi-
ble tone present and adjust FREQ pot
R6 to the center of the adjustment
range in which the ic responds to the
tone. Response is indicated by a dc
voltmeter connected to E3, which has
about 0.6 Vdc with no tone detected
and ground with proper tone detection.
To locate the exact center of the set-
tings which allow response, the sensi-
tivity pot may be reduced to the point
where the TD-3 responds only over a
very narrow range of settings.
Another way to check for response
of the TD-3 is to monitor the COS
voltage at the feedthrough cap on the
receiver enclosure.
Since the response bandwidth
(range of tone frequencies which will
be detected) is proportional to the
level of tone applied to U2, it is desir-
able to adjust the level for optimum
tone acceptance. If R4 is set too sen-
sitive (cw), then the bandwidth will be
fairly wide, up to three times as wide
as optimum. If R4 is set too insensi-
tive (ccw), then some stations may be
a little too weak or off the tone fre-
quency a little too far to access the re-
peater.
Normally, a system should be run
with about 300 Hz tone deviation on
the transmitters (mobiles using the re-
peater). Obtain a weak signal with a
tone a little below the low end of ex-
pected range of levels to be encoun-
tered in the system. For example, at
our factory, we usually apply an rf
signal of about 0.15 uV with about a
200 Hz tone deviation. Adjust
SENSITIVITY pot R4 cw from no tone
level to a level just high enough so the
TD-3 responds. Then, any signal you
normally expect to encounter will be
able to access the repeater.
We normally use a service monitor
with a subaudible tone synthesizer and
a deviation monitor. However, you
don't need to have a fancy signal gen-
erator. Just make sure the signal used
for testing has its tone encoder on the
proper frequency and at a relatively
low level of deviation. You don't
want to adjust the TD-3 to match a
tone which is off frequency. If you
use a strong rf signal or a normal tone
level rather than a weak tone as we
suggested, then adjust R4 just a little
further cw than the point at which the
decoder first responds. Then, weak
signals will be able to access the re-
peater too.
The only other adjustment is the
transmit audio level (R11) which is
discussed on page 2 under the section
on Option to Transmit Encoded Tone.
If you do not use this option, R11 can
be left in any position.
TROUBLESHOOTING.
The circuit is relatively simple.
With the theory of operation presented
on page 1 and the following list of dc
and signal levels, troubleshooting
should be straight forward. An oscil-
loscope is usually essential in trouble-
shooting, although it may be possible
to get by with just a voltmeter.
Unless otherwise specified, the fol-
lowing test data is taken with 13.6Vdc
at the B+ input (8Vdc out of the volt-
age regulator) and a 100 Hz tone ap-
plied from one of our receivers with
300 Hz tone deviation. That results in
about 200 mV p-p tone level at input
terminal E1. Current drain of the unit
is about 20 mA.
TABLE 4. TYPICAL TEST VOLTAGES.
Location Typical Indication
E1
4Vdc with 200mV p-p tone
Q1-base
4Vdc with 200mV p-p tone
Q1-coll
8 Vdc
Q1-emit
3.3Vdc & 200mV p-p tone
U2-1
5 Vdc
U2-2
5.8Vdc
U2-3
2Vdc (tone 75mV p-p min to be
able to decode)
U2-4
8Vdc
U2-5
7V p-p square wave centered at
3.7Vdc
U2-6
2V p-p triangle wave centered at
4 Vdc
U2-8 &
0Vdc with tone received &
Q2-base
1.2 Vdc with tone absent
Q3-base
1V p-p rounded triangle wave
centered at 1.3Vdc
Q3-emit
Same waveform at 0.6Vdc
Q3-coll
3V p-p rounded triangle wave
centered at 5.8Vdc
PARTS LIST.
Ref # Value (marking)
C1
2.2 uf electrolytic
C2
47 uF electrolytic