©1996 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 1/19/00
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the proper tone.
If you wish to have your repeater
transmit a CTCSS tone, do the follow-
ing.
a. Add a TD-5 module near the
exciter. In the REP-200 Repeater, the
TD-5 usually is mounted in the exciter
compartment, on the shield just to the
right of the exciter. Note that the
TD-5 cannot do transmit and receive
modes at the same time; so a separate
TD-5 must be used for transmit op-
eration.
b. Connect a hookup wire from
E7 on the TD-5 to the input of the
modulator stage in the exciter. The
exact circuit to connect to must be
determined for the particular exciter
you have.
For our TA51 (vhf) Exciter, connect
to the loop at the top of R22. This in-
jects the audio directly into the low
pass filter at the input of the modu-
lator.
For our TA451 (uhf) Exciter, con-
nect a 47K resistor from E7 on the
TD-5 over to the lead of C11 closest to
deviation pot R15. This also injects
the tone into the modulator, but the
47K resistor prevents loading of the
circuit in the uhf e xciter.
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 os-
cillator rf signal from getting out into
the audio circuits.
c. You must also connect B+ to
E2 on the TD-5 as explained earlier.
d. To set the TD-5 for transmit
mode, connect a jumper between E10
and E12. If you want to be able to
turn the transmitted CTCSS tone off,
you can connect a switch to discon-
nect the ground when you do not
want the tone.
Transmit Level Adjustment.
TONE LEVEL control R2 on the
TD-5 is adjusted for the de sired de-
viation of the subaudible tone. The
TD-5 has a resistor (R3) in series with
its output to avoid loading down the
exciter audio circuits. The tone out-
put level is adjustable up to 2V p-p
(710 mV rms) into a high impedance
circuit. The level will be somewhat
lower with a load.
If TONE LEVEL control R2 pr o-
vides too high a level, even with it ad-
justed to a low setting, additional re-
sistance can be added in series with
output terminal E7 or R3 can be made
larger.
Normally, the level of the tone
should be set for about 300 Hz de-
viation. That is sufficient for any
normal de coder to detect. (Good de-
coders normally decode anything over
50-100 Hz deviation.) Sometimes
people want to use much higher levels
on tones, and not only is this unne c-
essary but it causes the tone to be
heard as a buzz on the voice signal; if
set high enough, may even have a de t-
rimental effect on a touch-tone com-
mand system.
TROUBLESHOOTING.
GENERAL.
The circuit is relatively simple.
First, reread the theory of operation
on page 1. Because all major func-
tions of the unit are performed in ic
U1, troubleshooting is pretty straight
forward. An oscilloscope usually is
essential in troubleshooting, although
it may be possible to get by with just a
voltmeter.
CURRENT CONSUMPTION.
The TD-5 can operate on any
source of +7 to 15Vdc, and it normally
draws about 6 mA.
AUDIO LEVELS.
The transmit tone output level is
adjustable up to 2V p-p (710 mV rms)
into a high impedance circuit. The
level will be somewhat lower with a
load because of the resistor in series
with the audio in the TD-5.
Receive input sensitivity is such
that any tone level between 125-2000
mV p-p is acceptable (45-710 mV
rms). Output of the receive filter and
switch circuit at E4 should be about
the same as the input level.
VOLTAGE TESTS.
Table 2 gives dc and oscillator
voltages for the main ic (U1) measured
on a sample unit. These readings are
typical but may vary somewhat. As
always, such data should be used in
conjunction with a logical troubl e-
shooting procedure to avoid jumping
to conclusions if a voltage is a little
different in your unit. Some are de-
pendent on being in either transmit or
receive mode. Note that pins used as
logic inputs are not shown.
TALK-OFF PROBLEMS.
The response time and deresponse
time of the decoder circuit is set by
R4, R5, and C6. The purpose of this
integrator is to prevent bouncing on
and off any time a short duration of
low frequency audio is detected. In
commercial operation, the circuit
would normally be set up to respond
in 250 mSec. However, this short re-
sponse time requires that the trans-
ceivers have special audio filters
which prevent any voice audio from
ever being transmitted in the sub-
audible tone range.
Because amateur radio transceiv-
ers rarely have such filters, many are
capable of transmitting voice signals
with audio components easily getting
down into the 100 Hz range of the
usual CTCSS tone frequencies used in
repeaters. This can cause two prob-
lems: talk-off, which is voice audio
appearing out of phase with the
CTCSS tone and effectively canceling
it, and talk-on, which is voice audio
components appearing at the desig-
nated tone frequency, e.g., 100 Hz. In
the case of the latter, a man's voice
could occasionally trigger the CTCSS
decoder in a repeater, each time
causing the repeater to key up for 3-5
seconds until the tail drops.
Talk-on and talk-off can also be
caused by incorrect adjustment of the
modulation or the carrier frequency in
the transceiver. If the audio consis-
tently bangs up against the limiter be-
cause the mic gain is too high, the
squared off audio can cause low fre-
quency components which trigger the
CTCSS decoder. The answer to this
problem is for the user to properly
adjust the mic gain and limiter in his
radio. Likewise, off frequency opera-
tion of the transceiver causes distor-
tion by having modulation sidebands
extend over the sharp edge of the i-f
filter in the receiver.
To minimize these problems for
amateur radio service, we have chosen
values for the integrator circuit which
provide a slightly longer response
time, about 500 mSec, making it
much less likely a voice component
could be long enough to cause a re-
sponse. Of course, this may cause a
noticeable delay in pickup time when
someone keys the repeater.
If you prefer to shorten the re-
sponse time, you can lower the value
either of C6 or of R4 and R5, perhaps
cutting them in half. You can try the
new values for awhile and see if the
change is acceptable.
