©1999 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 2/4/03
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common "key" used with all com-
mands, and the fourth digit being a
function identifier. As soon as the
fourth digit is received, the selected
function is performed.
If some or all of the functions have
been modified for operation with a
smaller "key" or no "key" at all, then
operation will be different for those
functions. For security, the entire
command sequence must be sent
within about five seconds or a timer
will reset and the sequence must be
started from scratch.
Note that the "key" timer will keep
the key circuit enabled for about five
seconds once it is tripped. Therefore,
if more than one command is to be
sent in a short period of time, only the
final digit of the second and following
commands need be sent. If you re-
send the "key" digits during the five
second period, they will be taken as
function identifier digits and not "key"
digits and may cause erroneous op-
eration.
For example, using the commands
in our example, Table 1, if we want to
turn the autopatch on and then off
again within the five seconds, we
would send "* 1 2 3" to turn it on and
just "#" to turn it off, since the "* 1 2"
is still "in memory", so to speak be-
cause the "key" timer is still on.
TROUBLESHOOTING.
Tracking down trouble is fairly
straightforward. The
Theory of Opera-
tion
section describes the signal path
and what each circuit does. The only
significant voltages are CMOS hi's
(near +5V) and lo's (near ground) as
marked on the schematic by thelittle
pulse symbols. The only exception is
the latch output transistors, which
have about +0.7V at the base when
turned on. The collector is at ground
when conducting and open circuited
when off.
The operating voltage of the unit is
+10 to +15Vdc. Current drain nor-
mally is about 15mA, depending on
how many led's are lit, since they
draw more current than the other cir-
cuits.
Note that led brightness may vary
from one led to the next. To save
power and space on the board, the
led's are driven directly by the CMOS
ic's. In this mode, the ic's put out just
as much current as they can into
what looks like almost a short circuit
load. The amount of short-circuit
current varies from one chip to the
next, and so the brightness of the
led's vary. This should cause no con-
cern, since the function of the led is
simply to indicate whether the circuit
is on or off.
A logical troubleshooting proce-
dure would be to start by checking the
audio source to be sure valid tones of
proper level are being fed into the TD-
2. The range of acceptable levels is
100 mV to 2V peak-peak. Table 2
gives frequencies of touch tones. You
should check your tone pad if some of
the digits don't respond, to see if the
pad is sending tones on frequency. If
the U1 chip is not decoding touch
tones, check to see if its oscillator is
running and at the proper frequency.
You may need to use a 10:1 scope
probe for the frequency counter input
to keep from loading the oscillator cir-
cuit. The next step would be to check
the hexidecimal signals from U1 to U2
and check for a valid digit signal from
pin 15 of U1 (the valid digit led should
light).
Remember that the ic's are static
sensitive. You don't want to further
damage the board while troubleshoot-
ing. A ground wrist strap should be
worn when handling the ic's.
The next thing to check is the "key"
bus. Check to see if the output of
U6B at pin 13 goes hi when the "key"
digits are received. Next, check the
momentary outputs: R0, R1, A0, A1,
etc. Finally, check the 4013 latches to
see if they are responding and supply-
ing the required current to turn on the
output transistors. The led's can be
used as an indicator of latch status.
If falsing occurs with some com-
mands or the unit fails to respond,
you should check the twist of the in-
coming tones in addition to their fre-
quencies. Twist is the relationship
between the level of tones in the high
group to the level of tones in the low
group. There should be no more than
10 dB difference between the two tone
levels in any digit. That is about a 3:1
difference in voltage. Various factors
influence the twist of the tones, in-
cluding the tone pad at the transmit-
ter, coupling capacitor values in your
system, receiver de-emphasis, trans-
mitter pre-emphasis, and how hard
you drive the tones at the transmitter.
One problem with some transmitter
setups is that a ham will set his tone
level too high, trying to get full 5 kHz
deviation. The level is actually set so
high as to go into limiting. This may
cause the high and low tones to be
transmitted at the same level instead
of having the desired pre-emphasis. It
may also add distortion to the tones.
Then the receiver de-emphasis at the
other end causes the low tone to be at
a higher level than the high tone be-
cause the pre-emphasis at the trans-
mitter was wrong. You should
encourage system users to be conser-
vative in setting tone levels at their
transmitters.
Another cause of bad twist on
touch tones can inadvertently occur if
a sub-audible tone decoder is used in
the receiving system. The high-pass
filters supplied on sub-audible tone
decoder boards, usually connected in
series with the audio in the receiver to
get rid of buzz from sub-audible tones,
can severely degrade the levels of
lower frequency touch tones as well.
If you have such a board installed in
your receiver audio path, you might
want to check its effect on touch tone
twist. If a problem, you may want to
take your touch tones from a point in
the receiver unaffected by the high-
pass filter or even just not use the fil-
ter in your6 receiver. Generally, the
required level of deviation to make a
sub-audible tone system work is very
low, about 0.2 KHz or less, and some
people run the level much higher than
needed, which causes the buzz. Run-
ning the proper level may allow the fil-
ter to be removed with no great
problem.
Suggested Selective Calling Unit Circuitry