FM radio intercom
for motorbikes
Last month, we described the circuit and
gave the constructional details for our new
FM radio intercom. This month, we conclude
with the test and alignment details.
By JOHN CLARKE & GREG SWAIN
Before connecting power to the
intercom, you should first go back
over your work and check carefully
for wiring errors. Are all parts installed the right way around? Are
the parts values correct? Are there
any missed solder joints or solder
bridges? Some careful checking at
this stage may save a lot of frustration later on.
Once you are satisfied that there
are no errors, temporarily hook up
the loudspeaker and microphones,
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SILICON CHIP
then connect a suitable power supply. Either batteries (3 x 1.5V) or a
variable power supply set to 4.5V
can be used to power the unit.
Make sure that you don't set the
supply above 5V otherwise you
could damage the tr~nsmitter chip
(IC2).
Apply power and switch the intercom to the 'FM' mode. Now, using your multimeter, check that the
+ 4.5V supply is present on pin 4 of
IC1, pin 5 of IC3, and pin 9 of IC4.
If these voltages are all correct,
switch to the 'intercom' mode and
check that the transmit LED lights
when you speak into one of the
microphones (note: VR1 & VR2
should initially by set to midposition). Now check the voltage on
pin 15 of IC2 - it should be about
2.9V (not 2.5V as shown on last
month's circuit diagram).
Finally, check the voltages across
the supply pins of IC3 (pins 5 & 16)
and IC4 (pins 9 & 11) in the 'intercom' mode. You should get a
reading of about 0.7V when the
transmit LED is on and + 4.5V
when the transmit LED is off.
If you don't get the correct
readings switch off and check for
wiring errors. If you have problems,
there is a brief troubleshooting procedure at the end of the article, so
you should also refer to that.
L1
The alignment procedure involves making adjustments to the parts indicated here. Be sure to use an insulated
alignment tool when adjusting the coils (L1-L3). A metal screwdriver will affect the coil inductance and could
crack the ferrite slugs.
Alignment
If everything is OK, you are now
ready for the alignment procedure.
This can be done without the use of
any test equipment but does require
the use of a commercial FM
receiver (preferably one with a
digital readout).
You will also need a plastic alignment tool to adjust the slugs in the
coil formers (Ll-13). Do not use a
screwdriver here, otherwise you
could crack the fer rite slugs. A
metal screwdriver will also make
adjustments extremely difficult
since its presence affects the coil
inductance.
Make up a suitable alignment
tool if you don't already have one. A
plastic knitting needle can be easily
modified to fit the slot in the ferrite
slugs.
The slugs should be screwed into
11, 12 and 13 together with a short
piece of dental floss (available from
supermarkets and chemist shops)
or very fine elastic. This will ensure
that the slugs stay in position when
adjusted. Don't use wax to hold the
slugs in position - it won't work
and will make a mess.
Now for the alignment adjustments. The procedure is set out
below on a step-by-step basis to
make it easy to follow.
• Switch on your reference FM
receiver and tune it to a station
near the 88MHz end of the dial.
Rotate the dial on the intercom so
that the dial indicates the same frequency as the tuned station. Switch
the intercom to the 'FM' mode and
set the volume control to about 25%
of full rotation.
• Adjust 13 until the same station
is heard on the intercom. Now tune
the reference FM receiver to a station near the 108MHz end of the
band and set the tuning dial on the
intercom to the same indicated frequency. Adjust the trimmer
capacitor on the top of the tuning
gang (VCl) to receive the same station as the reference FM receiver.
Note that the trimmer requiring
adjustment is the one closest to the
front panel. The other trimmer is
not used.
• Retune the reference FM receiver to the station at the 88MHz.
end of the band and set the tuning
dial of the intercom to show the station frequency. Readjust 13 until
the station is heard. This done,
retune the reference FM receiver to
the station at the 108MHz end of
the band and set the intercom tuning dial to show the frequency of
this station. Readjust the tuning
gang trimmer so that the station is
heard.
The above procedure adjusts the
tuned circuit in the intercom's
receiver so that it covers the commercial FM band. If you like, you
can repeat the procedure once
more to ensure that everything is
correct.
• Tune the reference FM receiver
to 88.0MHz and check that trimpots
VRl & VR2 are at mid-position.
Select the 'intercom' mode and activate the transmitter by speaking
NOVEMBER 1989
65
The metal shield (at end of pen) must be installed before the alignment
procedure. Note how the O.lµF capacitor adjacent to L2 is installed.
into one of the microphones (the
transmit LED should come on). Adjust slugs 11 and 12 until your voice
is heard on the reference receiver.
Note that 12 sets the transmit
frequency while 11 forms part of a
bandpass filter. The two slugs
should be adjusted together so that
they are at about the same depth in
the slug core. Our prototypes tuned
to 88.0MHz when the slugs were
about 113rd of the way into the core
from the top. When the frequency is
tuned, adjust L1 for best signal
quality.
• Adjust VRl and VR2 so that the
signal does not sound overloaded or
cut out altogether. Rotating the
wipers clockwise (as seen from the
front panel) will reduce the signal
level. Both trimpots should be set to
the same position.
• That completes the adjustments
for the first intercom. Now you
need to adjust the second intercom
in exactly the same manner.
Testing
The two intercoms can now be
tested for range and clarity. Make
The coiled cord is clamped to the front panel using a cord grip grommet.
Don't forget the earth connection to the body of the pot.
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SILICON CHIP
sure that the antenna leads are
supported with the same orientation. We recommend that the antenna lead be clipped to your collar.
Some slight retuning of the
transmitters may be required for
best results. This is simply a matter
of tweaking 12 in each unit to give
maximum range (about 50 metres).
Don't touch 13 - it's best to adjust
the transmitter frequency only.
Note also that the components
located around tuning coils 11, 12
and 13 will alter the tuning if moved. These parts include the
capacitors and the tinplate shield.
If you do move any of these components, you may have to go back
over the alignment procedure
again.
As pointed out last month, the
O. lµF capacitor adjacent to 12
must be laid flat against the PCB. If
you don't to this, it will affect the
tuning of the coil, particularly with
changes in temperature.
Helmet wiring
The loudspeakers and microphones are installed in the helmet
as shown in Fig.1. Note that the
loudspeakers must be small enough
to fit behind the helmet lining. Very
small headphone loudspeakers can
be used if there is little space
within the helmet.
The electret microphones must
be secured in the positions indicated; ie, one directly in front of
the rider's mouth and the other
mounted to one side. Make sure
that the addition of these components will not cause injury if
there is an accident.
The leads running from the
microphones and loudspeakers connect to the 6.5mm stereo and mono
line sockets. These leads are run
beneath the helmet lining and
secured to the base of the helmet.
Note that if glue is used to secure
the wires and microphones, be sure
to use a type which will not affect
the helmet. Some glues, such as
contact adhesive, give off fumes
which may damage the polystyrene
helmet liner. Epoxy resin glues
should be safe, provided there is
adequate ventilation during the
curing process.
Once the intercom is completed,
it can be tested on the motorcycle.
The antenna lead should be clipped
to the collar of your jacket so that it
extends to maximum length. Of
course, common sense dictates that
you don't attempt to make any adjustments to the intercom while the
bike is in motion.
Troubleshooting
If it doesn't work, don't dive in
and replace all the ICs and transistors. That seldom fixes anything
and only wastes money.
Most faults in kits are due to faulty soldering, wiring errors and incorrect parts. The first step in
troubleshooting is to check all these
possibilities very carefully. In particular, check the wiring to switch
S1.
The diagrams show the wiring
for the Dick Smith Electronics
switch but if you use a switch from
another supplier, then the connections may be different. If necessary,
use your multimeter to determine
the switch terminals.
If all the wiring checks out, try to
determine which part of the circuit
is not working. For example, if the
unit won't receive FM stations, then
the fault could lie in either IC3 (the
receiver chip) or IC4 (the audio
amplifier). You can then check IC4
by applying a low-level audio signal
from an external source across the
volume control (VR3).
If the receiver circuitry works
but the unit won't transmit, the
fault could lie in the microphone
preamp circuit (ICla, IClb), the
VOX circuit (IClc, ICld & Q1-Q3),
or the transmitter (IC2).
You can easily check whether or
not the microphone preamp and
VOX circuit is working by selecting
the 'intercom' mode and speaking
into the microphone. If the transmit
LED lights, the VOX circuit is OK. If
it doesn't, there is a fault in the
VOX circuit or IC2 is faulty (or the
LED is wired back to front).
To determine where the trouble
lies, short the collector and emitter
leads of Ql. If the LED now lights,
the fault is in the VOX circuit; if
not, the fault lies with IC2.
If the VOX circuit isn't working,
use your DMM to monitor the
voltage on pin 8 of IClc. This pin
should normally be close to 0V but
should give a reading of over 1V
when you speak into the micro-
WIRING TO
- - - MICROPHONES
6.5mm STEREO
LINE SOCKET
ANTENNA
□--
LOUDSPEAKERS
-o
MOTORCYCLE
INTERCOM
-HELMET
Fig.1: this diagram shows how the electret microphones and
loudspeakers are installed in the helmet. Note that one of the
microphones goes directly in front of the rider's mouth while the
other is placed to one side. The speakers go behind the helmet
lining.
phone. If nothing happens, then the
microphone preamplifier circuit is
at fault.
Note: the output of IClc is actually a square wave which is averaged
by the DMM to give a reading.
If everything is OK here, check
the voltages on pins 12 & 13 ofICld.
Pin 13 should be at about 0.8V
while pin 12 should jump from 0V to
over 1V when you speak into the
microphone. At the same time, pin
14 of ICld should go from 0V to
about 3.2V.
Assuming ICld is working, transistors Qt, Q2 & Q3 can be checked
by measuring their collector
voltages. Normally (ie, VOX not activated & pin 14 of ICld low), Qt &
Q2 should be off and Q3 should be
on. Qt 's collector should be at
+ 2.4V, Q2's collector at + 0.7V
and Q3's collector close to 0V.
If this checks out, connect a
jumper lead between the cathode
(K) of D2 and the + 4.5V supply rail.
Qt & Q2 should both now turn on
which means that their collectors
should be close to 0V. At the same
time, Q3 should turn off and its collector should go to about 4.1 V.
Another way of checking the
transistors is to measure their baseemitter voltages. When the jumper
lead is connected, Qt and Q2
should both have a base-emitter
voltage of about 0.7V. When the
jumper lead is removed, Q3 should
have a base-emitter voltage of 0. 7V
(ie, the transistor is turned on).
Check the transistors carefully if
you fail at get the correct voltage
readings at any stage. Their correct
operation is crucial for supplying
power to the rest of the circuit
when in the 'intercom' mode.
~
NOVEMBER 1989
67
