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RADIO CONTROL
BY BOB YOUNG
Multi-channel radio control
transmitter; Pt.6
This month, we deal with the assembly of the
Mk.22 transmitter case and the sub-assembly
interwiring. The unit is housed in a sturdy
welded steel case with a powder coat finish.
The steel case helps in maintaining the very
good 3rd order intermodulation performance.
Before we start on the mechanical assembly, there are a few details which have arisen
in regard to the PC modules as a result of
experience gained over the past few weeks.
Firstly, I blew the power tracks clean
off an RF module because I forgot to insert
the insulator under the FET in one of the
modules. When I checked the RF module
assembly instructions, I found to my horror
that I did not stress the importance of this
insulator.
In fact, I did not mention it at all. The
insulator is supplied in the kit and is of the
type that does not need thermal goo to work
properly. Please make sure it is correctly
in place because if it isn’t the 10V rail is
shorted directly to the ground plane and
let me tell you, those tracks really glow in
the dark; for a while anyway, at least until
they vaporise.
Also do not fit or remove the RF module
while the power is applied in case the FET
accidentally touches the earth. For added
safety, always remove the power socket
before removing or fitting the module.
Secondly, TB29 is shown in the encoder
PC overlay (Fig.2, page 61 in the June 1996
issue) as a non-polarised 2-pin connector.
It should be a polarised connector, inserted
with the polarising keyway towards the edge
of the PC board.
Finally, TB30 was shown incorrectly as a
3-pin connector (Fig.4, page 62 in the June
1996 issue) for the encoder/decoder patch
cord. The amended drawing is shown in
Fig.2(a). Also TB10 was incorrectly referred
to as TB30 in the captions in the same issue.
Mechanical assembly
Let us start with the case. You can begin
by cleaning up the powder coating edges
and overspray. The cases sit flat on a tray
during powder coating and may pick up
some black swarf or rubbish along the
rims of the two halves during the process.
July 1996 77
Fig.1: details of the fixed wiring in the transmitter.
Using a sharp utility knife, trim off
any excess material and foreign matter. Check that the components all
fit easily into their respective holes
or slots. The powder coating goes
on very thickly to achieve the ripple
finish and this tends to close up any
openings in the case.
Remove any overspray or coating
where electrical contact with the case
is required; for example, the mounting
brackets for the RF module, top of the
threaded inserts (PC standoffs) etc.
Once the case halves are prepared,
mount the charge socket into the hole
provided at the bottom righthand corner of the front panel.
Now we can begin gluing down
all the sticky bits. The battery pack
is first. Using the double-sided self
adhesive foam-backed tape provided,
apply two strips along cells 2 and 7.
The battery goes in with the positive
78 Silicon Chip
terminal on the right as viewed from
the back of the case. It is a good idea
to remove any greasy residue from
the case and components to be glued
down by wiping them with a tissue
soaked in metho.
Do not use strong solvents on the
front of the case as they may damage
the surface sheen. Press the battery
pack firmly into place about 1mm to
the right of the charge socket hole,
with the battery sitting against the
case bottom.
Next, prepare the meter by applying
a very thin strip of ordinary contact
cement around the edge of the concealed face. Care is needed here to
ensure excess glue does not ooze into
the meter adjustment screw. This glue
strip is only to stop the meter moving
in the mounting brackets which will be
fitted later, so the glue can be applied
quite sparingly. Press the meter into
place and then remove it immediately.
Check that the glue has not migrated
under pressure and leave until the glue
is dry to touch. Then press the meter
firmly into place.
Remember here that contact cement
works best when the solvents have
evaporated and the glue feels dry to
touch.
Now very carefully apply a strip
of glue around the inside lips of
the control stick escutcheons and a
matching strip of glue around the corresponding edges of the large square
holes for the control sticks, taking
care to stay inside the boundary of
the escutcheon.
The bond must be good here for the
glue provides the only fixing for the
escutcheon. I find that some of the
exotic modelling contact cements such
as “Zapadapagoo” work very well in
this application. Again, wait until dry
and press the escutcheon firmly into
place, ensuring that the parallel sides
of the escutcheon are vertical and the
scollops (curves) are at the top and
bottom of the case.
Finally, using contact cement, fit
the antenna insulator into the top of
the case. Any excess contact cement
should be wiped off immediately with
metho (do not use thinners or anything
stronger).
Mount the slide switch, next ensuring the cover plate is mounted
correctly. There is a small pip which
indicates ON. This goes towards the
bottom of the case. Remember we are
assembling an Australian designed set
and in Australia, down is ON.
Wiring details
It is now time to fit the few hardwired links in the transmitter. Fig.1
shows the layout of the interwiring.
The hook-up wire provided is single strand 21/008, (21 conductors x
0.008mm, unplated, various colours
and red and black 14/0.01mm, tin
plated). Always twist the wires into a
cable form wherever possible for neatness and minimisation of RF pick-up.
The tin plated wire minimises the
“black wire syndrome” and must be
used for all positive and negative
power runs.
Begin by wiring the 6-pin and 10pin power sockets. The 10-pin socket
uses five pairs of two pins in parallel,
so gently bend each pair together as
shown in Fig.1. Tin and solder the
appropriate leads and then cover each
pin pair with the 3mm heatshrink
supplied. Twist the red/yellow/black
leads into a cable.
Do not include the white antenna
lead in this cable. It remains separate
and goes directly to the antenna tag.
The 6-pin socket is quite straightforward. Just block off pin 2 to match
the missing header pin on TB7 (on the
encoder module). The 10-pin socket
is plugged into the 10-pin header on
the RF module and a dot of paint is
placed on the right hand side of both
the socket and header. Just make sure
you get it right in the first place.
Next, wire the battery to the charge
socket. Note that the location of the
battery positive lead is important. It
must go to the terminal shown in Fig.1
as this is the terminal that mates with
the tip of the charge plug.
When wiring the switch, strip the
leads long enough so that the tinned
lead can be pushed through both
switch lugs as shown in Fig 1. This
connects the two switch poles in
parallel for added reliability. Connect
the meter as shown and the wiring is
virtually complete. The remaining
wiring consists of wander leads which
simply plug onto the appropriate
header pins on the PC boards. We
will deal with the final programming
next month.
Fit the handle, toggle switches and
auxiliary control potentiometers. It is
a good idea to wire these items before
mounting them – see Fig.2. Three-core
ribbon cable (blue/white/blue) is supplied for this task.
At this stage of assembly, most of the hardware is in place and the transmitter
and encoder modules have yet to be installed.
Auxiliary control pots
Wiring the auxiliary control pots is a
little tricky. In order to maintain servo
reversing and channel allocation on
these pots, it is necessary to solder the
4.7kΩ limiting resistors to the pot terminals as shown in Fig.2(d). Insulate
the pot back cover with a small piece
of tape. Once wired, coat the resistors
with contact cement and leave to dry
overnight. Finally, lash the wires to
the pot body with the small cable tie
supplied.
There is no polarity on these leads as
the plug is reversible. It is a good idea
to paint a dot on the right hand side of
the plug/socket as an aid to visually
determining if the plug is reversed or
normal. Also a small self- adhesive
label wrapped around the leads just
above the socket can be a great aid in
identifying each lead, as all control
Kit Availability
Kits for the Mk.22 transmitter are available in several different forms, as follows:
Fully assembled transmitter module......................................................$125.00
Basic transmitter kit (less crystal)............................................................$89.00
Transmitter PC board...............................................................................$29.50
Crystal (29MHz).........................................................................................$8.50
Fully assembled encoder module..........................................................$159.00
Encoder kit.............................................................................................$110.00
Encoder PC board...................................................................................$29.50
Transmitter case kit................................................................................$395.00
Full transmitter kit (includes all the above).............................................$594.00
Post and packing of the above kits is $3.00. Payment may be made by Bank
card, cheque or money order payable to Silvertone Electronics. Send orders to
Silvertone Electronics, PO Box 580, Riverwood, NSW 2210. Phone (02) 533 3517.
July 1996 79
at any time, all with no fuss. So the
leads must be long enough to reach
around the transmitter sides. Fig.2(b)
shows the leads for the toggle switches
and control pots at the top of the case
and their suggested length is 350mm.
All the other leads can be 270mm long.
This applies particularly when we
come to the configuration modules
and more especially when we use the
CROW configuration module. Because
the toggle jumpers have sockets at each
end, it is easy enough to make these
in various lengths to suit your application. The length shown in Fig.2(e)
is a suggestion only.
The knobs, toggle boots and Silver
tone label can be fitted at this point.
Control mode choice
Fig.2: details of the various wander leads used in the transmitter.
elements use the same type of lead
as shown in Fig.2(b). There are far
too many leads and all with variable
functions for colour coding to work
successful
ly, so I settled on blue/
white/blue ribbon cable with ID tags.
Fig.2(c) shows the details of the
toggle switch jumper lead. Fig.2(e)
shows the small patch cord for the
toggle switch programming. These are
required only if you intend to configure one or more channels as a toggle
switch channel. You need one lead for
80 Silicon Chip
each channel. It is a good idea to make
all of these leads at the one time.
Fig.2(f) shows the details of the
control stick wiring. Use heatshrink
sleeving where appropriate to protect
the solder joins.
You can tailor the length of each lead
to suit each control stick/slider/toggle/
pot location and thus minimise the
amount of loose wire hanging about.
However, the versatility of the Mk.22
lies in the fact that any lead can go to
any plug anywhere in the transmitter
Now comes decision time. As supplied, the control sticks are arranged
to provide Mode I (Throttle/Aileron on
the right stick). If you require Mode II
(Aileron/Elevator on the right stick)
then do the mode change on the sticks
before fitting them into the case. Note
well that the trims are on the outside
of the case contrary to normal practice.
This makes them much more readily
accessible in flight than the normal
layout. As before, it is a good idea
to wire the pots before mounting the
sticks. Use leads as per Fig.2(f). Lash
the wiring to the pot body using a small
cable tie and secure with a couple of
drops of contact cement.
Mount the sticks with the two
outside sets of screws (6BA x 6mm),
making sure that the correct stick location is observed for the mode chosen.
It is very easy to get confused when
working from the back of the case.
Next comes the preparation of the
antenna/RF module brack
et. First,
mount the antenna attachment screw
and insulating washers in the 6mm
hole provided. The screw protrudes
into the “U” and the antenna solder
tag goes under the nut on the inside
of the “U”. Solder the white antenna
wire to the solder tag and don’t forget
to fit the 3mm heatshrink tubing.
The two meter clamps and the RF
module bracket are fitted at this point.
The meter clamps simply go under
the stick mounting screws adjacent to
the meter. The RF mounting bracket
uses the two stick mounting screws
adjacent to the switch. Do not over
tighten the stick hold down screws
as you can crack the mounting lugs
on the control sticks.
Drop the antenna down through
the insulating grommet and screw it
fully home onto the antenna mounting
screw. Do not overtighten the antenna
for you may want to remove it from
time to time.
Mount the encoder module using
the 6BA x 6mm screws provided. Do
not over tighten as these screws are
slightly undersized to allow them to
pass through the corner holes. The
transmitter was originally designed
around 6BA screws but nutserts were
not available in these sizes.
The corner holes of the encoder
module should not be drilled out as
they are plated through. A small drop
of contact cement or nail polish will
serve to lock all of the screws into
place and prevent them from unscrewing with use. If you lose one of these
screws make sure the replacement is
no longer than 6mm.
Finally, mount the RF module using
the 3 x 6mm screws provided, taking
care to ensure that the mating surfaces
of the PC board and mounting brackets
are clean, bare metal. This is the heat
sink for the output FET and the earth
connection for the RF module and so
the surfaces must make good contact.
This is what your
transmitter should look
like when all the wiring
is complete.
Voltage checks
Disconnect any leads connected to
the PC boards. Switch the ON-OFF
switch to ON and check that the volt
ages are correct at the terminals on
the main power connectors for the
RF and encoder modules. If all is
correct, switch off and plug the two
power connectors onto their respective mates, observing polarity. Make
sure the crystal is in place and that
the RF module is programmed for AM
modulation.
Extend the antenna and, with a
sniffer probe held near the antenna,
switch on. You should see a modulated
RF signal on the scope’s screen.
If not, check the RF module programming and repeat the tuning sequence published previously. Check
the signal pin on the RF module to ensure that a data pulse train is present.
(Note: to make a sniffer probe, just
ground the tip of the scope probe – the
resulting loop works just fine).
Once you have the modulated RF
signal on the screen you may relax.
Congratulations you now have a working Mk.22 transmitter. Next month we
will show you how to align the system
and how to get the best out of it. SC
Close-up detail of one of the sticks and the power switch which is partly
obscured by the antenna. Note that both poles of the switch are wired in parallel
for increased reliability.
July 1996 81
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