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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