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REMOTE CONTROL BY BOB YOUNG How to service servos & winches This month, we will look at the problems encountered in servicing servos & winches. Normally, these are very reliable but like any electromechanical device they eventually wear & give trouble. The modern servo is indeed a marvel of electronic engineer­ing; compact, powerful, accurate, robust and above all, reliable. They are a far cry from the rubber driven escapements that I cut my teeth on back in 1955. They are even a far cry from the first servos that I built for the Silvertone Mark I proportional system in 1966. I am still amazed when I look back at the progress made in those eleven years. My first proportional servo was fitted with a discrete amplifier consisting of about fifty components, of which eleven were transistors, all jammed into a relatively large Orbit PS-2 servo. The next servo, the PS-3, was much smaller and we just managed to shoehorn the same electronics into a two-deck assem­bly. It was grumble, grumble all around. Assembly hated assem­ bling them and servicing disliked servicing them. They were however, very accurate and a good seller because of their size. We had hardly put that servo to bed when I received a phone call from the Orbit representative asking me to meet him at the airport as he had something very interesting to show me. I can still remember the stunned feeling when I saw what he had to offer – a new servo, half the size of the PS-3. How was I going to fit an amplifier into that? Luckily, in the other hand he had the answer – an in-house IC they had commissioned especially for this servo. There was nothing on the market like it at the time and I was doubly stunned. We had no sooner put the PS-4 into production than the big IC shortage caused by the arrival of the calculator hit us. The IC-maker could not deliver those ICs for two years. I had just set up a new, larger premises and employed new staff to accommodate the increased production called for due to the popularity of the new servo. There was no other alternative than to fit a dis­crete amplifier. It looked impossible and it almost was. They were difficult to produce and cost me dearly. I learned bitter lessons about single source supply from that exercise. However to return to the modern servo and the servicing thereof. The modern servo in concept varies little from our old PS-2. The housing contains a servo motor and gear train, which in turn is coupled to a potentiometer and an amplifier. Error amplifier This photo shows a servo made by Silvertone Electronics. Note the double-deck PC board with the parts crammed in to save space for the motor & feedback pot. 72  Silicon Chip The amplifier is in essence an error cancelling system which will always seek to find the null. If the control stick on the transmitter is moved, then the servo will move until the error is cancelled and the servo is in null once again. The only difference between a servo on the throttle which is position­ able and the steering servo which selfneutralises is that it is hooked into a channel which does not have spring return on the control stick. In the good old days of tuned reeds, we had two Fig.2: the circuit diagram of a typical FM receiver. Note the provision of a tuning point to aid the alignment process. separate types of servo, positionable and self neutralising. From a statistical point of view, mechanical damage is by far the most significant issue, with electronic failures few and far between. Jammed or overloaded servos probably account for the bulk of the electronic problems but we cannot judge this accu­rately as mostly the model no longer exists or the servos have been removed for servicing. Servicing equipment in a model is a real pain as the equipment is usually jammed into all sorts of difficult to get at places or sealed into a waterproof housing. I really discourage people leaving the radio in the model partly because I do not want to run the risk of damaging some of those beautifully built and finished models. Whilst on the subject of jammed or overloaded servos, one problem area is the throttle linkage Bowden cable. Fuel seeps down this fine tube and with time the castor oil solidifies and creates considerable friction between the cable and the outer casing. It pays to remove the cable from time to time and clean it and the inside of the tube. I have seen throttle servos stall because of this problem. Another potentially serious area in regard to the throttle installation is the problem of over-travel jamming the throttle servo up against the carburettor stops. This means that the servo is stalled on and so its current drain zooms up to around 600mA. Batteries do not last long under these conditions. Servo motors and amplifiers do not take to kindly to this sort of treatment either. Modern transmitters have a very elaborate servo endpoint adjustment routine for this reason. Make sure you use it and use it carefully. If your transmitter is not fitted with this facility, then use some sort of mechanical over-travel device. These consist of a clutch or some sort of spring arrangement which will absorb the over-travel without stalling the servo. Here again the compres­sion of the springs will increase the servo current, depending on the spring tension. The best method is to use an adjustable servo arm and a lot of care. Likewise, all flying controls should move freely without friction. Any friction in this area will tend to degrade the servo centring accuracy and again push up servo current. I have had Fig.1: this exploded diagram shows all the parts used in a typical servo control. The key elements include the decoder PC board (21), the motor (17), a servo feedback pot (18, 19), various gears & the output wheel or arm (2, 3). Modern servos are built around dedicated IC servo chips (eg, the NE544 from Signetics) & are very compact & reliable. March 1994  73 REMOTE CONTROL – Servicing the servos instances where aging batteries and slowly increasing servo current, due to degradation of the control linkages, have come together to such an extent that flyers who routinely flew eight 15 minute flights used up the receiver battery on their seventh, or last, flight. And it really was their last flight, with that model at least. Visual inspection I usually begin the servo servicing with a visual inspec­tion of the gear train, looking for broken gears, dirt ingress, etc. One important point to watch for in some servos is that control surface flutter in flight can cause enormous stress on the servo gears and in some cases excessively wear the teeth or, in extreme cases, actually melt them. I also check that the output gear over-travel stops are not bent or broken and that the holes in the servo case which locate the gear axles are not worn oversize. Check the outside of the case for broken mounting lugs or cracked case sections and if available check the output arm for cracks or splits around the drive ferrule. The twisting forces and down very slightly. The feedback potentiometer is particularly prone to damage from engine vibration. This applies particularly to the throttle and rudder servos, which tend to sit in the same position for the entire flight. The vibration on the pot wiper eventually drills a little hole clean through the pot track into the substrate. At this point the servo tends to sit chattering to itself and chewing up servo current. Replace the pot if this happens. For this reason, it is a good idea to routinely move the servos around the various locations. This spreads the pot wear over the full arc. The flying controls are not as prone to vibra­tion damage, because they are moving constantly into new sectors on the arc. However, they do wear the track in time and pot inspection is most important during routine maintenance. Horizontally mounting the engine in the airframe is the best method to minimise vibration damage. Vertical mounting, either upright or inverted, tends to resonate the wing skins and in­creases the overall level of vibration in the model. The pot wipers “Another potentially serious area is the problem of jamming the throttle servo up against the carburettor stops. This means the servo is stalled on and its current drain zooms up to around 600mA”. in a crash often split the drive ferrule or strip the output gears. Next, check the amplifier lead and the connector for nicks or broken wires. This is a most common occurrence in a crash. Clean the connector with a toothbrush and CRC-226 and check for loose wires or connections. During servo servicing, always be on the lookout for damage caused by engine vibration. Servos are often screwed down so tightly on the grommets that engine vibration can destroy compon­ents under severe conditions. The correct method of mounting is to screw down the mounting screw until it just touches the top of the grommet and then back it off about half a turn. The servo should then move freely up 74  Silicon Chip also tend to resonate with this form of vibration. Horizontal mounting absorbs the vibration into the length of the wing spar and dampens the level considerably. The pot wipers in the fuselage are usually at right angles to this form of vibration, hence there is less wear on the pot track. Any care which is applied to the engine mounting in regards to vibration will pay dividends in longer radio life and reduced noise levels. The modern sealed pot used in most of the Japanese servos these days seems to be much better in this regard than the old removable pot in the earlier sets. If you do have a problem with the sealed pot there is little that can be done other than to replace it with a new one. If you have one of the older replaceable elements, mark the location of the pot in the housing so that it goes back into much the same location. Carefully loosen the two screws holding the element in place and completely remove one of them. The other can stay in place, for the element will now lift clear of the housing. Keep the servo inverted while removing the pot element, because there is often a little carbon brush on the tip of the pot wiper which falls out when you remove the element. If you lose this, you are up for a new wiper assem­bly. Once the element is clear of the housing, clean the track with a cloth and inspect for holes or worn sections in the tracks. The pot wipers tend to wear a track through the resist and this shows up quite clearly under a magnifying glass. Once satisfied that the track is clean and in good condition, wipe a very light smear of Vaseline over it. Clean and re-tension the wiper and reinstall the pot element into the hous­ing. Make sure the marks on the element and the housing line up. If you have made a mistake here the servo may slam up against the end stops and strip the gears. Nip up the two screws so that the element can be moved. Now making sure that nothing is shorting to the servo amplifier, switch on the radio with the servo plug in and check the neutral. If it is out, move the pot element until the servo is in neutral and tighten the two screws. Some servos are fitted with a screwdriver adjustment for the pot, which is located at the bottom of the output arm locat­ing screw hole. If this is the case, then nip up both pot housing screws firmly and using a fine jewellers screwdriver, insert it into the hole in the output gear and adjust the neutral, again with the radio switched on. Inspect the amplifier for broken com­ponents and frayed wires. Often, wires get pinched between components during assembly and engine vibration can wear through the insulation in time. Spray CRC-226 onto the front and rear bearings of the servo motor and let them run free, out of the gear train, to allow the CRC to seep into the bearings. That’s it as far as the mech­anicals are concerned. Next month, we’ll deal SC with servicing the electricals.