Tosh McCaber Posted September 4, 2020 Share Posted September 4, 2020 I have, and still fly, with no problems, a couple of planes with Kraft Series 70 radio equipment controlling them. A slightly technical question- why do the early Kraft RC airborne sets have four, rather than three connecting wires?? Edited By Tosh McCaber on 04/09/2020 11:26:21 Quote Link to comment Share on other sites More sharing options...
Alan Gorham_ Posted September 4, 2020 Share Posted September 4, 2020 The fourth wire is a centre-tap from the 4.8V receiver battery to give a 2.4V reference voltage for the servos control system. Other manufacturers did similar - see early 1970's Futaba Digimac systems for example. Quote Link to comment Share on other sites More sharing options...
Andy Stephenson Posted September 4, 2020 Share Posted September 4, 2020 The centre tap battery was to reduce the complexity of the motor drive circuit in the servo down to two transistors from 4. Transistors were also expensive back in those days. Edited By Andy Stephenson on 04/09/2020 12:02:28 Quote Link to comment Share on other sites More sharing options...
Peter Christy Posted September 4, 2020 Share Posted September 4, 2020 The centre-tap on the pack provides two lots of 2.4 volts, and was connected to one end of the servo motor. The other end of the motor was connected via switching transistors to either the positive end of the battery or the negative end, allowing the motor to be driven in each direction as required. Later servo amps did away with this and used a "bridge" configuration to switch the motor direction. 3-wire servos use a different motor to 4-wire servos as the driving voltage is significantly different. You can't just put a 3-wire amp in a servo designed for 4-wire unless you change the motor as well. Strangely, initially 3-wire servos seemed to suffer more "connector" failures than 4-wires, despite the extra simplicity of 3 wires! This was largely cause by the habit we have of pulling the connectors out by the leads, rather than the actual plug. 4-wires spread the load more! The introduction of crimped connectors to replace soldered ones restored earlier levels of reliability... -- Pete Quote Link to comment Share on other sites More sharing options...
Peter Christy Posted September 4, 2020 Share Posted September 4, 2020 P.S. Should have added: A number of people have had great success converting these radios to 2.4 GHz! Checkout some of the conversions here: **LINK** -- Pete Quote Link to comment Share on other sites More sharing options...
Tosh McCaber Posted September 4, 2020 Author Share Posted September 4, 2020 The servos function extremely well. From what I'm reading above, I can't just chop off one of the central wires to use with 2.4Ghz? Quote Link to comment Share on other sites More sharing options...
Peter Christy Posted September 5, 2020 Share Posted September 5, 2020 No! The way the servo operates requires the use of a centre tap. Without it, the servo won't work at all! Whilst it may be possible to modify the servos to work on a 3-wire system, the cost / effort involved simply isn't worth it. Changing the amplifier isn't too difficult (if you can source them these days!), but you would also need to change the motor as well. Finding one that would fit your casings, and accept the matching pinion gear would almost certainly be problematic. In any case, modern servos of much better performance are readily and cheaply available. -- Pete Quote Link to comment Share on other sites More sharing options...
Andy Stephenson Posted September 5, 2020 Share Posted September 5, 2020 It would be possible to create a virtual centre-tap for the battery by using an OpAmp with a power boosted output. This would allow you to use those old servos. This is an extra complication, so new servos are a better solution particularly if you have changed to a modern receiver set up for 3 wires. A. Quote Link to comment Share on other sites More sharing options...
Tosh McCaber Posted September 5, 2020 Author Share Posted September 5, 2020 Definitely- Best plan is maybe to sell my Kraft gear as a oner to a vintage fan! I've just taken the Kraft gear out of my trusty CF Phase 4 (built in '74!), and am replacing with 2.4 equipment! All good things come to and end! Mind you, for simple 3-4 channel stuff, the Kraft gear still stands up! Quote Link to comment Share on other sites More sharing options...
Phil Green Posted September 5, 2020 Share Posted September 5, 2020 Another point is that a typical 2.4g receiver outputs lower voltage servo-pulses than the original Kraft receiver, the old servos may or may not have a problem with this, depending on which amp Kraft used (NE543?) What you dont want is for a servo to only just work to the lower level with no headroom - then one day, mid flight - it doesnt. I really like the older Kraft sets, I fly a Series 70, a Sport and a Gold Medal, all on 2.4 of course. Its because 'back in the day' ™ they were well outside our reach! Quote Link to comment Share on other sites More sharing options...
Peter Christy Posted September 5, 2020 Share Posted September 5, 2020 Phil: The NE 543 was 3 wire IIRC. I think it was the introduction of IC amps that allowed the use of 3 wire setups. All the 4-wire servos I've seen had discrete amps, all the 3 wires were ic. Another complication may be the decoder used in the Kraft (never owned one, so I don't know!). A lot of sets from that era uses SCS devices for the decoder, and these didn't like driving servos with low input impedances. SCS decoders were pioneered in the RCM "Classic" system, and for a short period became very popular as they were cheap and easy to implement. Unfortunately, SCS devices were very temperature sensitive, as I found out one day flying in sub-zero conditions! After that disaster, I went a bought a bag load of them from a local supplier, and spent a few evenings going through my two Sprengbrook receivers with a can of freezer spray, and replacing SCSs until I had both working well below zero! SCS shift registers were rapidly replaced once integrated circuit shift registers became available! No idea if Kraft used these or not, but IC servo amps generally have a high enough impedance not to load SCS decoders. Some systems also used negative going pulses instead of the usual positive. I think Kraft were always positive, but EK Logictrol and OS were negative going. Long time ago, now....! -- Pete Quote Link to comment Share on other sites More sharing options...
Phil Green Posted September 5, 2020 Share Posted September 5, 2020 Sorry I forgot we were talking 4-wire servos Pete Kraft decoders mostly used a 74164 TTL shift register so would output pulses at the full battery level (4.8v or more) and they were conventional positive-going servo pulses with 1.5ms neutrals. Tosh, you could make up a centre-tapped battery pack purely for a model using these servos, I do this anyway with lithiums though thats for balance charging rather than a mid-pack power tap. I use the signal wire on a conventional servo lead for the centre-tap as this means pos & neg are where you'd expect. You'd have to be careful never to plug this directly into a modern receiver and you'd need to make up a double-pole switch harness anyway to switch the battery pos and neg. It depends how determined you are, as retro enthusiasts we'd resurrect 50-year-old servos, but common sense says use new ones! Plastic parts become very brittle and may appear fine, but could easily fail in use. Cheers Phil Edit: why not convert it Tosh? its easy peasy and you get to keep the transmitter you love plus a few bells & whistles that the Kraft never had There are many, many examples over on mode-zero , come see! Edited By Phil Green on 05/09/2020 22:19:56 Quote Link to comment Share on other sites More sharing options...
Tosh McCaber Posted September 6, 2020 Author Share Posted September 6, 2020 Thanks for all the input, guys. If I had more time, I would certainly consider converting the box to 2.4Mh. However, I am now in my late '70s. I've had a great deal of value and pleasure from it since buying my Series '70 (in 1970, believe it or not!)!- although I've accumulated a few additional Futaba sets of equipment since then. I'll now concentrate on the vast amount of learning as to how to control my Spektrum DX9, to which I'm gradually converting my squadron of planes. (My Yellow Elephant glider's wings were built as part of my first propo glider built in 1966. The (massive, at the time, to accommodate the massive Orbit RC gear of that time) Elephant fuselage was slimmed down in 1980, and it has flown competitively in many club contests since then, with it's original Kraft equipment! But, now- it's time for the new! Quote Link to comment Share on other sites More sharing options...
Peter Christy Posted September 6, 2020 Share Posted September 6, 2020 Tosh: I'm sure there are plenty of people over on mode-zero that would love to give your Kraft a new lease of life! **LINK** -- Pete Quote Link to comment Share on other sites More sharing options...
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