jaymen

I received one of these from Bob in Orlando several years ago, and we would up deciding it would be far easier to replace the main board with a more modern one from another radio, as we could only get a modern receiver to follow the first two channels, and it would sometimes skip or jump channels as well, or quit altogether. Here is a write-up I did based on the radio and our experiences with it, plus my recollections of discussion I had with Don Mathes back in the 1980s when he worked with us a World of Robots:

The first KP-4 proportional by Kraft.

This radio, although it was not advertised, the KP-4 was Kraft’s first commercially available digital proportional radio. Compared to the Kraft-Pullen proportional radio that was in development around 1963, it was much simpler in that it used far fewer components. In addition, it was a true 4- channel full house system offering proportional control on all channels. The original version of this radio used quite a different arrangement in both the transmitter encoder and receiver decoder design, and as such, neither are compatible with the later KP-4B and KP-6B that followed. This was because the encoder used a self-triggering string of one shots, with no master oscillator/ multivibrator to control the frame timing. Instead, there was a start button on the front of the transmitter which when depressed would fire the first channel one shot which then in turn fired the second one shot and so on until all the one shots had fired. The last one shot would then re-trigger the first one and the cycle was then repeated. The pulse frame consisted of only four pulses instead of five, which is odd because conventional PPM digital proportional pulse frames consist of one pulse per channel plus an extra “sync” pulse at the beginning of the frame as a timing signal. Now how did Kraft then get four functions and use one less pulse than normally required? Well, there is normally a pause of about 6-10 milliseconds after the pulses, called the sync pause, that allows time for all the logic to reset once the channel pulses have been processed, this is done so the encoder and decoder can then reset and send another frame of information. What Kraft cleverly did was to vary the time of the sync pause and use it to convey a 4[SUP]th[/SUP] channel of information. The other trick was that by making the encoder self-triggering, it eliminated two transistors needed for a master multivibrator circuit to set up the encoder timing. The last trick was done as a cost saving measure.

Sound great, and very ingenious, doesn’t it? Well guess what, it was, in theory, but not in practice. One issue was drift, because without a multivibrator to act as an accurate master timing clock, the encoder lacked accuracy, therefore any small variations in each of the channels one shots were cumulative, causing some interaction, and drift. In addition, without a master timing clock that forces the start of each pulse frame, the encoder could, and would sometimes not self-trigger and stop, or “lock up” and quit sending pulses. This could, and did crash many planes. In theory, the encoder should have self-started when turned on, but this was not always the case, and hence the reason for Kraft putting a small “start’” push button on the transmitter face. If the transmitter quit, and if the pilot had fast reactions, he could possibly hit the start button in time to avoid a crash. On the receiving end, the decoder had an odd arrangement that allowed it to look at the variable synch pause and convert it to a pulse to drive the throttle servo.

Kraft quickly abandoned this design, and came up with a second generation version but added two more channels and called it the KP-6. Early Kraft price sheets reflect that KP-4 airborne equipment is not compatible with either the KP-6, or later KP-4B and KP-6B systems. Apparently however, they had produced enough of the original KP-4 systems to continue offering support in the form or replacement transmitters, receivers and servos. Once the KP-4B came out, it had a new circuit board in the transmitter that was similar to the KP-6 in that is used a master clock circuit. It must be pointed out however, that even the second “B” versions were still fairy crude in their design as Phil was still trying to use as few components as possible to keep the cost down. The KP-4B and 6B both used a single uni-junction transistor in place of a two transistor mutlivibrator for the master clock. This was common of early digital sets such as the Digitrio designed by Ed Thompson, which also utilized a uni junction master clock. To eliminate neutral drift and improve the linear response of the system, it became necessary to implement a two transistor multivibrator circuit using precision film timing capacitors for the master clock, and this then became a stand design feature of all digital proportional radios produced from about 1966 onwards.

Other than the obvious cost cutting measures employed by Phil Kraft which was based on a notorious technique called Muntzing, (remove as many parts as you can before it quits working), there was really no good reason that justified continued production of the original KP-4 design, as it proved problematic and hurt Kraft’s reputation for reliability. This probably explains the reason for them not being advertised, or sent out for review by the magazines either. At the Time, Phil had not only Jerry Pullen, but Don Mathes working on the proportional radio design, and Don’s experiences with his failed Digicon project had much to do with this. This is little doubt that he advised Phil to proceed cautiously and not do any press releases before they fully tested the new radio. One can also imaging that Don probably had his doubts and objections over the Muntzing of the KP-4, especially since his Digicon design had already proven the reliability and efficacy of a solid master oscillator encoder design. This is the typical age old battle between the engineers and the sales department which is waged over what is ideal, and what is practical. Don used to recount to us about how he would become frustrated by Phil’s continual Muntzing of his prototypes and designs.

What is quite interesting to ponder is why Kraft spent over two years of effort trying to develop an alternative technology to digital proportional after it came out. My own assessment is that Phil was driven by the desire to produce his radio at the lowest cost to make the best profit margin, and help it outsell other brands due to a lower price point, that, and the lack of a consensus at the time as to what design would prove to become the industry standard. Add to this the fact that Don and Phil were keenly aware of what had happened with the Digicon, and therefore were proceeding cautiously before going to the press with the KP-4. Inspite of having produced a significant number of these first proportional radios,Phil was able to overcome the setbacks with the first KP-4 for several reasons: First off, Phil Kraft was a well-established and respected R/C manufacturer; the KP-4 was not his only product and he was well entrenched in building successful single and multichannel equipment that had a very good reputation for reliability. Secondly, he did not advertise the first KP-4, even though he built and sold quite a few. He shrewdly did what today is known as a "soft" product release. Third, he had follow-up designs in the KP-6 and "B" series that were much more reliable by comparison. One must also keep in mind that virtually every digital proportional radio that came out between 1963 and 1965 had its share of issues as each one had its own quirks due them all being new designs using newly developed components as well. It was the accepted "norm" that you had to have three proportional radios: one in the plane, one in the repair shop, and one in the mail on its way to or from the shop! Another clue to the cantankerous nature of the equipment in those days was the practice of test flying most, if not every radio system. A little known fact is that the Ugly Stick started life as a test bed for radios systems. Phil had dozens of Ugli Stick planes made, and then he recruited a crew of pilots to use these plane to test his radios. In exchange, they got free planes, fuel, props and a free radio system as well.

Phil got the idea for the Ugli Stick from observing Zel Ritchies specially designed plane he used for test flying his Space Control 4 channel analog systems.