While continuing to wait for the encoder to arrive, I have been playing around with some other vintage Tx's to get a feel for things.

I received a Kraft KPT-7C yesterday with corresponding plug-in AM frequency module KPR-7C receiver. There were no batteries in the Tx so I tested things out with a spare set of Tx/Rx batteries to check the functionality of the electronics. The Tx seemed to be in order powering up with the RF meter also showing power output. The receiver however didn't seem to budge at all when powered up with a Kraft switch and servo which I know work. Either the Rx is shot or the module is. In either case, without being narrow banded, I think these AM RF decks are unfortunately of little use to us.

I worked on the Tx with the idea of keeping the original encoder and simply installing a low cost 2.4 GHz deck. The batteries in these radios were of the 2/3 sub-C NiCd type and took up a fair deal of space in the transmitters. I removed the plastic battery case which provided a base for mounting the encoder board (these are much smaller than the early 70's boards since they make use of 4 IC's) and soft mounted the encoder back against the front of the case. This will provide plenty of space to mount a flat LiFe or 8s AA pack against the back of the case with velcro. I also went ahead and removed the functioning AM RF deck module apparatus since it will no longer be used. I was hoping to see if there was a way to use the plastic housing for the module to accommodate the 2.4 module but I'm not sure this will work. Removing the module housing leaves the top of the Tx with a large rectangular opening in front of the holes made for the antenna. With the help of a little aluminum plate, this area could be useful to produce an interface area for the 2.4 deck bind and status module. While there is some concern (or is it disdain) for products made in Asia for a number of reasons, I have decided to give a 2.4 unit a try on this Tx since it is a bit of an experimental/learning exercise. Since one is modding this oneself, it is easy to build in an option to install another better known and trusted RF deck if the Tx is put into serious use.

I have located the battery power leads, the corresponding charge leads (these will display voltage when the Tx is off for charging and will have no power when on - reason for which DPDT power switches are used on Tx's) and have made sense of the 6 leads going to the AM RF plug-in deck. The positive (red) and negative (black) power leads for the deck are routed via the two outer pins. One lead is for the antenna and one for the RF deck power meter which left two leads for the PPM signal. There are two leads, one yellow and one orange/black and I haven't figured out the purpose for the orange/black leads since it exhibits no voltage signal when powered up. I'll have to dig a little deeper. In any case, the yellow pin 4 lead is the PPM signal as confirmed by the signal variation as the switches and sticks are moved. Total potential variation for each channel seemed to be 0.06V with 0.03V change in voltage when deflecting a stick from neutral to full travel. Although I didn't check, trims are probably on the order of 0.01V or less (~25 to 30%).

Armed with this knowledge, if one takes the yellow, red and black leads and feeds them to the 2.4 deck, one should be golden in the conversion. There is presumably also a PPM I/O signal at the trainer/charge/Rx voltage multi-pin jack in order to receive singals from a buddy box and pass them on to the RF deck if the Tx is being used in master mode. Come to think of it, this is what the orange/black lead may be for. When the trainer switch is pulled, the RF deck no longer is fed from the resident encoder via the yellow lead and instead it receives the PPM signal from the slave (student) Tx over the multi-pin jack and sends it to the RF deck over the orange/black lead. I should have taken pictures of the rear of the encoder prior to affixing it against the front. It's just double sided tape so it can be done if needed.

The trainer jack consists (should be past tense) of 6 leads too. Two of them as mentioned are for battery charging (blue+ & purple-) and I imagine also to send current to the slave transmitter encoder and four other leads are, and I'm kinda guessing a bit here, for encoder send (yellow/bk), encoder receive (orange/bk), and, encoder enable - trainer switch powers it up (gray/wht). The fourth lead is black and is ground.

There is also a 4 pin jack in the bottom left of the Tx and judging by the colour of the leads, this not only can be used as battery charge jack as it is used in the sport series Tx's but it also might serve to pass Rx battery voltage to the Tx for display on the meter. In fact, it may also act as a direct servo controller (DSC). If it is, I'm just not sure how the signal is passed on to the Rx since the switch interface which presumably does the PPM I/O to the Rx would need to be at least a three pin connector. I received no switch with the Tx/Rx and the switch that comes with sport series radios is a two pin (+/-) Rx battery charge connector.

I'll have to do a little more voltmeter prodding to see what's what. In any case, the long discourse above regarding PPM outputs is also for the purpose of installing a simple 1/8" mini-jack for PPM output to a simulator. JR uses a very simple PPM out DSC/trainer interface with a 1/8" mono jack tip to pass PPM and somehow it also manages to engage the encoder without having to turn either the master Tx on (DSC function) or the slave Tx on (trainer function). I'm not sure how that is done but perhaps someone like Andy could illuminate here so we may implement the same simple means using a 1/8" jack. I have a particular dislike for those multi-pin cables. I don't know why.

With that said, I should be able to enable this Tx in legacy hybrid mode upon obtaining a fresh battery for it and a 2.4 deck for broadcast. It would be nice to add servo reversing as well as dual rates which is something which can be done on this encoder. This might require a little further inquiry with those who have narrow banded servo reversing D/R versions of the KPT-7C.

Here are a few pics I took while sorting things out.

David.