Most of us will have come across the use of a trainer lead, or buddy box set up at some time, either as a student or as an instructor. It is a very effective system - the instructor's transmitter flies the plane then, by pressing on a spring-loaded switch, the instructor can activate the sticks on a student's transmitter, connected by the trainer lead. The key thing is that, whichever set of sticks is being twiddled at any moment, it is the instructor's transmitter that actually transmits and the slave transmitter remains switched off.
Recently though, I discovered that things are not always as straightforward as we may like to think. When new member, Keith Young turned up with his brand new Alpha 180 and Hitec Laser 4 transmitter, he posed a couple of new challenges. Firstly, my trainer facility is based on two Futaba FF7s, with a converted trainer lead originally bought for use with an FF7 and an old Futaba Gold series Tx. This lead does not have as many wires in it as the modern FF7 to FF7 lead, but it seems to work, so I have never enquired what the other wires are for! Keith had been assured that the Hitec Tx was compatible with the Futaba trainer socket, so I was interested to see whether it would work with my slimline lead. Somewhat to my surprise, it did. One potential problem dealt with.
The second problem was that Keith's transmitter was mode 1 and I fly mode 2. This meant that I had to use my FF7 Tx as master, and of course I didn't have a Futaba transmitter crystal on Keith's channel (72). We first tried the Hitec crystal in the FF7 without success, so Keith bought a Futaba 72 Tx crystal and we tried again.
This is when things got very confusing! The FF7 was switched on and - nothing. Extending the aerial did bring the plane into life but, since the aerial-down range was zero, I concluded that the crystal must be faulty. However, unplugging the trainer lead brought everything back to life and a full range check was then okay. What was going on!
This is the scary bit. We removed the crystal from the Hitec transmitter and bingo - everything worked, even with the trainer lead connected. I have done no further tests to confirm this but it does seem that when the Hitec transmitter is connected via the trainer lead it actually transmits even when switched off. Of course, if the two transmitters are on different frequencies, you wouldn't notice - until you shot down some poor soul flying on the slave transmitter's frequency! With both Tx's on the same frequency of course, they will interfere with each other, until the aerial is extended to enable one Tx to dominate over the other. Now, I have often used my two FF7s connected together in trainer mode, both fitted with ch 77 crystals, and not encountered this problem. Whether it is intrinsic to the way the Hitec transmitter works, whether the Hitec/Futaba compatibility is not as total as we were led to believe or whether it is something to do with the missing wires in my trainer lead, I don't know. If you can shed any light on this, please let me know, so we can all learn about it.
However, the moral of the story must be to remove the crystal from the slave Tx or take pegs for both transmitter frequencies unless you are absolutely sure that the slave Tx is not transmitting. As I said, scary!