he liked to build them big . . .

This thumbnail picture (click it for the big view) appeared on the club scene page of ModelFlight 14 when I visited Craig Trickett's club, the St John's R/C Flyers, Newfoundland, Canada. It was one of the large collection of photos on their 'gallery' page and I expressed an interest to know a bit more about the model and its owner.

tranny trouble 1

by Gordon Cook

fault occurring with Futaba FF8 transmitter

Fault: with the transmitter switched on, the audible alarm sounds briefly for about 1 second when the aerial is extended.

The cause is due to one or more fractured solder joints between the RF module and mating printed circuit board. This may be either the joint failed between the male 5-pin post header and the brown pcb in the transmitter or in the RF module itself between the 5-way socket and pcb.

location of pcb  pcb topsidepcb underside showing solder joints

The problem is due to relative movement between the pcb post header and RF module socket due to the brown pcb moving towards the top of the case as the aerial is extended. Repeated aerial extension and retraction causes fatigue failure of the joints until - as happened to me, but fortunately on the ground - total RF transmission is lost.

In my opinion, the cause is from two sources:-

1. Insufficient strength in the aerial post and relative loose fit of other parts which allows a small amount of bending and 
2. with time and use, the aerial sections gum up with engine oil so that the force required to extend the aerial increases significantly.

The aerial mounting post is not adequately stiff in the extension direction and is also vulnerable from failure as noted in the other report (see below). Cleaning the aerial with methylated spirit makes a very noticeable difference to extraction force and therefore load on the aerial mount and RF module connector. So for safety's sake - and not only for optimal transmitter range - keep the aerial joins sliding freely. Actually, last time, I also lubricated mine with WD40 which seems better still.

Remember, if you get an odd bleep when you extend the aerial, get the transmitter checked out before complete failure occurs!

tranny trouble 2

This is another symptom of the problem identified by Gordon in the above item. I heard something rattling about inside my Futaba FF8 transmitter and actually shook this little bit of black plastic out through the battery compartment. The transmitter was still functioning perfectly o.k., but when I showed this to Gordon, he immediately recognised it as evidence that I was heading towards the same failure that he had previously experienced some time ago.

When we opened the case of the transmitter and lifted off the printed circuit board which covers the aerial and carries the RF module connecting pins, we could immediately see that this plastic support was part of the aerial support and also carried the pcb which registered over the little pegs. The pegs fit into those two holes in the pcb, and the movement of the pcb as described by Gordon above had flexed this plastic support to the point of fatigue and breakage. If you look at the larger view of this picture, you can also clearly see that the support on the left is similarly damaged, and it, too, fell away even as we positioned the transmitter to take the photograph!

the repair

I must say, with both sides of the support having broken, I do wonder how long the repair will last but the alternative seems to be a replacement case.