Happy silver anniversary.Transmitter channel 5 can be modified by a new switch and voltage divider resistors.

Edit.
I make out receiver FP-Rxx .
Futaba and Spektrum are makes that used < 3 volt servo pulses.

Some details
Transmitter FP-T7FG/K - 1983/84 (according to the date on a Futaba Calalogue that I had kept with the instruction booklet
Receiver FP-R107M (4.8V, 15mA) using I/C IR-2501

Too late home from work for testing!

I suggest starting a new thread elsewhere with photos of the handset internals.It is likely to have the NE5044 seven channel encoder chip.You may be lucky because that may allow substitution of the switch and no more.If there is an NE5044 and switch centre connection is removed and the wire voltage goes to 2.5 volts,that is the case.The channel 5 servo would centre with the centre cable disconnected.

If not,you could trace the switch cables to the circuit board,note the coloured bands on the connecting resistors or measure their resistance.

There may be up to five resistors connecting the switch to 0,5 volts and the decoder.
http://images.google.com/images?hl=e...tor+Color+Code
Note: surface mount components were introduced at about the time of that set.They were used in the following Futaba models

OK Thanks,
I was thinking along the same lines. Dismantle the Tx and get a photo or two of the particular area and current switch.

It may not require further input.

Signs to look out for:
If there are three wires to the switch and disconnection of the centre causes channel 5 servo to centre and fight attempts to reposition it then channel 5 is getting a 1.5 ms pulse,as it should.If all other channels are OK then do a straight swap with a centre-off switch.

If there are two wires to the switch and an NE5044 encoder it is possible with photos or if you have a buddy who has basic rc electronic knowledge.
If all channels misbehave the job requires additional bias resistors.
If only channels 5,6,7 misbehave then transmitter probably has a digital encoder and requires additional bias resistors.

I've done this before.
NE5044.pdf<br type="_moz"/>

Stmjxxi
All done and working. Thanks for the great advice.

The switch had 3 wires - see photo 1. I replaced it with the centre-off switch and turned the switch so it is orientated sideways. I did a bit of multimeter testing before I swapped the switches. With this arrangement the servo works very well. Left - off - Right. Perfect.

Photo 2 shows the front of the Transmitter with Ch.6 (previously modified as a throttle).

Photo 3 shows the circuit board where the Ch.5 wiring from the switch is connected.

I have also modified the switch on Ch.7 and put in a joining bar to use both Ch.6 & Ch.7 together. I have seen this arrangement on real boats so it is realistic. The joining bar can be removed if required. Photo 4. The Ch.5 switch is on the top right of the transmitter.

I put all the radio components back in the boat and they work well in conjunction with the motors operating. No cut-outs.

Thanks again.

It just requires a bit of fortitude sometimes.
Fleet radios allowed both throttles to switch to one stick.The problem is you forget if you don't have an led to remind you.
Based on your experience,I'd say your radio has the NE5044 encoder and Futaba has bias resistors on ch 5,6,7 and you could replicate that with only a switch.