My son uses the Futaba 6EX 2.4 Ghz in buddy pair mode flying using futaba WTR-7 through my Futaba 7C 2.4 Ghz.
For my Micro RC Flying, I use Plantraco 900 Mhz transmitter with my micro receivers. Since we have only one 900 Mhz transmitter, Instead of going in for a second Plantraco Transmitter for my son, I decided to go in for modifying the existing Futaba 6EX for 900 Mhz Plantraco buddy box COB to be operated in dual band both 2.4 Ghz and 900 Mhz.

I would like to share the modification details with you all.

Old Plantraco buddy box comes in a plastic box casing to be used through the trainer port to convert any transmitter into 900 Mhz system. The New Plantraco buddy box is supplied covered in heat shrink wrap. Both are supplied with only a DIN 6 plug. As such it required to be modified with the compatible square plug (Futaba) or round to be used with modern transmitter through a trainer port.

I decided to mount the COB inside the transmitter.

Items required for this modification are:
1.   RC Transmitter any band any brand (in my case Futaba 6EX 2.4 Ghz)
2.   Plantraco Buddy Box  approx $ 50 (http://www.microflight.com/Online-Catalog/Radio-Systems/Monolith-Buddy-Box-Transmitter )
3.   Two way - centre off toggle switch ( Rs 10) 01 Nos
4.   Some wire, soldering iron, drilling M/C, cutting tool, file, glue gun.

Reference http://www.rcgroups.com/forums/showthread.php?t=723846

The Buddy box has only three wires

Red wire               -   +VCC supply (5V to 12 V) to be connected to switched +VCC pin from main switch.
White wire            -    PCM signal input -  to be connected to PCM out pin of the trainer port.
Black/ Green wire   -   Ground  -  to be connected to grnd pin of the trainer port.

Even the old Buddy box COB can be removed from the Plastic box and used.

I used a Futaba transmitter with a square port so I am giving the pin assignment of Futaba square port photo and connections.

First step is to open a Transmitter. Remove the RF module from its location to expose the soldering side of the RF circuit board.

Solder a white PCM signal wire to the PCM out pin (PIN 3 square port). Solder a Black/ Green wire to Ground pin ( PIN 2 Square port).

Remove the red wire  +VCC switched supply to the RF module from the main control circuit board.

Re –fit the RF module back in its place.

Warning : Please do not power up  the transmitter with its antenna Pig tail connector removed during this process, which will damage the RF module.

Drill a 4 mm hole for the indicator LED at the location shown in the back plate body shell.

Mount the Plantraco buddy box COB with its indicator LED through the hole as shown in the photo graphs.

Mount the Centre off Two way toggle switch in a slot cut in the back plate body shell as shown in photographs.

Switching scheme of the Centre off Two way toggle switch is a shown in the Photo graphs.

Solder a Red wire from the Switched +VCC of main control circuit board and solder it to the centre pin of the toggle switch.

Solder one side pin of the toggle switch to the RF Module +VCC supply and solder other side pin of the toggle switch to the Buddy box module COB + VCC supply.

Solder the white PCM signal wire to signal input of the buddy box and black/green ground wire to the ground of the buddy box COB.

Though the buddy box COB has an inbuilt antenna it has been disconnected by Plantraco and supplied with an additional Dipole antenna. Fix the antenna, COB, Toggle switch using Glue gun. And close the TX.

Secure the loose cables/wires with cable ties.

http://lh3.ggpht.com/_qHf-gMAcV8Y/TNFtn8t2OyI/AAAAAAAAAFU/vNTZWqMWeMs/s800/WIRING%20SCHEME.jpg

Wow Sunderam sir !! This is one of the best Ts DIY/Hack threads I have seen just not on this forum but anywhere else.

Great going. Do post some video with the Tx in action!

Great Col  very interesting content thread after a long time 

May be I missed it between the lines, but this gives you a second 900Mhz transmitter, right ?  How do you buddy it with the other (unmodified) Plantraco transmitter itself.

Thank you for your compliments Mr Anwar, Mr Sai, and Mr Ujjwaana.

@Anwar The hack with monolith buddy box COB converts the same existing Futaba 6EX 2.4 Ghz to be used also in 900 Mhz mode compatible with all Plantraco comapatible 900 Mhz receivers. It gives me a second 900 Mhz Tx.

with the toggle switch you can switch between 2.4 Ghz band and 900 Mhz band and in center off position with both RF module switched off, it can be used as a simulator with USB simulator cable through the Futaba square port.

The standard Plantraco Tx box can also be used as a buddy box to transmit in 900 Mhz with any model Tx with a RJ11 cable modified with Square plug through trainer port.

However standard Plantraco Tx is not compatible to be used as a buddy trainer system since its RJ11 port has only PCM in. On detecting signal on "PCM in", joysticks are disabled. Further PCM out is through 3.5 Mono Jack SIM port. In effect if I fabricate a buddy cable to use the Plantraco Tx as master its joystick will be disabled and if I use the Plantraco Tx as trainer through sim port then transmission will not be 900 Mhz but that of Master Tx.

However I can use my Hacked Tx Futaba 6EX 2.4 Ghz/900 Mhz as master to transmit in 900 Mhz buddy paired with standard futaba square port buddy cable or WTR-7 with another futaba Tx.

Quote from: anwar on November 04, 2010, 09:01:56 AM
How do you buddy it with the other (unmodified) Plantraco transmitter itself.

I will post update for this hack also. However it cannot be used as buddy trainer system.
With the hack unmodified Plantraco Tx can be used as a slave 900 Mhz tx with any Futaba Tx to transmit in 900 Mhz.

Quote from: ujjwaana on November 04, 2010, 01:14:55 AM
I have seen just not on this forum but anywhere else.
Do post some video with the Tx in action!

Yes Ujjwanna rcgroups does have a internal mounting thread.
http://www.rcgroups.com/forums/showthread.php?t=632626

Instead of bumping 2007 year old threads there on this, I thought I should post it here and link it there.

Videos follows

This hack of internal mounting does have a range reduction issue with certain Tx which have their outer body RF shielded. In case range reduction is noticed then two antenna wires of length 78 mm has to be got out and fixed outside instead of fixing it inside. Will range test and post update. 

After I have sold my 6EX modified to a friend I have now modified the Futaba 7C 2.4 Ghz for dual band 900 Mhz operation.

One more 900 Mhz Plantraco module available for sale which will be posted separately as a sale thread with complete details