Teach an old Tranny new tricks

My Cockpit SX is not that old, but still.. As my understanding of the 2.4GHz technology grows, so does the urge to go the full way.
As you know I like to throw planes now, and I really like the small Multiplex SX. It seems to be built for the job (it actually is). My last session out was with the new Blaster and a cranky 35Meg receiver. In all honesty, the less then perfect performance of the radio was largely due to an empty-ish Rx battery, but still. Of course a plan had already been hatched long ago, so this was just the excuse needed to fire up the soldering iron.

The Plan was to take a Spektrum DM8 module, and connect that to the Multiplex SX. Of course fate has it that Dave had one in his van this weekend. Many people have done this trick to the Royal Evo series, therefore I could not see any immediate reasons why it would not work on an SX, other then the fact that you spend £100 on a £70 radio. Maybe SX owners aren’t that adventurous?

Anyway, the conversion was actually largely a non event. Hardest job was to find a decent place for the bind button. (I do not like to have that at the rear).
So in short, all you do is open the Tx module, remove the bind button, add a few wires and a push button switch, add some wires to feed the module. Your battery voltage does not really matter, since the module has it’s own 3.3 Volt regulator. And the rest of the TX is also happy with a 2s lipo, it has it’s own regulators to make 5 volts where needed. Of course there is always a small gotcha, this one being the fact that the module wants positive pulses, rather then the neg ones that MPX uses. But nothing 1 transistor and 2 resistors could not fix in 5 minutes. (The ‘conversion’ pcb is hidden in the white heatshrink.)

While I was at it, I replaced the battery (NiMH) with something ‘bigger’, since I noticed I was running out of battery juice too fast.

I sometimes see moans about the fact that ‘2,4 hardware should be improved because it uses so much current. Yep, probably 8 times as  much as a 35Meg TX board. Funny how most people will understand that an engine running 100000 RPM will use more fuel then one running 5000 RPM (if they were the same )  High frequency electronics is more power hungry  just because it does a lot more! 

I just put the bottom casing of the module in the cover, it saved me figuring out a place for the famous orange led, and at the same it it now looks as if the module is really designed to be there.

If and when the mood is right I’ll add a few more technical words to this prose, including why I believe this is a proper and legal conversion. I know the BMFA is in general trying to discourage this type of behavior, but it’s really like trying to convince engineers that ‘steam is best, honestly, believe us’.

..more to follow, first a few flights.

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Update 12th July 09:

In the meantime  I’ve had probably over a hundred flights with the MultiSpek and there has not been a single problem. I’ve hooked a few thermals (rare event  here) and never had the feeling that I lost control at any time. In short, it’s an easy upgrade to perform!

The careful reader will notice that the latest pic shows the module in a sligthly different location as the first pics. This was done for no other reason as to make it more obvious that this really is a Spektrum module. Totally original, nothing changed. Plug and play.

Some thoughts: It is generaly accepted that a radio can be marketed with a provision to allow RF modules to be changed. It is accepted that radio’s can have a buddy lead system. OFCOM (the guys that look after this stuff) is not in the business of testing each and every setup, they leave it to the person assembling the bits to make sure it works as advertised. To translate the official words into something more user friendly: ‘It’s up to you mate, good luck if it works, as long as you don’t interfere with others, we are happy.’

My Cockpit SX has a budy lead, which allows you to turn off the RF section and use the RF module of the other Transmitter. All I do is put the ‘other’  Transmitter in the same box. Quite a bit safer and more secure than having an additional RF module dangling from a bit of wire.

Someone can argue that when I plug in my trainer lead, and connect the RF module that way, I do not modify my transmitter and hence it is perfectly legal. If, on the other hand I use the same plug, but route the cable to the inside of the box, it becomes illegal. In law-speak that might be true, but technically it’s nonsense. A properly executed internal modification is always more secure than dangling wires.

The authorities worrying about these things are only concerned about the RF side of the radio installation. No matter what people might want you to believe, they do not care about the batteries, colour of your neckstrap or stickers on the box, providing it does not change the characteristics of the RF output.

In years gone by, RF sections of radio’s were very easily upset, and yes, they could be detuned by holding you hand on the back of the box, holding your antenna(!), dirt on the antenna, temperature, and a whole host of other unknowns. Engineers knew this and designed stuff so it would ‘mostly’ work. Because this almost always resulted in ‘I’m gonna make sure my box works ok’ the authorities had to come up with rules to make sure we could all live together. And their rules were needed to make sure that stuff worked within certain boundaries.

2.4GHz has changed the landscape: The electronics used are the exact same chips used for wireless computer networks. As such, they have to be designed and work in such a way, that there is NO WAY any enduser can mess about with anything that might upset the tuning or other characteristics. (simplifying here) They are the ultimate black-box building blocks: Information goes in, information comes out at the receiver. There’s magic in between and we do not worry how it works. It works or it does not. That’s another feature of 2.4 GHz technology. It will selfdestruct if there is something wrong in the RF hardware.

So, back to our modules: They consist of the little metal box (that’s the actual RF transmitter) and an interface board. The interface board in the case of this Spektrum module (others are similar) contains a voltage regulator to make sure the RF board gets it’s 3.3 Volt and an microprocessor to translate my pulses from the transmitter into something that can be sent across.

RF modules will have an FCC number on it. It is this number that lets you trace the technical specs and license. It is only this bit that Authorities worry about. (You can search the FCC site if you like, I just googled and this is the first one that came up.) You will also find that ALL Spektrum transmitters have the same RF system. Again, the RF system can’t care less whether you send 5 channels, 9.5 channels or wathever. The link has nothing to do with the message.

Here is an interesting view, I know, it’s in America, but in this case we tend to follow FCC regs.

EDIT 17 June 2006 – AMA Postion on this:
(revised to include email header and date, etc., per a suggestion from a member)
————————-
From: Steve Kaluf
To: Larry Hilgert
Sent: Wednesday, April 12, 2006 11:15 AM
Subject: RE: XP6102DX

Larry, here is AMA’s official stance regarding this issue.

Recently, there has been a flurry of activity with regard to various modifications of the Spektrum RC radio systems. Some of the modifications involve rather complicated issues and we’re working diligently to research them completely to assure compliance with Federal Regulations, safety and standards that are being developed for the usage of the band. AMA is doing our best to balance the interests of all involved.

In particular, the removal of the Spektrum RF section and it’s antenna from the DX6 and replacement in the JR6102 has captured the interest of many. After much research with all parties involved, including review of FCC regulations, it has been determined that it is legal to make the modifications under the following conditions by FCC regulations. The complete Spektrum DX6 RF section can be removed along with it’s original antenna and placed into the JR 6102. The FCC identification number from the Spektrum DX6 transmitter must be transferred and prominently displayed on the new transmitter as well. Although the AMA and the manufacturer, Spektrum RC, do not recommend, nor encourage this practice, it falls under FCC regulations as acceptable under the preceding conditions.

Anything more than this immediate modification is beyond this scope of this statement and is being dealt with on a case-by-case basis and will be pursued as resources allow.

Regards,

Steve Kaluf
Technical Director
Academy of Model Aeronautics
United States of America
Ph: 765-287-1256 x 230
Fax: 765-286-3303
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EDIT 17 June 2006 – My position on AMA’s postion:
————————-
In my opinion (make you own determination) one can take this module (the DX6 radio) and connect it to anything you want to provide the power and control signal – it does *NOT* have to be an XP6102. Part 15.23 allows individuals to do so for up to 5 units for personal use, without any type testing or certification required. The DX6 radio, when used with it’s supplied coax and antenna, has already been tested and certified. People get confused (including the AMA apparently) that this radio is NOT covered under Part 95 rules, but IS covered under Part 15 rules, which pretty much encourages the use of spread spectrum technology (with tested and certified radio systems) in many different devices.

In fact, many people have done this with other radios, not just JR radios, and they should be fine under Part 15.23 just as long as the original radio board, coax and antenna is used.

..to be continuead and revised..

References, not complete yet:
OFCOM OfW311
UK RCC
UK RCC-2.4
UK Frequency Allocation Table
Interface Requirements
CAP393
OFCOM IR2030
It’s like this one?!
Juno module
RC-CAM
RC-Groups