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Buddy Box Training Radiomaster Tx to Spektrum DX9

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To Set The Scene ...

My fellow club instructor and myself both have 'Radiomaster TX16S' Tx.

I have an ancient, (first edition probably), Spektrum DX6i. He has a new Spektrum DX9 to set up on behalf of a new trainee in our club.


I successfully set up a wired lead buddy box connection to my DX6i using a twin mono plugged 3.5mm lead - DSC socket to DSC socket in both radio.


In a recent 'rainy day' session, we successfully also set up a Radiomaster to DX9 buddy-box connection using a stereo twin 3.5 plugged lead.

(We believe this is a Spektrum brand original lead but it has no distinguishing labels or marks on it, just it's a stereo plugged lead).


Wanting a degree of back-up for the working lead used in the DX9 we tried the mono lead which had been used successfully with my DX6i buddy-box set-up.

No joy, yes it powered up the slave Tx but no control signals were possible either way.

So we tried another (different) stereo lead thinking that was probably why it didn't work, again power up of the slave Tx but no transfer of control possible.


Puzzled, we tried several other mono and stereo 3.5mm plugged leads, all which work perfectly in their audio operations with phones, headphones etc.


Returning to the 'Spektrum' stereo lead, we also discovered it would only work in one direction i.e. the same 3.5mm plug had to be inserted in the same Tx DSC socket, swapping ends and nothing happens. (However, there are no arrows or 'insert this end' etc. on the lead, we only discovered this by pure chance!).


Trying one of the other leads again, we discovered a very flaky/intermittent operation could be established if we jiggled the plug in the DX9 socket, almost half way out but this is obviously not satisfactory for safe, reliable training purposes on the flying field.


Yes, we know it is possible to go wireless with a module in the Radiomaster but we're trying to keep costs down for the trainee and keep things simple too.


  • So, Is the required lead wired differently pin to pin in the plug(s) and/or DSC socket(s)?
  • Does it really have to be a (comparatively expensive, circa £20-00) Spektrum one or can we make a reliable working one out of either the stereo or mono leads we have or similar that we can obtain at any component shop or even phone spares outlet?
  • Has Spektrum or Radiomaster for that matter, now have poor quality DSC sockets? (Remember my ancient DX6i works perfectly with the TX16S and with a mono lead!)


Apologies for the lengthy post but I wished to attempt to show we tried our best to cover the obvious solutions such as trying other mono & stereo leads and swapping around.

Thanks in anticipation of the knowledgeable advice I know this community can deliver.



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Thanks one and all for your valuable info.

However ... today the puzzle goes further ...

We buddy lead coupled the Radiomaster TX16S (Master) to the DX9 Sektrum using the (one) lead that worked.

Before flight we taxied our club trainer fitted with a Spektrum AR620 Rx, (that's the external aerial-less one), to extremities of our strip just to verify control.

We kept getting 'signal critical' announcements and the RSSI bars drop out on the TX16S Tx.

So one of us picked the plane up and held it at waist height whilst repeating the merry dance at range around the boundaries of our patch.

Still getting some 'signal critical' but not quite as many.

Plucking up courage, (or foolhardiness!), we flew a tight fairly low level circuit or two; ditto, still 'signal critical' call-outs from the TX16S.


Following a hurried but safe precautionary landing, I directly bound the Spektrum DX9 to the Spektrum AR620 Rx and flew again.

The signal bars, on the DX9 fluctuated from full to virtually none so again a landing was soon made.

Re-checking the Spektrum AR620 instructions apart from the obvious no fitting inside a carbon fuselage, no orientation or special installation instructions are given.

Our AR620 was in the balsa/ply fuselage of a 'Boomerang II' trainer, mounted securely and as far away from motor/ESC as any installation normally allows.

What more should/could we do??


We are now baffled with the Radiomaster and Spektrum buddy box combo and the AR620 Rx, at present we have little confidence in risking a model further.

I am not a Spekky basher, (I own two Spektrum sets and have flown successfully with these for years). 

They are, whether you/we like it probably the market leader/most commonly available R/C gear now that Hitec, JR and even Futaba are less frequently seen or indeed available anymore. 

Users of AR620 Rx, what are we doing wrong or should do? Your experiences please.

Many grateful thanks to all that respond with positive and helpful info.

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If the same sets bind in DSMX mode to other RXs successfully then it would appear the problem is a faulty RX. To be sure bind up an alternative RX to the same TX and install it identically in the same model then retest as before. If all is well in that setup you have your root cause.


Edited by MattyB
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From checks with other Rx e.g. Radiomaster R168, once 'Signal Critical' is announced the next announcement can be 'Signal Lost' and all control is lost until you either come back in range or have fine tuned the Rx. Hence, our heeding caution and not continuing our flight trials for fear of such a terminal announcement.


Frame losses, please enlighten me, do I activate the 'Flight Log' - 'Frame' & 'Hold' in the Tx menu set-up to record these? I see 'Tone or 'Tone+Vibe' in my DX8 menu (yes, trying the AR620 with another Spektrum radio).

You also suggest we swap roles and enable the DX9 wireless set-up and use the TX16S as a Rx(?)


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47 minutes ago, Denis Watkins said:

Just check Capt K, that your Rx is not mounted flat between the servos, as is handy on the Boomerang,

but mount it high on the inside of the the under the wing line

where it is in the open and not in the shadow of the servos.

this Rx, like any other, needs mounting in the most open area in the fuz.


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I have always been baffled why Spektrum release RXs with short range telemetry only when no other manufacturer does so. What is the point? If the model is within range to control then I want the telemetry data;  when you are talking signal strength and your initial install has been validated it is only really useful at the edges of range anyway as an indicator that it’s time to turn back.

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15 hours ago, Capt Kremen said:

Early Bird,

From the Spektrum AR620 User Guide, quote:

' The Spektrum AR620 receiver is a full range telemetry receiver featuring DSM technology and is compatible with all Spectrum aircraft radios that support DSM2 and DSMX technology' .

Looks like the full range is 300-400 feet if you want telemetry. In you test is this the range you were at, if so then the RSSI alarm will trigger. 


I am surprised but I now remember a club mate being told that the AR620 is intended for indoor and park flyers. Don't fly far away! Keep it close! he was told just as he took off. 


Even without telemetry 800 feet? 😲


Does this answer the RSSI issues?

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Thanks to all for their learned replies.

Digging around t'internet I find on one of the official Radiomaster pages the following in their 'FAQ' section (See below):


DSMX Range issue?


Many DSM receivers have what is known as "fly-by" telemetry, this is a very short range telemetry system intended to give telemetry data only at close range. The problem presents its self most noticeably in small BNF models which may have as little as 30m of telemetry range. This issue does not however effect the range of the actual RF link and the telemetry warnings may lead some pilots to think they have a range issue with the model when in fact they do not. There is a setting in OpenTX that allows for these telemetry warnings to be disabled and the topic is covered on the Multi-module website.

Disable the telemetry warnings with the check box below.

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13 hours ago, Capt Kremen said:

"fly-by" telemetry

Good find and explanation. 


One of my club mates, despite the keep it close warning due to the 300-400ft fly-by range, had to be rescued with his Riot rapidly becoming a dot in the distance. There was full control at a distance which could have been something like 1000m. Whatever it was, was well beyond the 800 ft range. 


I am now led to believe that they are full range, which to me means if you can see the model then it's in range. Confirmation would be good though.



Edited by EarlyBird
ft not m
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