Turnigy 9x module short to ground

[JustinTime]

In order to paint my transmitter I had to take out the original module. To take it out I had to de-solder the antenna wire. That's all I did to the module. When it was time to reassemble I decided to put a u.fl plug. After soldering the plug I checked to see if there is a short to ground from the central pin. Low and behold, there was one. I was sure I introduce it with some lead flowing under the plug and causing the short. I removed the plug, which is no easy task, and cleaned all the lead from the pads on the board. It is CLEAN! I checked the pads for continuity to the ground and there is. The pad that is the antenna central wire shows short to the ground. Is this normal to these modules? I checked a DSM2 module and there there is no short to the ground from the central pad. As I said, I have done nothing to the module except remove the antenna wire.

[jhsa]

I was caught as well 2 years ago..
It's normal with the turnigy/flysky module. I remember spending a couple of hours trying to fix a problem that wasn't there..

Do a good range test before you fly.. I mean a good one to make sure all is ok..

Good luck

Joao

[JustinTime]

Thanks, Joao. I lost the u.fl plug after I de-soldered it and had to buy a new one. Didn't want to solder it before I verified that a short is normal.

[ShowMaster]

Guys!
Rf and antenna feeds are not like wiring up a simple led DC circuit!
RF is AC and AC can deal with what your DC meter thinks is a short!
If you're going to work on you antenna systems and "shunt fed" transmitter connections, you'll have to throw away your "DC" meter and trust your soldering!
What's a "short" to a DC meter may be a perfect 50 ohms "impedance" at 2.4G!
As with writing the complex programming code like er9x or open9x requires an understanding of how it works, so does antenna theory and transmission theory.
Ask any ham radio operator. The test we take requires this knowledge to pass it and it also make sure that at 1000watts you don't cook yourself or or others or worse, die!
Your DC ohm meter is useless for rf and antenna trouble shooting.
On the plus side, it good to read that you got it working and your good to go.
SM



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[ReSt]

And because you can not simply verify your soldering, do a range check afterwards.

Reinhard

[JustinTime]

Thanks for all the input, guys. Glad I've asked. Obviously I have no theoretical idea, or knowladge on how antenna works, which is why I've asked. The new u.fl connector will go on the board today!

[ShowMaster]

Because you asked and for educational purposes only,

Most RC flyers don't understand antenna theory because we usually don't work on our TXs. Most big brand systems won't even sell parts unless its a simple PNP part or antenna replacement. One reason is if the output power stage is in anyway modified from the original circuit it can possibly cause unwanted emissions of spurious signals that can interfere with other systems and also void the original type acceptance that countries "FCC" have for the equipment. Just adding a connector in the antenna line has voided that certification. The coax used and each added connection can cause a reduction in the TX or RX ability of the system. Remember most 2.4G systems TX and RX at the same time. Each added antenna connection can add a 3db loss of efficiency. 3db can be 1/2 power out.
Many times due to the nature of the coaxes velocity factor in relation to the antenna used, the actual antenna itself is modified in length, or the coax length is modified to cheat the physics to get maximum antenna performance.
A different brand or type of coax and connector added to the same antenna may actually produce performance!
This was never an issue in the past since the TX came with a long whip and the RX if not messed with had a long wire antenna tuned for the frequency.
Now with so may 2.4G spare antenna parts out there everyone's messing with their antenna systems. Most violating the certifications. Usually the risk is less range however!
To quote from posted antenna theory, not my words,
"An antenna will still transmit even if the length is not ideal for resonance. However, less of the power input to the transmitter will actually show up as useful output signal. In other words, the efficiency of the system will be significantly lower."
So mod all you want but do a serious range check before and after your mods to at least your normal line of sight flying.
SM
As an added note of info,
You'll read about how the silly RX manufactures sell their RX with an antenna length shorter or longer than the math says it should be?
Then it's recommended to alter the length?
Depending on the snap connector at the RX end if used, not soldered, and the velocity factor of the coax used, the length is usually set at the factory using high end test equipment. The antenna is then cut for the center of the band since most systems hop, to ensure a consistent power out or reception. It's also done to insure minimum transmission of spurious harmonics to be certified. Sometimes a retuning can generate more power out in the tx case, but it can also cause more in band emissions that can cause trouble for other flyers close by or they to you.
Using a "booster" will certainly cause trouble for others close!
It's all good if you're aware of it all.

BTW, all this is covered elsewhere and required knowledge when you're a licensed rf repair person and have to sign off your work like Futaba, Spektrum, and the other repair stations must do. I'm just pointing it out for safety's sake. That 100+ mph plane going by my head hopefully is safe!


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[ShowMaster]

I have a question I'm sure I know the answer to but I can use a second or third opinion in case I'm missing something.
I working on a Turnigy module that had the antenna removed from the Turnigy tx case and relocated (epoxied) to the back of the Turnigy module. The antenna coax was soldered as one melted strand to the rf chip output pads.
This is the interesting part. We all have learned that the rf chip output pads read a short to ground with a DC ohm meter.
Before I resoldered the coax back and had good clean antenna wire shield and center conductor I measured it with my ohm meter. To my surprise it reads a short? Not knowing what's been done to the antenna itself it's either damaged internally or the coax is pinched?
I don't have a Turnigy antenna of my own to unsolder and measure to see if this is normal. Anyone have one that's not connected they can measure and report their findings?
I'd hate to return it not knowing if it's damaged.
I'm sure something is wrong with it.
Thanks
SM
SM


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[ShowMaster]

Got the answer to my own question after close examination of the coax at the fold over section slot. The coax has been stretched to 1/2 it's normal size and is shorting out at that spot. So the answer to my question is yes, the coax should read open from shield to the center conductor when not soldered to the module output RF IC pads.
Now him am I going to fix this?
You don't just solder busted coax back together. If I didn't know better I'd do it and tell him it's fixed,... it's a joke!
Anyway mystery solved!
SM


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