Two dodgy FrSky XM+ receivers in a row - the hunt for answers

CurryKitten · January 29, 2018, 03:57:30 PM

Hello all,

I just want to sound off here to see if I'm missing anything. I've got the little Diatone GTR-90 quad which I'd initially fitted out with a FrSky XM, and it fell out of the sky with 0% RSSI at about 100m.

I swapped it over to an XM+ - having had fantastic range on these before. Same thing, the RSSI dropped off like a stone at around 100m once again. I had a close look, and couldn't really see anything wrong, but I had another XM+ spare so decided to try that. Much the same thing (at this point instead of flying, I'm walking downstairs in my house - which drops it to about 50% whilst on another quad with another XM+ it's at 90+)

I decided to desolder it from the FC, and then shorten those wire and resolder - but same issue. So my next move was going to be to throw those wires away and use some others.

Obviously, it would be useful to stick this XM+ on another quad to see if I had a different result, but slightly more of a faff. Seems unlikely that I've had 3 bad rx's in a row. For the benefit of this testing, the rx is actually outside of the frame and so should be more free of any electrical interference.

So is the wire swap a reasonable thing to try next, or is there something better to do ?

Loopdreams · January 29, 2018, 04:00:42 PM

Was one of the XM+ RXs you tried a known good one?

CurryKitten · January 29, 2018, 04:25:52 PM

One of them had solder on the pads so I assume this was ok. It would be useful to check that obviously but I was avoiding pulling another quad apart to check. I might have something to hook it up to and check to see if the rssi is normal on the test.

ched · January 29, 2018, 07:29:12 PM

Have you checked the supply voltage is holding above 3.7v? I assume the regulator is providing enough current for the rx?
If it's happening to different rx sounds like a power supply issue.

CurryKitten · January 29, 2018, 07:31:26 PM

Quote from: ched on January 29, 2018, 07:29:12 PM
Have you checked the supply voltage is holding above 3.7v? I assume the regulator is providing enough current for the rx?
If it's happening to different rx sounds like a power supply issue.

Yes - I did check it was putting out 5v, but I'm unable to test that whilst it's under load

ched · January 29, 2018, 07:36:06 PM

If you solder the 5v pad that is supplying the XM+ to the vbat pad you could monitor the voltage under load via the OSD?

Schalonsus · January 29, 2018, 07:44:13 PM

Did you flash them with latest genuine firmware from www.frsky-rc.com?
These receivers are often flashed with crappy chinese firmware that does not work well.

Or maybe a problem on the transmitting side. Broken internal antenna on the Taranis?

CurryKitten · January 29, 2018, 09:01:44 PM

Quote from: Schalonsus on January 29, 2018, 07:44:13 PM
Did you flash them with latest genuine firmware from www.frsky-rc.com?
These receivers are often flashed with crappy chinese firmware that does not work well.

Or maybe a problem on the transmitting side. Broken internal antenna on the Taranis?

Yes - reflashed with RSSI firmware from FrSky

Taranis fine - It's how I could compare other quads running XM+'s to see these ones.

I'm going to have to put a known good XM+ from another quad on there to see what happens. I'm starting to think it's an issue on the FC now

English Turbines · January 29, 2018, 09:38:50 PM

Are the RX antennas parallel to each other..? I know on a Telemetry RX it has a very detrimental effect.

I know its best to have some separation and with ideally a 90 deg angle between the two antennas. I would agree though, you should measure the 5v supply the (FC?) is supplying to run both RX and FC when its under load perhaps..?
When I measured one of mine with a linear regulator onboard the FC, it measured only 4.75 volts.

The XM+ is reported to have amazing range, certainly more than 100 mtrs.....Seen one doing 5kms and still more to go, even with a standard TX antenna being used....On a plane though, not a noisy quad.
The other thing I do, is I always filter my VBATT supply to the FC....which can only be a good thing..?

:vulture:

Coyote · January 29, 2018, 11:17:13 PM

Unless being inside the frame has effectively created carbon / lipo Faraday cage around them I would look at another voltage source simply because 3 unit all being faulty would be a bit of a tall order.

CurryKitten · January 30, 2018, 01:39:07 AM

Plan of attack:

I'll swap XM+'s between the ones currently on the Tyrant S (working well) and the GTR-90 (not working) That will tell me if it's the quad of the rx.

If the GTR-90 is still bad with a known good XM+, then my thoughts are that it's the FC. I'm getting a constant 4.95v reading from the current power source, and as I can drop the RSSI to zero within my house without putting any load through, it seems less likely to be voltage.

What I can try (I think) is to take a line into the uninverted SBUS signal on the XM+ into UART6 and try getting the signal from here. I just chatted to Oscar Laing on Facebook to quiz him on inverted/uninverted signals as he's done a few blogs on it - but he did also mention he had a failed FC in a Diatone quad which was giving some bad readings - so certainly not an unprecedented thing.

If nothing works (I don't mind working around if UART 6 works) then I'll attempt to get a new FC to replace this one.

English Turbines · January 30, 2018, 02:00:56 PM

Still sounds like a bad receiver then to me, because of the obvious loss of RSSI. Rather than the FC re-booting itself...?

You can imagine they dont test these tiny RXs at a tenner a go...?

:vulture:

CurryKitten · January 30, 2018, 02:33:57 PM

Quote from: English Turbines on January 30, 2018, 02:00:56 PM
Still sounds like a bad receiver then to me, because of the obvious loss of RSSI. Rather than the FC re-booting itself...?

I'm not suggesting that the FX is rebooting (I would have seen that - as I'm carrying a quad around my house watching the RSSI drop as I move away from the radio) I'm just suggesting that there is *something* not right with it.

I'll know once I swap to a know good working XM+

CurryKitten · January 31, 2018, 03:42:24 PM

Well, this is a turn up for the books. I remove the dodgy rx and then moved over the known good one from the Tyrant S and put it in the GRT-90. I also installed the (what we think) is a dodgy one from the GTR-90 into the Tyrant S.

The result: The GTR-90 now has good range, and the Tyrant S gets 0 RSSI within my own house. So I've got at least 2 dodgy XM+ receivers. A bad batch, or a formware issue. Usefully I don't know *exactly* which version of the firmware I that's giving good range, but I did notice it had RSSI on ch 8 (instead of 16) whilst the others had it on 16, so I desoldered and reflashed it to the latest FCC version with RSSI on ch 8 and it's still crap.

So that's crap isn't it. I've just had another XM+ arrive today as I was out of them (even more so now) I hope this one isn't bad, I'd also considered them ultrq-reliable previously

Here's a video of just how bad the range is

! No longer available

The real annoyance here as well is that I was using a DShot beeper (as a normal beeper wouldn't fit) but because the RSSI was zero and stayed that way until I managed to fluke along close to it some 15 minutes later, the beeper wouldn't sound until I came within very close range.

Loopdreams · January 31, 2018, 04:07:10 PM

It might be worth swapping the antennas between the good one and the duff one. Also are you changing the TX mode before binding after swapping between 16 and 8ch firmwares?