Hi
I have a FRsky X20 Tandem with R9 and R9sx receivers.
Both registered and bound easily having Access firmware with the tx internal module updated and with the internal antenna set. But with a few servos plugged in I get no control or very delayed control and the tx announced loss in telemetry every few seconds as the Rx led goes from green to red. Any ideas how to deal with this?

Maybe a disconnected internal antenna?

From your description it looks like your receiver is momentarily / intermittently losing binding with the transmitter.

Are you testing this on the bench? If so, make sure you have at least 1-2 metres distance between the transmitter and the receiver. If they are too close the high powered RF signal in close proximity to the receiver can overwhelm it and cause the issues you are seeing. It might not be the cause of your issue, but moving the transmitter and receiver further away from each other will either confirm or eliminate that as being the root cause of the problem.

I've tried moving them apart after binding and the problem is the same until 5 metres at which point all control is lost.
There is an internal antenna. Is there a chance that it's the wrong type so 915 MHz not 868? It is still glued to the RF board. I'm going to try an external antenna when I get one.
When I updated the Access firmware for the rxs there was no option for EU lbt nor when I updated the internal module.

The internal antenna is tuned to ~900MHz which then universally supports both 868MHz and 915MHz applications. While it's not perfectly tuned for either one or the other, the small difference will be negligible unless you are doing long range flying in which case you are advised to use an external antenna anyway. TBS Crossfire is the same. They only manufacture a single antenna for use with both 868MHz and 915MHz systems.

If you haven't already done so, I would open up your FrSky Tandem X20 transmitter and check to see if the U.FL connector for the 900MHz antenna is seated properly onto the board of the internal module. If that looks to be OK then it might be a problem with the R9SX receiver. Do you have another of the same receiver you can try? If you do, that will at least then narrow down or eliminate the original R9SX as being the root cause of your issue.

Ok thanks for the information. I checked the internal antenna, ordered an external type. In the meantime I reflashed the recs ensuring I got the lbt version and that seems to have put it right.
Thanks for your help