i have the same symptom like yours with the unmod & standard firmware flysky 9x TX, i manage to get in control around 30 meter on ground test only!! if more than 30 meter, servo start to get unresponsive & most of the time no movement.
i have also open up the Tx & Rx to check the antenna cable for cold joint but it all checks out with good solder connection!
Like u, i have also check the Tx & there was no resistance to be measured on the PCB between grnd and the coax positive, even after I'd unsoldered my connections. But on Rx, i have a very high resistance reading.
So, my question is, have u found your solution or repair the Tx module? if yes, can u share / teach me how to repair this 9x tx.
best regards
ktgoh
From Malaysia, KL
ktgohdt125