FrSky telemetry not working on some pixhawks

Hello all

i have 2 pixhawks both are CUAV Pixhawk 2.4.6 with MCU rev3 (2m flash size)
both flashed arducopter 4.0.5 as fmuv2, on first one FrSky telemetry is working but second one is not working

im using rs232 as converter with this tutorial thats working perfectly on first pixhawk
SERIAL2_PROTOCOL = 10 (telemetry 2)

what i tested :
arducopter 4.0.5 fmuv2
arducopter 4.0.5 fmuv3
arducopter 4.0.5 pixhawk-1m
arducopter 4.0.5 pixhawk

testing with other serial ports

any help is highly appreciated

Problem solved by adding 2 1n4007 diode before VCC from FC to RS232