Basically, using the normal procedure, install the FW version that has the APP version you want to keep. Example: APP 1.1.9.
Then connect to your X1600 via WiFi and using the necessary tools (example, WinSCP) just go to
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/usr/firmware
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and copy to the above folder the FW BASE versions that you want to keep in your radio.
You need the following credentials:
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Here's the content of /usr/firmware on my X6100:
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The above xgf files can be extracted from the the ZIPs that have the FWs made available by Xiegu. You can use a tool like WinZIP to open and extract each off the above (except the _160 files; those are the patched ones).
If you do the above and go the FIRMWARE UPGRADE option on the radio, here's what you'll see:
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Now just pick, from the list, the BASE FW you want to keep and choose the UPGRADE option on the above screen.
Hope this helps.
73.
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Joao, CT2HME
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PS: Proceed at your own risk. This is not described anywhere on the X6100 manual.