With information from KK7QLA we I now know that the LCD backlight control signal is on MCU / CPU pin PC6, this pin is used as the PA band select control in the original UV380
At the moment I don't know what pin now controls the PA band select, but assuming that the LCD backlight and the PA band select pins have been swapped,
I have build a version where the PA band select now uses pin PD8, which is my current best guess about what pins are controlling the PA band select
See
downloads/MDUV380_DM1701/Firmware/Exper ... 0_PLUS.bin
Note.
The file is .bin and not .zip, hence you need to select "Legacy firmware files (*.bin)" in the firmware loader to load this file.
I'm almost certain this fixes the backlight problem, but somone needs to check the power levels when transmitting on FM on VHF and UHF, because even if the backlight and PA band select pins have only been swapped, there is a possibility that the polarity of the signal to select VHF vs UHF may have changed.
If the power looks correct on both bands, then PA Band select is definitely on PD8.
If the power is only correct on one band, or is wrong on both bands, then its likely that PD8 is not being used for the PA band select
It should also be noted that if you load this firmware into a 10W radio, the output power should be approximately 2 x the value displayed on the screen, i.e 5W should output approximately 10W
For anyone with a 5W PLUS version which seems to have the backlight etc problems, the the power is probably going to be as displayed on the screen...
Edit
I just did some tests on my RT-3S aka MD-UV380, and reversed the polarity of the PA Band control signal, so that the VHF PA was selected for VHF and the UHF PA was selected for VHF, and the results are somewhat interesting
If the band select is inverted, then VHF Tx seems to work OK and produces more power (5.5W) than when the band select is configured correctly
However no power was produced on UHF.
This seems to indicate that the tuned circuits in both PA's operate as a Low Pass Filter.
ie. on VHF the UHF PA LPF was enabled, and the reason that more power was observed is that the thrid harmonic of the VHF frequency, ( 145Mhz x 3) = 435Mhz is being mesaured as well as the main signal on 145Mhz, hence the power output was 40% higher than it should be.
This could be confirmed by using a SDR or a Tiny SA, but I've not had time to do that today
On UHF, the radio hardware would enable the VHF PA, which would greatly attenuate the 435Mhz Tx signal. Which doing a rough calculation would need at least 30dB attenuation to reduce 5W to < 0.05W that my power meter can measure.
So. If the UV380 PLUS version, I linked in this post works on UHF and also VHF with consistent power levels on both bands then, the PA band select pin has been guessed correctly and the polarity is the same as in the RT-3S / normal UV380.
If however there is little or no power on UHF then by guess is wrong.