Problem with brightness controls

[natgwil]

After installing Kubuntu 18.04 on my Fujitsu AH532 I have had problems with controlling the brightness of the display. Initially I had no brightness controls at all after installation. I edited /etc/default/grub and changed the line which read GRUB_CMDLINE_LINUX_DEFAULT="quiet splash" to GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor". Now I have brightness controls again in "Battery and Brightness" and I can also control it with my function keys F6 and F7.

However there is still strange behaviour. If I put the slider up to 100% it is not very bright, it's actually maybe 70%. But if I adjust the slider down, as expected it gets dimmer, and as I reach about 10% the screen is completely dark. However when I reach 0% the screen is suddenly at full brightness!

What can I do to make the brightness slider (or the function keys) operate as they should? Thanks in advance for any suggestions.

[kc1di]

Please install inxi via command

sudo apt install inxi

then issue this command in the terminal and post the results here.

inxi -Fxz

[natgwil]

Here are the results of inxi -Fxz

System: Host: natgwil-LIFEBOOK-AH532 Kernel: 4.15.0-20-generi
c x86_64
bits: 64 gcc: 7.3.0
Desktop: KDE Plasma 5.12.4 (Qt 5.9.5)
Distro: Ubuntu 18.04 LTS
Machine: Device: laptop System: FUJITSU product: LIFEBOOK AH53
2 serial: N/A
Mobo: FUJITSU model: FJNBB2A serial: N/A
UEFI: FUJITSU // Phoenix v: Version 2.08 date: 10/25/
2012
Battery BAT1:charge: 32.4 Wh 74.2% condition: 43.6/44.3 Wh (
99%)

model: PAC BAT1 status: Charging
CPU: Dual core Intel Core i5-3210M (-MT-MCP-)
arch: Ivy Bridge rev.9 cache: 3072 KB

flags: (lm nx sse sse2 sse3 sse4_1 sse4_2 ssse3 vmx)
bmips: 9977
clock speeds:max: 3100 MHz 1: 1220 MHz
2: 1415 MHz 3: 1366 MHz 4: 1324 MHz
Graphics: Card: Intel 3rd Gen Core processor Graphics Controlle
r
bus-ID: 00:02.0
Display Server: x11 (X.Org 1.19.6 )
drivers: modesetting (unloaded: fbdev,vesa)
Resolution: 1366x768@60.00hz
OpenGL: renderer: Mesa DRI Intel Ivybridge Mobile
version: 4.2 Mesa 18.0.0-rc5 Direct Render: Yes
Audio: Card Intel 7 Series/C216 Family High Def. Audio Contr
oller
driver: snd_hda_intel bus-ID: 00:1b.0
Sound: ALSA v: k4.15.0-20-generic
Network: Card-1: Realtek RTL8111/8168/8411 PCIE Gigabit Ethern
et Controller
driver: r8169 v: 2.3LK-NAPI port: 3000
bus-ID: 02:00.0
IF: enp2s0 state: down mac: <filter>
Card-2: Intel Centrino Wireless-N 2230
driver: iwlwifi bus-ID: 08:00.0
IF: wlp8s0 state: up mac: <filter>
Drives: HDD Total Size: 500.1GB (60.1% used)
ID-1: /dev/sda model: WDC_WD5000LPVT size: 500.1GB
temp: 23C
Partition:ID-1: / size: 49G used: 5.8G (13%)
fs: ext4 dev: /dev/sda2
ID-2: /home size: 354G used: 269G (80%)
fs: ext4 dev: /dev/sda5
ID-3: swap-1 size: 6.29GB used: 0.00GB (0%)
fs: swap dev: /dev/sda6
RAID: No RAID devices: /proc/mdstat, md_mod kernel module p
resent
Sensors: System Temperatures: cpu: 47.0C mobo: N/A
Fan Speeds (in rpm): cpu: N/A
Info: Processes: 349 Uptime: 20:05 Memory: 3180.7/5818.4MB
Init: systemd runlevel: 5 Gcc sys: N/A
Client: Shell (bash 4.4.191) inxi: 2.3.56

[natgwil]

In addition to my original post another problem has developed. Now, each time I reboot the brightness starts at the lowest level, so that I can barely see anything.