Hi,
Doozer is correct in my opinion.
I don't use ubuntu but from what you write I deduct that the running kernel in the 18.10 distro doesn't support 15khz and truth be told I find it odd the 14.04 one does.
I don't understand what you mean when you say you built a 14.04 one - you mean you built a kernel? Because that's what you need to do for your radeon to work.
A while ago I built myself one and I modified the diff's for the radeon to work with that kernel's sources. I'll attach them. To my knowledge they are the most current ones that exist.
I documented it for myself and I'll copy paste the md file I created.
It's not exactly copy paste for the reader but you should be able to work your way through and you'll need all the build dependencies.
You don't need to use a custom x11 cfg - follow the custom edid instructions in the attached archive
###begin md
TODO : doesn't work with kernel 4.15
The diff files have been modified to patch a 4.14 kernel from kernel.org
#fetch the archive
wget
https://cdn.kernel.org/pub/linux/kernel/v4.x/linux-4.14.tar.xz#extract it
tar -xf ./linux-4.14.tar.xz
#navigate to extracted directory
cd /home/********/kernel/linux-4.14.14/linux-4.14.14
#apply patches
patch -p1 < ../../linux-15khz-patch-master/linux.diff
patch -p1 < ../../linux-15khz-patch-master/ati9200_pllfix.diff
patch -p1 < ../../linux-15khz-patch-master/avga3000.diff
#edit its makefile so we will be able to see the modified kernel name in the grub menu
nano MakeFile
* change EXTRAVERSION = to EXTRAVERSION = -15khz
#obtain super user rights
su
#install kernel header files from currently running kernel to obtain .config
apt-get install linux-headers-$(uname -r)
#locate the contained .config file
find / -name .config
#and copy it to the location where we extracted the kernel from kernel.org
cp /usr/src/linux-headers-4.9.0-5-amd64/.config .
#check if the following option is ON or we won't be able to load our custom edid
cat .config | grep CONFIG_DRM_LOAD_EDID_FIRMWARE
CONFIG_DRM_LOAD_EDID_FIRMWARE=y reads y? -> ok
#compile the kernel
make -j$(nproc)
* answer defaults to all .config new parameters (just enter all the time)
* kernel compiles - took 90 minutes on test system
make modules_install
make install
#follow instructions in linux-15khz-patch-master/README.md file to apply edid when loading
###end md
If you modify the diff's to work with the latest stable 4.19.6 I kindly ask to post them back please.
Also, use the edid_arcade15.bin as it is the most flexible one for a scart tv. You'll need it for groovyMame
kind regards