Prevent HDMI from losing signal after lock/blank screen

This worked for me:

xset -dpms
The -dpms option disables DPMS (Energy Star) features.

For permanent solution:

echo "xset -dpms" >> ~/.xinitrc
If it doesn't work or want more user friendly way, add xset -dpms to:

System Tools → Preferences → Startup Applications

(Thanks goes to Ingo Karkat and user.dz)

Reference:

• man xset
• https://askubuntu.com/questions/696738/prevent-monitor-from-losing-signal-after-screen-saver-lock-activates