prevent pi4 hdmi shutdown while printing

Well, really I'd like to prevent it from going into power save mode/screen blanking in general, but mostly during prints.
I tried following the directions in http://raspberrypi.stackexchange.com/questions/47229/ddg#100347 though now it's an all black screen instead of powered off, so there must be something else that I'm missing.
Raspberry Pi Stack Exchange
Using the Raspberry Pi 3

My Linux

It's a Yocto image with Sato desktop

root@raspberrypi3:~# lsb_release -a
LSB Version: core-4.1-noarch:core-4.1-arm
Distributor ID: poky
Description: Poky ...
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