Interesting. It is difficult to find the source of these types of problems even with the hardware on hand to poke at, so it's pretty much just conjecture on my part since I do not own such a supply, but I've seen these kinds of issues before.
Can you tell me what monitor you are using? CRT or LCD? Does it have an old-timey RF input? Antenna? What kind of power connection does it use? Two or three prong?
And is everything connected to the same AC mains? Like on the same power bar or somehow maybe the monitor is plugged with a 30 foot extension to the next room? With this type of issue, gotta know that.
And is the maker of this supply aware of the problem? Do they acknowledge it? Are they working on it?
My theory, for what it's worth, is that the VIC-20/monitor combo is showing on the screen the noise that the 5V module is injecting into the DC side rather than draining it on the AC ground/earth side.
Perhaps page 4 of this will explain
https://www.mornsun-power.com/public/up ... 3BxxR2.pdf
The AC side of a switching power supply is quite noisy in the electrical sense, you can consider it like a small AM radio transmitter sending out a noisy signal that consists of the frequency of the switching power supply which can be in the 100s of KHz these days, maybe even close to 1MHz (maybe not in these cheaper modules but still).
For many applications it is acceptable to dump this energy on the DC side and it's fine.
But as you can see in the very analog world of NTSC video hooked up to a monitor, there is no difference between the noise and the real video signal and the TV shows it as a hum bar or herring-bones or whatever crap.
The only way to really handle this is with the circuit from page 4. It's either a re-design of the supply or a rework of the present one.
I think by adding an earth ground to the power supply and the right filtering, the issue can be solved but you will need to dig in the supply and add some parts. And from what I see it's a tight fit ... yikes.
, now what?"