This is common with bad or cheapo electric installations.
With a good electric installation such normally does not happen.

To make sure you are not in danger of fire or electrocution, I'd suggest you to ask some knowledgeable friend or an electric contractor to check your electric installation for proper grounding, filtering etc.

Yup, this is possible. My father is a ham (radio amateur) and experienced a lot of weird stuff with RF.
From what I know or learned so far (incomplete)..

a) Modern devices do care little about RFI; they're produced cheaply and thus do have little to no shielding
b) CRTs are sensitive to magnetic fields; their flyback units are akin to strong electro magnets
c) Noise can crawl along the cables; socalled "coat waves" (or mantle wave)
d) Ferrites (ferrite clips) can block coat waves (install them at the ends of the cables)
e) The protective ground on power outlets is not always clean, sometimes it causes radio noise to travel in
f) Some external, higher quality power outlets do have built-in AC filters (caps, coils etc)

Anyway, these are just my two cents. Just read an old article about Atari ST vs C64 in the shack.
Turned out the C64 had a bad shielding; the keyboard was the culprit of noise. Some hams used the shielding of their RG58 cables to fix the C64.
They removed the shielding net/grid safely from the cable and imposed it over the keyboard cable (and grounded the shelding.)

Btw, something similar was mentioned in the book "Radio&Computer" by Mr. Gongolsky.
At the time (1990s). he wrote that cheap PC keyboards do have noisy keyboard controllers inside. No shielding, at all!

Edit: Added some pictures of the ferrites that I meant.
One is built into a good USB cable (orig. printer cable), the other one was attached manually to a cable modem's PSU cable.
From what I read in a HAM magazine, some people reported that noise went down by 36db after applying such a ferrite to the PSU cable of a DSL router.
Not sure if the numbers are right, but attaching such a little ferrite shouldn't hurt (please prove me if I'm wrong).

Edit: The microwave problem is special, though, I believe.
Such a beast draws about a kilowatt, which can cause extreme fluctuations in the mains. That's not very friendly to a PCs swichting PSU, either.
There must be a reason why some step-down / step-up converters are especially labeled "PC compatible".
I assume these PC PSUs (and the monitor) were constructed with a more or less clean sine wave on the AC in mind.
So the mircrowave oven is essentially the worst case scenario here, being a giant RF generator.
On the other hand, both devices were meant for home or office use, so there must be a way to solve this conflict.
I'd check if it is possible to install some filters between the power outlet and the PC.
Or I'd check if the PC's or microwave's AC plug is correctly installed in the outlet (phase on wrong pin ? / hot vs cold)

Here are some links, that I found while searching:

https://www.quora.com/Why-doesnt-it-matter-wh … he-power-outlet
https://www.thespruce.com/checking-for-incorr … -wiring-1152518

The Seven Types of Power Problems
https://www.apc.com/salestools/VAVR-5WKLPK/VA … WKLPK_R1_EN.pdf

That being said, please be careful with the mains. Don't do any tests/experiments when you're alone.
Some households around the world still have no residual-current circuit breaker as standard, also.

Many, many years ago I worked in a car audio shop and my manager brought in a PC running DOS and Blaubox for us to use when designing custom subwoofer enclosures. I had completed inputting the Thiel-Small parameters for every single sub we sold when he brought in an old microwave for us to use at lunch if we needed. Space was tight, so he put the microwave on a table next to the desk the PC was on. About a week of using the microwave, the PC refused to boot. I brought in my copy of DOS, reformatted the drive and installed it and in less than a week of heavy microwave usage I had o do it again. It seems the old microwave was screwing up the hard drive badly every time it was used. We ended up relocating the PC to the front of the store and after a reinstall it was working fine again.

This issue is almost always caused by improper/insufficient grounding.
It is just impossible to give advice without knowing the electric earthing system in effect.
I suppose you have a TN-C system, as I have. With bridges between N and PE in the wall outlets.

You can set up a small TN-C-S subnet by supplying *all* computer related stuff via a single outlet. But you must take extra care that you do not have any PE to N connection to the rest of the electric installation. This can for example make necessary to cut patchcable shields on one side etc, to avoid PE-N shorts.
This is the way to avoid such problems you see and reduce the fire hazard (for example through having neutral currents flowing through cable shields).

So I solved the otherwise "normal" problem you experienced.
And just to mention, my Sanyo microwave oven stands in a distance of 80 centimeters of my main PC, and it gets fed by the same circuit (on the TN-C part). Haven't had any display distortion problem since I split up the distribution as described.

^Thanks for sharing that experience of yours.

Personally, I was almost going to recommend getting a UPS (uninterruptable power supply) for filtering and such,
but since I had a bad experience with a very cheap UPS once (or "USV" as it's called in my place), I didn't yet. 😅

Yup same here. Only when I had the monitor with it's back to the wall where opposite the microwave was running.
Funny thing: when my previous dog barked (giant dog, loud bark) in the room the monitor is in, the screen of the CRT shaked. Pretty crazy right? No idea how it's possible. My CRT tv doesn't have this, only computer screens.

I also had computer speakers where I could sometimes hear radio broadcasts from, when the PC was powered off.

I do have an expensive power conditioner, called a "Monster Power", that I am not using. I might as well just plug all my retro computers into it. it will even protect them from power surges, so I can feel comfortable leaving them plugged in all the time (though I probably still won't). In addition, I'l have my electrician friend check my wiring. With modern, cheapo wiring, anything is possible. Thanks for all the advice!
When it first happened, I thought the monitor was failing, but there were no weird smells or sounds, and if it were a monitor issue, the lines (or another sign of personality) would have come back when I turned it back on. They did not. I also recall the lines starting at the exact time I heard the microwave beep.

I might power the 425 on, and make my dad microwave something (this could be an excuse for him to make my dinner!! 🤣) so I can observe the CRT. 100% chance it'll do it again.

AC line suppression prob- definitely exists in the 425's PSU, but I'm pretty sure that's for the ground output, as that's what I observed in the shipping-damaged 433's PSU board. Might also be as per the FCC rules that claim the device must accept any interference... blah blah blah.

I'll also have to see if it affects my CRT monitor. Just for accuracy, I'll plug it into the same outlet the 425 usually receives its power from. It's newer (circa 2000 is my guess, cryptic date code, wanna help me decipher it?) so it may have some form of input suppression, but it's a cheap monitor, so maybe not. I'll also see if the 425's behavior-with-microwave is any different when plugged into the expensive power conditioner - it will be, but I just want to see. 😀

Also, the dog barking thing is interesting. Maybe your dog was emitting electromagnetic energy!! 🤣

Yes, ask your electrician friend where ground (PE) gets separated from neutral (N).

Correct is to do this before the electric distribution main box, after which there must be no connection between N and PE.
That frequent "solution" bridging neutral and PE in the wallplug is a poor and dangerous "retrofit", as it leads to different PE potentials at every wallplug. When I was young I learnt how dangerous this is, when I had an electric heater on, and noticed that the printer cable was scorching hot. This was because of a considerable part of the heater current flowing through it.
This is extremely dangerous. In particular patch cable shields are in risk to get so hot to cause a house fire.

When part of the microwave current draw flows through the shield of your VGA cable, such distortions are just normal consequence. This should not be taken lightly.

Another dangerous consequence can happen: electrocution in case of a neutral conductor break...

Edit: Coils and some kinds of capacitors can develop microphonic effects, which can get very annoying.

Ultrax wrote:

I do have an expensive power conditioner, called a "Monster Power", that I am not using. I might as well just plug all my retro computers into it. it will even protect them from power surges, so I can feel comfortable leaving them plugged in all the time (though I probably still won't).

Oh no, Monster's marketing strikes again...

Monster isn't a technically good brand, just one that invests heavily in retail presence and bonuses for salespeople foisting their mediocre products onto unsuspecting customers. That's what you paid for, not any intrinsic quality of the device.

Brand issues aside, "power conditioners" are hopelessly overrated. They can smooth out some ripples, but they don't even marginally protect against massive overvoltage due to eg. lightning strikes (the main reason people buy them) and they can't protect against dips in supply due to brownouts or large appliances. All they can do is protect you from little peak voltages when bad-behaved devices turn on or off.

If you want something that actually guarantees correct voltage, go for a UPS. Even that won't really help against lightning, but you'll not be affected by surges or dips anymore.

Of course, it's entirely possible here that the EMI is over the air so not at all related to shared power lines in the first place. If you're used to TFT, it can be a shock how sensetive CRTs are to anything inducing a magnetic or electric field. That's what you get for essentially basing your display technology on a low-end particle accelerator 😜

I read the title of this thread, at first, as "Microwave oven interface with CRT?!". I was thinking, "What's this guy got cooking?"

dionb wrote:

Oh no, Monster's marketing strikes again... […]
Show full quote

Ultrax wrote:

I do have an expensive power conditioner, called a "Monster Power", that I am not using. I might as well just plug all my retro computers into it. it will even protect them from power surges, so I can feel comfortable leaving them plugged in all the time (though I probably still won't).

Oh no, Monster's marketing strikes again...

Monster isn't a technically good brand, just one that invests heavily in retail presence and bonuses for salespeople foisting their mediocre products onto unsuspecting customers. That's what you paid for, not any intrinsic quality of the device.

Brand issues aside, "power conditioners" are hopelessly overrated. They can smooth out some ripples, but they don't even marginally protect against massive overvoltage due to eg. lightning strikes (the main reason people buy them) and they can't protect against dips in supply due to brownouts or large appliances. All they can do is protect you from little peak voltages when bad-behaved devices turn on or off.

If you want something that actually guarantees correct voltage, go for a UPS. Even that won't really help against lightning, but you'll not be affected by surges or dips anymore.

Of course, it's entirely possible here that the EMI is over the air so not at all related to shared power lines in the first place. If you're used to TFT, it can be a shock how sensetive CRTs are to anything inducing a magnetic or electric field. That's what you get for essentially basing your display technology on a low-end particle accelerator 😜

Well, it was given to me. Glad I wasn't the one who paid for it! 🤣

I have an APC UPS somewhere. I'll have to find it, or buy a new one. Preferably one with a true sine wave inverter.

This particular Monster Power has actually saved the TV in the living room on a few separate occasions. Even still, it might not be what I need. Plus, with a UPS, I can play Wolfenstein when the power goes out! 🤣

retardware wrote:

Yes, ask your electrician friend where ground (PE) gets separated from neutral (N). […]
Show full quote

Yes, ask your electrician friend where ground (PE) gets separated from neutral (N).

Correct is to do this before the electric distribution main box, after which there must be no connection between N and PE.
That frequent "solution" bridging neutral and PE in the wallplug is a poor and dangerous "retrofit", as it leads to different PE potentials at every wallplug. When I was young I learnt how dangerous this is, when I had an electric heater on, and noticed that the printer cable was scorching hot. This was because of a considerable part of the heater current flowing through it.
This is extremely dangerous. In particular patch cable shields are in risk to get so hot to cause a house fire.

The "correct" way depends on country and the type of installation:

https://en.wikipedia.org/wiki/Earthing_system … voltage_systems

Furthermore, in some countries it is (or was) not mandatory to have grounded outlets in every room. On top of that, sometimes grounded outlets are installed even if no grounding wire is present (usually bad DIY) or the wire gets interrupted somewhere.

Update to this: I just acquired a Compaq 151FS. Totally different monitor, made by a different manufacturer inside. Guess what? My dad turned the microwave on and the exact same lines appeared on the screen. I can only hope it's going through the air and not my wiring. So, the good news is, my 425s monitor is not failing. The bad news is, microwaving will be prohibited during vintage computer time. 😜

What brand is the microwave? Sometimes the cheapy ones skimp on shielding and power filtering.

It's a Panasonic "Inverter" 1200 watt, made almost entirely out of metal. Maybe the higher than usual power has something to do with it?

No. You have already been told the cause of the problem, but you seem to prefer denial than facing the reality that your house's power distribution has (possibly dangerous and/or costly to remedy) issues.

Jo22 wrote:

b) CRTs are sensitive to magnetic fields; their flyback units are akin to strong electro magnets

I had a CRT so sensitive that it needed degaussing after rotating the monitor by 90 degrees on its stand.
My theory was that sudden change of monitor's orientation with respect to Earth's magnet is causing picture distortion. Does that sound plausible?

Yes, in fact early color TVs had to be either manufactured on the same hemisphere they were to be sold in, or have the tube magnetic controls adjusted again after transport.

The noise visible on CRT can be down to subtle changes in line voltage at the moments a high-current device is turned on. Nothing to do with RF and so neither EM shields nor cable-mounted ferrite chokes / spike supressors are going to help much. Back in the day my own CRT Samsung would start wobbling just a tiny bit in upper left corner every time the electric grill in the kitchen was turned on. And the grill is just a dumb heater element. Ain't nobody going to rip out the cables out of reinforced concrete walls in multi-story building to "fix" that. It's mostly down to subpar PSU in the monitor rather than the wire diameter anyway.

Deunan wrote:

It's mostly down to subpar PSU in the monitor rather than the wire diameter anyway.

Using subpar stuff greatly enhances ones chances to encounter surprises, indeed.
And IBM PS/2 (speak PS half) monitors are grossly subpar. They were a real shame for that companys' name.