That's true and not true all at once. It depends which monitor you're talking about, how big a sync range it has, and how it handles signals further from its ideal rate. The MS8, and maybe it's only specific MS8's, are quite sensitive. That's about as limited as it gets for anything made from about 1980 onwards, at least that I'm aware of. A TV chassis newer than about 1990, on the other hand, is happy to try and sync to just about anything. Results can vary when you're not specifically at PAL or NTSC frequencies, but normally it's not an issue.
Most monitors are going to fall somewhere between these two extremes. I get the tuning fork analogy, and that it's going to push the system harder to operate away from a set ideal frequency, but nothing is ever dead on including component values, this is the real world. Old arcade games weren't universally synced to 15.75kHz/59.9997Hz anyway, they were grouped around that. Operators would swap 15kHz jamma boards in generic cabs all the time.
We mostly talk about arcade chassis here, true. And multisyncs are more fragile than the average 15kHz-only monitor, at least according to everything you hear. It only makes sense that different chassis are going to perform differently in the long term, the question is how. If you're sure the MS9 won't tolerate the ranges discussed above, that's valuable information, no need to beat around the bush
And if you have any specific information or specs regarding other monitors, i'll happily start recording it because it's good to know.
EDIT: Is the difference going to be PAL/50Hz frequency tolerance? I had assumed in those cases that the horizontal coil wouldn't much notice - it's still pinging away within it's sync range, it's just the vertical section that's running slow. And there's much less power going through the vertical section, isn't there? I've always found vertical issues easier to diagnose and fix, certainly. I guess i've never seen any specs for a newer 15kHz CRT that didn't list a ~48Hz-65Hz vertical refresh range. I just looked up my Sega MC-2000S and it lists 50-60Hz, but does state a 15.75kHz horizontal scan specifically.
EDIT: sorry, i'm rambling, but i think this is worth talking about. I found several 2014 quotes from Jomac in a thread about MS8's which i'll take as gospel on the subject:
The MS8 has a very tight frequency band in 15K mode , absolute min of 15.450khz and Max of 16.050Khz , optimal is 15.750Khz , it also has a tight vertical range between 54Hz and 60Hz.
When ever they are operated outside of this range there is push and pull amplifier ( part of the little Hybrid Sync Module )that attempts to clamp the frequencies within the safe range , so if it is being run and absolute min or max listed above the amplifier / clamping section is working at maximum , if you exceed these limits these chassis most of the time are able to cope when the chassis is in perfect working condition.
I have measure on a CRO watching all sorts of Mame setups that are supposed to be blocking everything outside of 15K but at startup before there is even anything displayed I have seen anywhere from 13K to 17K before they settle to somewhere in the 15K range.
By no means am I saying don't use these monitors , I am only stating that 99% of all the failures I see with the Sync section on these are from machines running mame of some description.
There is no gradual or accumulative damage , they just work as normal and suddenly fail as with the chassis in this thread.
These monitors are great and fine to use as long as the everything has finished stabilizing before the power is switched on , it seems to be that most have failed during the bios screen but plenty more have died from soft15k.
It's the frequency changes they are not designed for that causes the damage , the games they were designed for are Xtal locked to a set frequency that never changes , I don't recall this problem ever happening with these when used on a genuine arcade board.
you have to keep in mind that most monitors have a wider tolerance than this model Nanao so it's not often an issue
Guys I'm not really sure how many other ways I can say this, I think I have more than made it clear that anything non genuine can and often does damage the MS8-26 and MS8-29 models ,( Not MS9 ) having the power off during boot up helps a lot but is not a guarantee.
And regarding the MS9's having the same issues as the MS8...
Not a problem at all with the MS9 , but you can still cause the horizontal drive to go out of spec and blow the horizontal output.
https://www.aussiearcade.com/archive/index.php/t-68425.htmlThis doesn't address the MS9's 31kHz sync band, of course, so more information in that area would be nice