I bought a green F28T12 yesterday, and I've been unable to turn it on. There's some blackening around one side of the electrodes, but I can still measure continuity across it.
I've tried both preheat and electronic ballasts with no luck. With preheat there's literally nothing, whereas with electronic (instant start I presume), the side with blackening glows a very mild green without any discharge being formed.

Is there any way to get it to work? It's supposedly a new lamp but it's completely generic with nothing but a G marked on it.

May be the tube has leaked? Do you have a source of high frequency high voltage like a tube checker or just a plasma globe to check if the fill is still OK?

Sounds like it has some sort of contamination

The tube works but won't strike. I don't have a lamp tester yet, but using a "400kV" (around 15kV) HV converter I'm able to see it glow a dim green.
My test was between two opposite terminals so it won't confirm filament strength and such but the lamp doesn't have vacuum loss.

Well, both ends should measure about 10 Ohms cold on both ends. Without both filaments intact, electronic ballast won't work. By the way, your title says T5 and you name the tube F28T12. What is correct?

28W T5 tubes have really high burning voltage of 167V and high striking voltages of 425V hot/650V (!) cold. Sure your ballast is rated for these voltages?

And this tube certainly won't run on preheat until you will arrange a special circuit with high OCV

By the way, your title says T5 and you name the tube F28T12. What is correct?

That's T5, not T12, my bad.

The problem is solved: the side with the blackening has an open circuit and measures infinite resistance.

I had no idea F28T5s run at such a high voltage but it's a philips fixture, so I assume the ballast is rated for it. My preheat ballast, on the other hand (I used 36W for a quick test) is absolutely not rated for this and I have no way to get such a high voltage, because my mains top out at around 245V.

What led me to believe I could do it was, I've succeeded in running F14T5s on electronic as well as preheat setups just fine, with no apparent issues and manual preheating worked fine with almost every tube I've tried so far.

Problem solved though, thank you!

That's T5, not T12, my bad.

The problem is solved: the side with the blackening has an open circuit and measures infinite resistance.

I had no idea F28T5s run at such a high voltage but it's a philips fixture, so I assume the ballast is rated for it. My preheat ballast, on the other hand (I used 36W for a quick test) is absolutely not rated for this and I have no way to get such a high voltage, because my mains top out at around 245V.

What led me to believe I could do it was, I've succeeded in running F14T5s on electronic as well as preheat setups just fine, with no apparent issues and manual preheating worked fine with almost every tube I've tried so far.

Problem solved though, thank you!

thats probably what blew the cathode, the preheating/short current of a normal F36T8 choke can be anywhere from 600-900mA or there abouts which is *far* too high for a 28W T5 tube and I can speak from bitter experience that such preheating currents will blow the cathodes of a F28T5 in short order

@LightBulbFun
The blackening was prevalent even before my test, and I didn't use a starter with the 36W ballast. As with most of my fluorescent tests, I used a wire to manually preheat it and despite that I saw no glow of the filament. If this is what killed it, I'm mad at myself... I should've used the right gear.
Lesson learned, I guess. The hard way.

I think if @SoxFtw really blew one of filaments because of overcurrent, he should see a bright flash at the moment of preheat. I once had a 9W PL lamp (starter integrated) grossly overloaded due to its ballast choke got partially shorted. It started and lighted up (all length) with unusual bright blue-green light (for 827 color tube) but both electrodes survived a preheat phase and a few seconds of this run, until I noticed and switched the lamp off.