Reassembled mainboard and turned on a monitor, heard arcing noise from deflection coils and instantly powered it down. Disconnected horizontal deflection coil and arcing stopped, but there was no heater voltage present on the neckboard (thus no vertical white line) though G1, G2, RGB voltages were present. Tested heater filaments, no open or short there, I know for sure that heater voltage should be present, regardless deflection coils are connected or not, strange... Heater voltage comes from LOPT 9th pin, current path to fillaments is OK, no open resistors etc. I think that HV might have also been absent.

Charred windings at the very beginning of the coil, though no dead short, LOPT tester wouldn't detect such failure.

Gradual horizontal coil failure most likely caused original HOT to blow.

Good news everyone!

Thanks for all the suggestions and help 🤣

Uploading pictures of plastic latches for future reference, it can be pain in the arse to remove back cover when you don't know how these latches work.

And of course:

Hey, I just started trying to repair a non-working IBM 8518 and the pictures of the plastic latches were very helpful. I'm also curious about what was the fix in the end in your case. Can you explain what was the problem and how you fixed it?

Hello, both times the circuit failure was caused by a bad horizontal deflection coil. The first time I replaced only the horizontal output transistor, the second time there were more minor faults like open resistors and shorted low power BJTs in the horizontal centering circuit, and somehow a XOR chip responsible for H / V synchronization managed to fry itself.

Thank you! I'll update this thread if I can fix my monitor.

What fault symptoms does your monitor exhibit?

I suspect there's some short circuit somewhere that causes the monitor to not even start up, since when I plug it in and turn it on the power LED doesn't even turn on. I suspect there's no high voltage at all since all the usual CRT start up sounds are missing, and the only sound I hear is a repeating "beep" like sound, that seems just like some over-voltage or over-current protection kicking in.

I had an exact symptom the 1st time, power was cycling and you could hear a chirping noise coming from the PSU, though LED light was also cycling in my case, you might have a short somewhere in one of the PSUs rails, check which rail powers the LED.

Fixed my 8518 monitor back then, all it had was main capacitor that bloated.

Much better monitor than 8513 which I knew well were high failure rate and poorly designed.

Cheers,

Fixed!

The only problem was a shorted horizontal output transistor as well. Also re-soldered what seemed like a cold solder joint on a capacitor near the failed transistor for good measure. I hope the root cause isn't a bad deflection coil. SSTV2, is there a way to test this hypothesis?

Great job!

Was that capacitor with a cold solder joint connected between ground and the HOT collector? If so, it means that the transistor has shorted out because a snubber (or safety) capacitor was lost. The purpose of that capacitor, together with a damper diode, is to protect the HOT from a high inrush current/voltage spike during retrace period, when the magnetic field in the primary winding of a flyback TR colapses.

Once I tried to adjust the horizontal width of one monitor by trying different capacitance snubber caps and accidentally turned monitor on without one in place, needless to say, HOT shorted out immediately.

That's exactly it! It was the big 1600V film cap. Well, that's one mystery solved. Thanks for all your help! Now I need to clean this thing to make it look more presentable.

Situations like such illustrates well why a shorted HOT or any other active component can't be blindly replaced before figuring out why said component had failed, you got really lucky this time 😀

So, I may not have been that lucky after all. When I had it open everything was fine, and even after I put it back and cleaned all the plastics it was still working fine apparently, but then I noticed that if I gave it a nudge the image would briefly flash brighter and with horizontal and diagonal lines which wasn't a good sign.

I think there may be another cracked solder joint somewhere since the bright image now just comes and goes randomly but tapping the monitor definitely either fixes it or causes it temporarily. Any ideas where I should look for before I take it apart again?

Try reseating CRT's neckboard in the first place, if it doesn't solve the problem then sligtly tap or turn "screen" adjustment pot on the flyback transformer, if that doesn't help either, inspect brightness and contrast adjustment circuitry for cold solder joints, neckboard or area around flyback. Make sure that the flyback transformer is properly soldered.

Alright, I think those tips helped. I didn't end up needing to check the flyback transformer since just securing the neckboard better helped. I noticed that one of the metal stand-offs that attaches the metal shield to the neckboard wasn't attached at all, so I attached it and that made a big difference. The image is stable now when adjusting the tilt, swivel and when lightly tapping on the screen.

Here it is in all its glory: