Hey, CRT nerds! Any ideas on what could cause this sort of blotchy dimness on parts of the screen?
I want to apologise for insulting French Telecom engineers. The minitel crt is really nicely designed for servicing and has its own stand built in.
@th From the second image it seems like there might be a person stuck inside the computer; could be related
@th Hmm, is this stable? I hope the phosphor is not damaged due to a failure of deflection at some point in its history.
@artelse it appears to be getting worse. I’m disassembling it now to check on the caps and rest of the components.
@th something seems wrong with the deflection circuit. Have you checked stability of power to the deflection circuitry?
@th @cibomahto The Minitels I have are designed by Philips and has an 80C31 in it. Interestingly has an eprom on board that can be used to run ones own code as some did.
@th that really is a nice stand! Is it steel?
@th +1 with @jelora and friends, capacitors might cause this, if you can read French, there are some threads about this kind of problem, like this one :
https://forums.futura-sciences.com/depannage/866536-crt-minitel-bande-noire-horizontale-a-l-ecran.html
Have a look on the forums of the Musée du Minitel (hi @cquest 🤗) : https://forum.museeminitel.fr/t/reparation-minitel-besoin-daide/593
@scalonnec @jelora @cquest that futura thread looks like the exact same problem! although unfortunately they weren't able to fix it by swapping caps...
@th definitely goblins
@th That's very weird, Smells like damaged phosphor flaking off the inside of the tube, but I've never seen a failure quite like that before. Possible there's a hairline fracture in the neck of the tube and it's gassy?
@ieure I'm worried it might be a tube fracture -- it started after I was flipped the device upside down and on its side to remove the top cover, so maybe this jostled something in a bad way.
@th Well, if the problem gets worse and the picture eventually goes out and the guns in the neck spark, those are all signs of a tube that's lost vacuum.
@th Grimflits. They infest old hardware. :P
@th I am a tube guy, and i have to say this is an really interesting fault that i have not yet many ideas for explanation, other than perhaps a broken anode connection inside the tube
Do the spots change when you touch the front of the screen inside or around them?
Do the spots move together with the picture content if you bring a magnet near the tube?
@crazy_pony the magnet test is a good idea! the dim spots have grown and joined together to cover almost the entire upper half of the screen, and they do move around when I bring a magnet nearby.
@th thats now seriously weird interesting
When the spots move with the picture content we can rule out screen issues (and static charge becuase of broken contacts)
But its also very unlikely an electronic issue as this would likely not blank the retrace lines
@th just beeing curious if there is anything new?
@th phosphor burned or aged with time
There is also a few decades of dust on the tube and the caps (“condos”) might be in bad shape.