IBM 5151 Monitor, bright spot when turning off

[retrogear]

I've always thought of RESET as hardware reset and c-a-d as software reset

 

[SpidersWeb]

Maybe not. I have just acquired a 5151 (the one I had died) and it has a severe phosphor burn in the middle-bottom of the screen.

In that photo, all I can see is a pen mark?
Is that to cover the burn, or is the photo of something else?

 

[retrogear]

Looks like a burn mark to me but with the CRT off, if you look with a strong magnifier close up off to the side of the mark, it's easy to tell if it's a mark on the glass or the phosphor.
I looked at my 5151 around the center with a magnifier and see no marks (yet)

PS - do in good room lighting, not with a lamp as direct light will cast a shadow of the mark onto the phosphor.

Larry G

 

[nestor]

In that photo, all I can see is a pen mark?
Is that to cover the burn, or is the photo of something else?

It is not a pen mark, what you see is the burn. When this 5151 arrived I cleaned the screen in depth, at first I thought it was a mark in the glass but looking carefully... it's inside.

 

[SquallStrife]

Maybe not. I have just acquired a 5151 (the one I had died) and it has a severe phosphor burn in the middle-bottom of the screen.

Such severe burn-in would take an extremely prolonged period of displaying that line, no question.

If that kind of burn-in occurred from a momentary flash, then the whole monitor would be damaged from text being displayed while the user reads it for a little while.

 

[SpidersWeb]

It is not a pen mark, what you see is the burn. When this 5151 arrived I cleaned the screen in depth, at first I thought it was a mark in the glass but looking carefully... it's inside.

Interesting. I have a couple of screens with burn in, but I've never actually seen a black mark like that before.

Edit: although googling it shows lots of dark examples, I've just never come across it that bad before.

 

[retrogear]

I'd seen several with phosphor burns in center of screen doing service on them for years. I remember a crt gun that was shorted and would intermittently snap. The center of the screen was peppered like a shotgun blast of buck shot.
In theory, I think it was pieces of oxide coming off the cathode being hurled at the screen.

Larry G

 

[KC9UDX]

Such severe burn-in would take an extremely prolonged period of displaying that line, no question.

If that kind of burn-in occurred from a momentary flash, then the whole monitor would be damaged from text being displayed while the user reads it for a little while.

Yep, something's fishy.

But I have to disagree that the power-off beam-on phenomenon is always very quick. Maybe with solid-state monitors it is, I guess I never tried to find out. But on old black&white TVs, and some colour ones, the time of the beam-on state can be shown by bringing a magnet near the neck of the CRT, and moving it around. One set that I tried that on long ago had a dwell of two to three seconds (mind you the intensity diminished greatly). Still not enough to cause burn-in. But, the way I understand, all phosphor excitation causes some miniscule amount of burn, so the repeated spot over time (maybe too long to matter) will eventually burn in.

I have seen one black&white TV with an unfocused spot burnt in, but that may have been due to loss of deflection for some other reason. Oh, and that whole myth about colour monitors never burning in is just that. I've seen hundreds of colour CRTs with severe burn-in.

I'm not sure how to explain that line in this case. That doesn't make a whole lot of sense.

 

[retrogear]

Are you talking about phosphor persistence? If you want to see that demonstrated, turn off a crt television while watching a show in a dark room and see how long you can see the last image on the screen.
As far as how a short vertical line could be burned in the lower half of the screen, I remember the really old b/w televisions when turned off the spot would actually travel down the screen before extinguishing.
I'm guessing that's due to gravity because the ions have mass. Maybe I read that in a book somewhere because I have no clue how I came up with that one ...

 

[bobba84]

see how long you can see the last image on the screen.

At one stage, I had not turned on my 5151 for MONTHS (maybe even a year) and, while searching for something in the cupboard I had it stored in, I shone a torch at the screen. XTreeGold showed up legible enough to read the last few folders I had visited on it!

 

[KC9UDX]

Are you talking about phosphor persistence?

No, I'm talking about the total time that you see a "spot" when the power is turned off. Of course, when you turn the power off, three things turn off that affect this: the horizontal deflection, the vertical deflection, and the B++. Usually, the horizontal and vertical discharge pretty quickly, due no doubt to the low impedance of the deflection coils. But, the B++ takes a while to discharge. There are two timed events there: One is the time it takes for that to discharge, and then, when that is done, there's the time of persistence. The first can be observed by deflecting the beam, and the second can be observed only visually.

As far as how a short vertical line could be burned in the lower half of the screen, I remember the really old b/w televisions when turned off the spot would actually travel down the screen before extinguishing.
I'm guessing that's due to gravity because the ions have mass. Maybe I read that in a book somewhere because I have no clue how I came up with that one ...

Some of them did (do). Most that do that squiggle. I'm sure you know what I mean; the horizontal deflection is still barely running, and gets smaller exponentially, whilst the vertical falls. That in itself demonstrates the beam is still active, and that we're not just dealing with persistence.

On some sets, if you recall, the picture reduces to a spot, which stays in the centre, and then becomes severely unfocused before extinguishing.

As to why the beam falls in some, I have no explanation for that, you'd sure think it would migrate toward the centre. It can't be gravity, I don't think. Else the beam would fall any time you kill the vertical deflection. All those TVs with open vertical output transformers with that telltale horizontal line would have that horizontal line somewhere well below centre.

Also, turning a TV on its side would make it fall the other way!

In any case, if the beam is falling at power-off, I still wouldn't expect to see burn-in like that, due to the fact that the beam energy is greatly diminished by then. But who knows? I sure don't have a good explanation for why it's burnt like that. At least if it was a straight line, I'd say it's possible someone was monkeying round with it and killed the horizontal deflection and turned the vertical size down and off-centred it.

Hmm... I wonder if a momentarily greatly increased line voltage (line spike? lightning?) would cause the horizontal deflection to fail. That might explain it, rather remotely. But, if the horizontal oscillator stops, there shouldn't be any HV...

 

[KC9UDX]

Heh, here's another common one: Vertical collapse and horizontal expansion. (faked of course)

Now I'm going to have to make videos of all my TVs turning off.

 

[SquallStrife]

Yep, something's fishy.

My suspicion is that in a previous life, the monitor failed, and was left running in its failed state for some time before somebody noticed it, had it repaired, and the rest is history.

If there was some piece of text down at that position on the screen, and the horizontal coil became disconnected somehow (bad solder joint perhaps), then you'd get a super-intense vertical strip in one spot. It would only take a minute or two to wreck the phosphors in a scenario like that, I hazard.

 

[retrogear]

"Applying line frequency to vertical deflection"

Ah yes, the "hum" test. Dang, the memories that are coming back. When I was a young pup technician, I was taught while troubleshooting a vertical deflection problem,
a quick way to isolate whether the problem was in the vertical oscillator or output stages (I think it was all tubes in the circuit) was to actually hold a small value capacitor
(like maybe .01uf ?) in your fingers and touch the grid of the output tube. If the raster partially fills, the output stage was ok, go after the oscillator. Our small shop
couldn't afford the fancy test equipment in your video

PS - my morning caffeine is slowly awakening my memories. I think it was touch a .01 uf cap between the grid and a 60Hz AC source. I do remember just using my fingers if the
grid was a neutral voltage, though. Good thing my heart was young back then because I lived dangerously at my workbench.

PSS - it was to the filament AC voltage !! Ok I'll stop ...


Larry G

 

[KC9UDX]

My suspicion is that in a previous life, the monitor failed, and was left running in its failed state for some time before somebody noticed it, had it repaired, and the rest is history.

If there was some piece of text down at that position on the screen, and the horizontal coil became disconnected somehow (bad solder joint perhaps), then you'd get a super-intense vertical strip in one spot. It would only take a minute or two to wreck the phosphors in a scenario like that, I hazard.

I keep thinking like a TV, and in that case, it couldn't be that simple. But you're right, in the case of a monitor, with never moving text in that part of the screen, it does make sense.

I'd still expect to see a very straight vertical line, which that one doesn't look like to me. But, it must be, and just doesn't look like it from here. Because what you're describing is the only thing that makes sense.

 

[KC9UDX]

I just explained that video and all my follies with the hum test and my post vaporised. Well I'm not typing all that again.

But I'll post the sequel video because it's fitting .

What did I do to anger the forum software???

 

[retrogear]

I went thru a period of time losing posts but then suddenly had no problems. To play it safe, I do the cut and paste to notepad if the post is long enough. I'm thinking it was a funky drop
of internet connection maybe even at my end. Sucks, though ... Would like to have read your post. I know we are comrade TV repairmen. Maybe it's the vodka

Larry G

 

[Stone]

This forum automatically saves unfinished/mid-stream posts and will restore them upon user request. As I am typing this the yellow 'Auto-Saved' indicator is intermittently flashing in the lower righthand corner of the text box. I know it works as I've used the restore feature many times.

 

[KC9UDX]

I must have a different text box.

Actually, probably so. I think I changed editors to be able to use IBrowse.

 

[Stone]

Everything comes at a cost!