Fuzzy corners on an old CRT TV

[treismac]

I just snagged a 27" Toshiba CRT (tube screen) from my buddy who is doing some early Spring cleaning. The picture looks great other than the corners which are noticeably blurry. Below is a photo I snapped of Tetris being played on it.



Note the blurrier lines in the corner.

I assume this is a "natural" (technologically speaking, of course) degradation that occurs over time. The TV must be close to ancient since it only has one video input. Is there any miracle/magnetic cure to this condition? Thanks for any info!

[RP2A03]

That is normal for many cathode ray TVs since most of them do not have dynamic focus. If you are feeling brave, take off the back(if necessary) and look at the flyback. It will have two or three pots. If I recall correctly the top one is usually G2 which affects overall brightness and, in your case, should be left alone. If you only have two pots, then that leaves only one pot for focus which means that can only have either a sharp center or sharp corners. Since you can not have both, set the focus to the best compromise between the two. If you have three pots; one is for G2, one for horizontal focus, and the other is for vertical focus. Horizontal focus will appear to mainly affect the edges while vertical focus will appear to mainly affect overall focus.

To adjust the focus with a three pot flyback you will need to display a cross hatch on the screen, then adjust the horizontal focus so the the vertical lines are sharpest at the edges. Next, adjust the vertical focus so the the horizontal lines are sharpest at the center of the screen. You will have to repeat this procedure several times as the two controls interact with each other.

I should also note that you must be very careful when working inside a CRT as you can receive a lethal shock if you touch the wrong thing. Use insulated tools and keep one hand in your back pocket.

[bust3dstr8]

Is there any miracle/magnetic cure to this condition? Thanks for any info!

Why yes...yes there is a magical magnetic solution for this...
they are called yoke tabs, convergence strips, or permalloy strips.

You slide them behind the yoke and stick them onto the tube/neck of the monitor.

You will need to find a test pattern generator that displays a pure white grid.
With the cover off and the pattern running (be careful the tv may have a live chassis)
slide the strips behind the yoke and slowly move them in the corner until they converge
the RGB lines and make white. Then when you are satisfied with the display stick the strip to the tube.
Beers and buddy spotter highly recomended.


Monitor with very poor edge convergence.


Permalloy strips on tubes with and without yoke.

[Tokimemofan]

That is normal for many cathode ray TVs since most of them do not have dynamic focus. If you are feeling brave, take off the back(if necessary) and look at the flyback. It will have two or three pots. If I recall correctly the top one is usually G2 which affects overall brightness and, in your case, should be left alone. If you only have two pots, then that leaves only one pot for focus which means that can only have either a sharp center or sharp corners. Since you can not have both, set the focus to the best compromise between the two. If you have three pots; one is for G2, one for horizontal focus, and the other is for vertical focus. Horizontal focus will appear to mainly affect the edges while vertical focus will appear to mainly affect overall focus.

To adjust the focus with a three pot flyback you will need to display a cross hatch on the screen, then adjust the horizontal focus so the the vertical lines are sharpest at the edges. Next, adjust the vertical focus so the the horizontal lines are sharpest at the center of the screen. You will have to repeat this procedure several times as the two controls interact with each other.

I should also note that you must be very careful when working inside a CRT as you can receive a lethal shock if you touch the wrong thing. Use insulated tools and keep one hand in your back pocket.

10 to 20Kv brave if you weren't aware. CRTs are no joke.

[APE992]

10 to 20Kv brave if you weren't aware. CRTs are no joke.

I've been told it is acceptable to discharge the capacitors by shorting the terminals with a screwdriver though to me this sounds like it might be a bad idea. I've monkeyed around with CRTs plenty of times without discharging it and have yet to be shocked but I'm sure if I keep doing it my luck will run out eventually.

@treismac
Out of curiosity, how old is this display?

[dendawg]

Nevermind..nothing to see here

[fluid_matrix]

I've been told it is acceptable to discharge the capacitors by shorting the terminals with a screwdriver though to me this sounds like it might be a bad idea. I've monkeyed around with CRTs plenty of times without discharging it and have yet to be shocked but I'm sure if I keep doing it my luck will run out eventually.

@treismac
Out of curiosity, how old is this display?

I've cheated death enough times. Discharging a flyback doesn't sound like something I would even consider.

[treismac]

@treismac
Out of curiosity, how old is this display?

APE992, it says it was manufactured in March of 1999.

[APE992]

APE992, it says it was manufactured in March of 1999.

Then my only thought is likely irrelevant. Bad capacitors are known to cause all sorts of fun problems for CRT displays and recapping is a fairly lucrative business for some people. Not sure if I'd ever have a display I love so much as to pay out the nose for someone else to recap it but hey there is a market for everything.

In this case I doubt the caps have gone bad. Might be getting close (10-15 years) but not likely today.

[Tokimemofan]

I've been told it is acceptable to discharge the capacitors by shorting the terminals with a screwdriver though to me this sounds like it might be a bad idea. I've monkeyed around with CRTs plenty of times without discharging it and have yet to be shocked but I'm sure if I keep doing it my luck will run out eventually.

@treismac
Out of curiosity, how old is this display?

The problem is that the CRT its self acts as a capacitor. I had a neighbor who once worked on these and he would use a alligator clip lead attached to case ground and a well insulated screwdriver to short it out. Just be careful as you will probably reflexively jump due to the loud pop. The dangers are a lot higher than other common multi Kv power supplies like helium neon laser equipment. That is because some CRTs regain a charge even months after being unplugged and discharged, so they should be considered live unless known otherwise. That said for $20 you can buy a late model TV at a thrift store with only a day's search.

[Niku-Sama]

Then my only thought is likely irrelevant. Bad capacitors are known to cause all sorts of fun problems for CRT displays and recapping is a fairly lucrative business for some people. Not sure if I'd ever have a display I love so much as to pay out the nose for someone else to recap it but hey there is a market for everything.

In this case I doubt the caps have gone bad. Might be getting close (10-15 years) but not likely today.

actually the caps could be going bad. i've had to replace caps on 3 to 5 year old LCD monitors and they use less juice and have a lighter duty cycle than a CRT.
i was reading in an electronics book that one of the major cap makers had some one steal their design and take it to taiwan or thailand or some place and mass produce the same caps for extremely cheap and saturated the market but when the design was stolen there was a flaw that wasnt corrected untill after all this happened. obvioudly the flaw shortens the life of the cap but the point is they are still working off of this stock pile of bad caps and other companeis are still producing them with the flawed design.

i guess its cheaper that way....i always wondered why i never say bulging or leaking caps in the old electronics i have (80's and before)





also, tetris

[Tokimemofan]

actually the caps could be going bad. i've had to replace caps on 3 to 5 year old LCD monitors and they use less juice and have a lighter duty cycle than a CRT.
i was reading in an electronics book that one of the major cap makers had some one steal their design and take it to taiwan or thailand or some place and mass produce the same caps for extremely cheap and saturated the market but when the design was stolen there was a flaw that wasnt corrected untill after all this happened. obvioudly the flaw shortens the life of the cap but the point is they are still working off of this stock pile of bad caps and other companeis are still producing them with the flawed design.

i guess its cheaper that way....i always wondered why i never say bulging or leaking caps in the old electronics i have (80's and before)





also, tetris

Yes, it was a missing stabilizer that has made more than a few mini grenades. Another problem is that high frequency electronics use them in ways that were never accounted for when the aluminum electrolytic was designed.

[Orion Pimpdaddy]

If you haven't already thought about it, check to make sure you don't have anything magnetic (e.g., speakers) close to the TV.

[APE992]

Yes, it was a missing stabilizer that has made more than a few mini grenades. Another problem is that high frequency electronics use them in ways that were never accounted for when the aluminum electrolytic was designed.

Never seen any of these guys perform their feats in person though I've known people who have had to replace every cap on entire motherboards because of the stolen formula.

Lately though I've been having to replace caps on late 80s video gaming hardware more and more often. GameGears are notoriously bad to start with but I always figured it was to due to the use of SMD capacitors.

[Tokimemofan]

Never seen any of these guys perform their feats in person though I've known people who have had to replace every cap on entire motherboards because of the stolen formula.

Lately though I've been having to replace caps on late 80s video gaming hardware more and more often. GameGears are notoriously bad to start with but I always figured it was to due to the use of SMD capacitors.

Yeah, that is what killed my mom's computer. I popped it open, saw a dozen or so that had erupted, put the side on and asked if I could have the hard drive in exchange for free data recovery to her soon to be new computer. ^.^