1084S monitor unsharp in the corners

[8bitbubsy]

So the picture on my 1084S-P1 (PAL) is a little bit unfocused in the corners while the picture in the middle is crystal sharp. I think this started to happen this year, but I'm not sure.
I know this is a common problem, but what I do not know is how to solve it.
I have no big experience fixing monitors, and I know it's really dangerous, but if you have a concrete and known solution for the problem then I might man myself up and try it. It's a lovely monitor, it's worth fixing it

Here's some things I've done to it earlier:
* Reflow all dried solder joints
* Check all cables and connections

I haven't changed any capacitors though (I don't want to change the inrush current too much), nor have I touched any of the adjustment pots inside the monitor.
I know that I have to discharge the HV anode and the main buffer caps before working in it.

[T_hairy_bootson]

Originally Posted by 8bitbubsy

I know that I have to discharge the HV anode and the main buffer caps before working in it.

I didnt know this a few years back and boy did I get a nice zap for my efforts.

[8bitbubsy]

Originally Posted by T_hairy_bootson

I didnt know this a few years back and boy did I get a nice zap for my efforts.

When I was a kid I opened monitors and touched all kind of components.
I never got zapped, I must've been the luckiest kid ever. I did know to wait a little after unplugging the power, but I waited only a few minutes.

[T_hairy_bootson]

I tried to swap the tubes from one 1084 to another. while dismantling the first I tried to remove the suction cap looking thing and zzzzzzapp. I didnt make the same mistake on the second one. :P

[8bitbubsy]

You mean the HV anode cap. There can be aprox~ 26.000-28.000 volts in there, although the ampere is the most dangerous factor.

[stefcep2]

I opened up our ancient 63 cm TV as a kid, ignoring the "dangerous voltage inside" labels, because the TV wasn't plugged in to the power....

[rkauer]

Let me put this thread back on topic: my recommendation is to check the capacitors only if the image quality changes from the time the monitor is turned on (from crisp to blurred). Then go for the capacitors (all!).

Power resistors are also not in the scope of this defect. What is left?

Syncing circuit, PLL chip and the coils around the neck of the tube.

All you have to check is the kind of dust/grime affixed on the coils and try to clean they. IPA is good for this purpose.

If no avail, live with the problem until the day the monitor finally pack up.

[pandy71]

There is some special potentiometer to adjust voltages on CRT electrodes - in better monitors this adjustment is dynamic (optimal focus depend from place of electron beam)
Search for focus in areas CRT board and area of https://secure.wikimedia.org/wikiped...age_multiplier - but remember - this can be dangerous for Your life.
Assume also that with time CRT loose efficiency and degradation of focus is one of the most important symptoms for this. I don't have schematics for Your monitor so my advice is find some schematics, analyze circuit and then slightly change level for focus if it helps - good for You - if not then...

[Loedown]

The three areas of danger in a TV are;

1. Main power filter, a large capacitor rated to 400 / 450V in the power supply, most are fitted with bleed resistors which bring the stored charge down in a few minutes. The only time this can pose a problem is where you plug the set in and it doesn't switch on but turns on when you measure the voltage on the filter cap with a multimeter, this fault is the startup resistor in the power supply.

2. The tube can store charge both in the ultor cap ( suction cup ) and on some of the pins on the neck board, the ultor cap can be discharged by using two screwdrivers, place one on the grey section of the picture tube ( aquadag ) and crisscross a second one making sure of connectivity and then sliding it under the ultor cap until you hear an electrical crack, do this two or three times to be sure. Picture tubes can also hold charge due to light on the front of the screen, this happens slowly and will not occur over a matter of minutes.

3. Triplers use a series of diodes and capacitors to generate the very high tension used by the ultor cap, they can be shorted out by placing the ultor cap suction plug onto the grounding pin of the tripler, this is usually unnecessary because when you short out the tube with the ultor cap it shorts out the tripler as well. Modern CRT designs use a cojoined flyback and tripler in one unit, the 1084(s) has this arrangement, Philips K9 / K11 TV chassis use the separate tripler.


As for the original question, unless the unit has taken a decent knock ( shadow mask ) or been too close to a high magnetic source, I don't know what would cause the unsharpening in the corners, most likely a cap that is in the focus circuitry around the flyback circuit. For a completely fuzzy picture with the focus control on the flyback ineffective I have found that the focus resistor(s) have gone out of specifications, usually 10Mohms or thereabouts.