Filter CAPs - When to replace, possible to increase value?

nate1981s

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Filter CAPs - When to replace, possible to increase value?

Some of the 25" wells gardner K7000's have raster size change with a bright screen which is driving me crazy. I have fully recapped the chassis's in question except for the large filter CAP on each one. Is this the most likely cause of the bright screen image size increase? Some of the tubes are weak and I am slowly trying to find tube donors to deal with this.

Does a weak tube also increase this affect due to the higher driving voltages?
Most of my other games that are 19" are k4900's and they do not seem to have the problem really at all.

Are k7000's or 25" monitors more likely to have this bright screen raster growing problem?

I know the voltage of the CAP must be the same or higher but what about the value? Is increasing the value possible and will it minimize the effect? I ask because I have been removing some expensive large capacitors from dead APC UPS power supplies that I found but probably is best to replace with the same value I assume.

Buffett - what do you think if you see this post? Do you have any for sale and what is your opinion?

thanks,
Nate
 
You could try replacing the filter cap, and a size increase probably won't matter much, although i'd try to keep the microfarads the same.

however, your problem is most likely with a leaky diode...
 
A certain amount of picture size changing is unavoidable. It's caused by poor voltage regulation. Some monitors are designed better than others. The K7000's usually have some of this size changing - when the beam current is higher (white screen) the picture tends to bloom bigger, and when it's lower (darker screen), it shrinks. You might be able to help this by beefing up the B+ cap IF that's what's causing it.

Monitor the B+ voltage and see if it sags - it's *supposed* to be very well regulated, but if it changes with screen content, then you've found your problem. Put a meter on it, and display a black screen. Now switch to a white screen and see if the B+ dips. If it does, try increasing the filter cap size.

It could also be poor high voltage regulation, in which case there isn't a lot you can do about it without digging further into the HV circuits. Do you have a HV probe? You can do the same test while monitoring the HV.

Again, some of this is simply the way the monitor was designed, and you won't be able to fix it. But if it's the B+ supply that just needs some stiffening, or the cap is weak, then you can improve it.

-Ian
 
i find by replacing the filter cap on theses chassis helps it alot. i do it to every rebuild i do.

also if you adjust all the colors and brightness and contrast correctly it is almost gone, or gone entirely pending on the game of coarse.

i also find if you run it just a tad darker then it does not do it at all.

now on games like MVC2 it will do it as they change from lite to dark alot. and it is there if you pay attention, but it's not bad as most times i dont even see it.

if you want your tubes checked i have a rejuvenater that will also determine how bad your tubes are and if your tubes are tired or not.
some may need a little help.

also dont trash the tubes your swapping. i need tubes with burn or no for my games and test monitors. i would like them please.

Peace
Buffett
 
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