Monitor Focus Seems Out of Whack

[titan91]

Since I'm having so much good luck with free TVs lately, I recently picked up a Symphonic/Funai 24" that has not only an aggressive low pass filter built in to the chip, but also a sharpening filter I can't turn off. Fun!

One of the other issues this TV has is after a few minutes of operation the focus was a little blurry with the factory focus pot position on the flyback. I adjusted the focus and found adjusting it fully to the right makes the image a little blurrier, and adjusting it fully to the left makes it as good as possible. I feel the focus can get a bit sharper but that's all I can adjust on the flyback.

To my knowledge this is not normal. The focus should be blurry at either extreme and should be sharp when the pot is centered. The set was made in 2005 with what I would anticipate only about 4 years of use on it. It was used to receive over the air TV until I assume 2009 when high power analog TV transmissions ceased to exist. It's very clean inside.

I have not re-capped it. But to my knowledge the focus winding on the flyback just goes through an internal resistor and of course, the focus adjustment pot. The socket on the neck board is clean and secure. The TV did not belong to a smoker.

Crappy flyback? I noticed the screen pot started causing the picture to pop in and out of brightness a little bit after I adjusted it.

[vol.2]

Crappy flyback? I noticed the screen pot started causing the picture to pop in and out of brightness a little bit after I adjusted it.

So hard to know. It's a budget brand, and that's the end of the prime capacitor plague era, so I wouldn't rule out extensive cap carnage either.

No, it absolutely should not focus best on the extreme of the focus pot. That means you have an issue with one or more things.

This stuff is all pretty dangerous to diagnose because focus is 2kV and up in most cases. You need a high voltage probe and you should be using an isolation transformer to isolate the DUT from the power grid (properly modified IT that has the earth ground REMOVED from the circuit)

The good old repair faqs has some advice about bad focus you can exhaust, but keep in mind that bad components could be pulling voltages down (or up) in unexpected places. Finding a low focus voltage does not necessarily mean your flyback is pooched

https://www.repairfaq.org/samnew/tvfaq/tvbadfocus.htm

[tongshadow]

I had an old LG that did exactly that, only a new flyback replacement fixed the issue.

[titan91]

That's a great article, thanks for the link. I don't really intend on troubleshooting the chassis board since I'll be swapping it. I just wanted to make sure the tube wasn't shorted. It was operated for about 5,000 hours if used 4 hours a day for 4 years as a very rough guess. I doubt it's that many hours, the only dust anywhere is around the flyback.

The chip on this one is pretty limiting with some filtering that is always on that softens the image at 480i resolutions. 240p (double strike) resolutions look ok. I have a box of WG K7500 chassis boards someone gave me and I may try and see if one is the P538 variant for 25" tubes that I can repair and test.

https://forums.arcade-museum.com/thread ... ch.389265/

I realize the whole art of tube swapping is an iceberg in and of itself...

[vol.2]

I realize the whole art of tube swapping is an iceberg in and of itself...

It is. You are most likely going to run up against incompatibilities between the yoke and the new chassis board, or the new chassis board's yoke and the tube

I've seen it done many times with monochrome tubes with little to no fanfare, but newer and color tubes are more difficult

And you can't really know if the tube is shorted or pooched without either first ruling out the chassis board (and neck), or using a tube tester. Sometimes tube testers can also lie, but they generally can tell you if you got a real issue or very weak tube on your hands

BUT, my instinct in you case is that the tube is okay and you have bad component(s). I think usually if the tube is super weak, you won't get focus anywhere along the travel of the pot. In other words, you will get "best" focus somewhere in the middle, but it will still be fuzzy because you have to turn up the screen too high to get a raster

[titan91]

I agree. I had a Samsung tube from a late model TV develop a blue cathode short to ground. That tube would not maintain stable brightness between dark and light scenes, and the focus would drift with brightness. That TV had about 30k power on hours from my rough math. Used all day every day in an office for many years.

Fortunately this Funai tube doesn't have any of those symptoms. And I'm aware of potential yoke resistance/impedance matching issues with a replacement chassis. There's also a new one (albeit from China) I could get for less than $100, replace the capacitors with quality ones, and adjust it to match the horizontal yoke differences. There are different tap points on the flyback secondary side to accommodate multiple yoke measurements. And the new replacement chassis supports multi-region s-video, component video, and has a full service menu by design.

I think because I've gotten the TVs I have, and the Wells Gardner boards, for free I'm overdue for spending some actual money.

https://www.youtube.com/watch?v=MoqflFfvkR0

[titan91]

I tried cleaning the pots with DeoxIT Fader and while they seem to turn smoother, limited focus adjustment is still the same. Very little change from one extreme to the other. It does change, but it only gets "good" at the lowest end, and not "great" like it should halfway.

Would I be off base if I were to accuse Funai of using an underrated transformer for this 24" tube? Here is a JZ or JF (can't tell) branded JF0501 series transformer from a 20" RCA TV. Its part number starts with 19.

Spoiler

And here is the same brand and series flyback from this 24" Funai TV, about a quarter of an inch larger in diameter. Its part number starts with 21.

Spoiler

Could 19 and 21 indicate the designated CRT sizes? A couple of JF0501 schematics I found and their voltages:

Spoiler

[vol.2]

I tried cleaning the pots with DeoxIT Fader and while they seem to turn smoother, limited focus adjustment is still the same. Very little change from one extreme to the other. It does change, but it only gets "good" at the lowest end, and not "great" like it should halfway.

I wouldn't expect that to help. If there's something wrong with the flyback, it's either a partial short in the windings or a leaky voltage divider between the screen and the focus. Cleaning it shouldn't help

Would I be off base if I were to accuse Funai of using an underrated transformer for this 24" tube? Here is a JZ or JF (can't tell) branded JF0501 series transformer from a 20" RCA TV. Its part number starts with 19.

i doubt it. it probably worked alright when it left the factory