Need to swap the yoke?

[ccie38296]

Hi gurus!

I have a 25" K7000 that came with a game I bought. It doesn't focus perfectly and the red is dim even when the gain/cutoff are turned up.

On a whim I bought a cheap 25" TV off FBMP this morning after looking up the typical tube from that model. I pulled the tube from the old TV, connected a chassis to it, and it works, but the image is way too short vertically.



Here are the specs:

Original K7000 Tube: Zenith A63ADG25X
New tube: Zenoth A63AGD01X
(Yes, one is "AGD" and one is "ADG")

Original Tube Yoke:
Red/Blue: 2.2 Ohm / 0.952 mH
Yellow/White: 7.8 Ohm / 12.45 mH

New Tube Yoke:
Red/Blue: 2.1 Ohm / 0.88 mH
Yellow/White: 5.4 Ohm / 6.4 mH

The image only fills about the middle 1/2 of the screen from top to bottom through. If I max out the V-Size pot, the image folds over and gets really stretched out, and still only fills maybe 90% of the screen. However, the colors and focus are great, much better than the original tube. You can see that here. The fold over is at the bottom… I had to reverse one pair of yoke wires to flip the image and hadn't done that at the time I took the pic.



I'm guessing the image size is a sign that the yoke is too far out of spec for the chassis and I will need swap the yoke if I want to use this tube, is that correct?

I've seen a Reddit thread where someone claimed to swap this same tube on a K7000 and it worked fine, and a KLOV thread from some time ago about the same tube where the poster had to change the width cap but no indication of vertical height being an issue.

 

[UFighterX]

There are a few components in the vertical section that you might be able to adjust. This might point you in the right direction.

 

[ccie38296]

Thanks! Worth pointing out that with the other tube the image size was fine. I just couldn't get it focused well anywhere in the flyback's range and the red gun was pretty weak.

The focus on this tube is razor sharp and the colors look great, the image just isn't tall enough. So I have a feeling maybe the yoke values are just too far off…

 

[ccie38296]

OK, I played with the 50/60 pot and that seemed to help. With that close to its limit in the CCW direction, and the V.Size pot maxed out, it's now filling all but about an inch on the top and bottom of the screen.

That then sounds like what @zenomorp talks about in his video, but R80 is measuring (in circuit) at 4.4 Ohm. Which is extra confusing to me because unless I'm colorblind the markings on R80 look to me like Orange, White, Gold, Gold which my trusty secret decoder ring tells me should be 3.9 Ohm. I haven't looked up the 25" schematic so maybe it's a different spec for R80 than the 19". I'll look it up shortly.

 

[alggamer]

k7000s are usually pretty forgiving on the yoke specs. Dumb Question: Nowhere in your previous posts does it mention you adjusting the Vertical Positioning pot. Have you tried that?

 

[ccie38296]

k7000s are usually pretty forgiving on the yoke specs. Dumb Question: Nowhere in your previous posts does it mention you adjusting the Vertical Positioning pot. Have you tried that?

Fair question. I did, and that will move the image up and down some but doesn't change that with the V Height pot maxed out it's only filling maybe 90-95% of the screen, leaving a good inch on the top and bottom.

 

[DuffCon]

25" K7000 full vertical size with pot maxed out

Oh great collective, This is a new one as far as I have researched. 25" K7000, 1990 rev board. General overhaul, new caps, reflow, etc, etc. B+ perfect at 130VDC Everything is awesome. As I mentioned in the general thread, I couldn't get the vertical size beyond 50% or so on a TV yoke that...

forums.arcade-museum.com

 

[ccie38296]

25" K7000 full vertical size with pot maxed out

Oh great collective, This is a new one as far as I have researched. 25" K7000, 1990 rev board. General overhaul, new caps, reflow, etc, etc. B+ perfect at 130VDC Everything is awesome. As I mentioned in the general thread, I couldn't get the vertical size beyond 50% or so on a TV yoke that...

forums.arcade-museum.com

Thank you. This may be a good direction. C51 does appear to be the right value (marked .68), but I put my Peak ESR meter on it and the ESR was 12 ohm. Capacitance measured 710nF on the meter, which is within 5%. I didn't take it out of circuit to verify (yet) but I tested the same component on a known good K7000 and it read C51 at 680nF and an ESR of about 1.1 ohm. So definitely possible C51 is just bad.

Looks like APAR has it: https://www.arcadepartsandrepair.co...acitors/0-68uf-35v-tantalum-capacitor-mc1036/

 

[jjhudgins]

r80 for a 25" should be 3.2 or 3.3

 

[ccie38296]

r80 for a 25" should be 3.2 or 3.3

Schematic here says 3.9 ohm R80 for a 7191, which is what mine is marked as:

https://www.wellsgardner.com/wp-content/uploads/2016/08/K7000_25_27_inch.pdf

My R80 is measuring 4.4 in-circuit, which is just out of spec high.

 

[jjhudgins]

Trust me. A little does a lot. Plus removing r80 is way cheaper and easier than a yoke swap. Video

 

[andrewb]

Schematic here says 3.9 ohm R80 for a 7191, which is what mine is marked as:

https://www.wellsgardner.com/wp-content/uploads/2016/08/K7000_25_27_inch.pdf

My R80 is measuring 4.4 in-circuit, which is just out of spec high.

Dumb question, but are you removing the resistance of your DMM leads?

 

[ccie38296]

Dumb question, but are you removing the resistance of your DMM leads?

Not a dumb question at all — something that never occurred to me I had to do. Leads measure 0.6 ohm, so retesting R80 it is coming in right at 3.9 after accounting for the lead resistance. Which does match the marking.

 

[ccie38296]

Hey all!

Realized I never followed up with a resolution on this. But for those doing future research, I followed the advice from @jjhudgins and changed R80 to 3.3 Ohm, and also replaced the shot C51.

After doing that, I was able to get the image to about 95+% the full height of the screen with good geometry and pretty good linearity, there's just a sliver of blank above and below the image, which was good enough for me. This is going into a T2 and will be reflected off a mirror so I don't think it will be too obvious if the image is just slightly smaller than the actual tube. Maybe bumping R80 down a hair more would finish the job but this is going to be close enough for me I think.

Thanks for all the help on this one!