Neotec NT-2501 Focus question

[MrArcade]

Started with a ticking monitor. Replace shorted HOT, C308, C314, C315, C422 and C316 which is a 25v 22uf bipolar. We don't have any 22uf bipolar caps and the parts chassis are missing C316 already. I made a 22uf bipolar by soldering the negative leads of two 47uf caps together and installed it.

Powering up showed a solid red screen. Pulled chassis and found one leg of R948 had lifted. I soldered it back down.

Powering up showed game video (California Speed), but focus was horrible. I think this game had focus issue in the field.

Adjusting focus on flyback changes focus from HORRIBLE to playable but still out of focus.

Is this definitely a bad FBT or can the focus circuit be getting a low/high voltage that keeps it from adjusting to the correct focus?

If it can be voltage please let me know which caps etc to check. I don't know where the focus circuit is on the schematic.

Thanks!

MrArcade

[MrArcade]

As the game warmed up the focus improved, but turning the focus knob all the way one direction still can't bring the picture all the way into focus.

After running overnight I noticed vertical foldover at the bottom of the screen this morning. As I looked at it, the screen jumped to fill the screen vertically. I watched it a while and it jumped at least once more to vertical foldover than back to filling the screen. I imagine it is a cold solder joint somewhere.

I pulled the chassis and looked through what I am guessing is the vertical circuit and the caps etc around the flyback that might affect the focus.

I replaced C424 and C504.

Focus issue remains. I will let the game run and watch for the vertical foldover issue.

[Ken Layton]

In general, focus issues on the 2700 series is usually the flyback. So I would assume the 2500 series to be the same.

[lilshawn]

focus is controlled by a resistor ladder inside the flyback that feeds off the HV anode voltage. not serviceable.

check your B+ and make sure it's not too high (can cause focus issues) and also try turning your screen control down (less voltage into the focus circuit.)

dont forget this is a CRT tube. it's not supposed to have retina buring brightness like modern displays do.

[MrArcade]

I checked voltages.
26v good
12v good
165v was 147v

I realized this being the third neotec monitor I worked on within 2 weeks that I thought I had checked the b+ but I had not.

b+ was only 117v so I adjusted it up to 125v

165v is now 157v

Monitor is now in focus but still at the very end of adjustment to be in focus.

Brightness is already turned up all the way. I adjusted the screen on flyback down as much as possible without making screen too dark.

Game is playable now. I am guessing the flyback is on its way out. If the problem comes back will probably order new flyback.