WG 4900 - The Split Screen Pot

Phetishboy

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WTF is this pot for? I went to do some adjusting on my Qbert and have no idea what pot was doing what, as I didn't have a mirror at first. I was messing around trying to adjust colors, brightness and contrast and accidentally hit a sync/hold/centering(?) pot that basically spits the screen in two and puts a black dividing line between the 2 screens. Went and got a mirror, continued to rotate the pot further and the black bar widens. Go the other direction and it narrows then disappears. What was the purpose of this pot?
 
That's not what the pot is supposed to do. That's what it does when you have an issue with the chassis.

I believe it is either the vert damp or the vert size pot that causes that to happen. If you are having an issue in your vertical deflection circuit, that is a symptom. It's not easy to track down. Sometimes a cap kit fixes it and sometimes it doesn't. If you can get it to a point where it looks normal and big enough to suit you, then leave it alone...
 
That's not what the pot is supposed to do. That's what it does when you have an issue with the chassis.

I believe it is either the vert damp or the vert size pot that causes that to happen. If you are having an issue in your vertical deflection circuit, that is a symptom. It's not easy to track down. Sometimes a cap kit fixes it and sometimes it doesn't. If you can get it to a point where it looks normal and big enough to suit you, then leave it alone...

Yeah I've already capped it. Chris says it was a flaw in the early design of the 4900 that was fixed in later runs with a blue vert damp pot. I can adjust it out and then the image is perfect. I will leave it as is for now.
 
Yeah I've already capped it. Chris says it was a flaw in the early design of the 4900 that was fixed in later runs with a blue vert damp pot. I can adjust it out and then the image is perfect. I will leave it as is for now.

True. I though about mentioning those early runs in my response, especially as they need a specific yoke, but they aren't seen often so I didn't think you had one...
 
True. I though about mentioning those early runs in my response, especially as they need a specific yoke, but they aren't seen often so I didn't think you had one...

What a weird phenomenon though. It really just shocked me when I came around to the front and saw it, then thought "Now what?!"
 
Yep. I remember way back when and I didn't know that a later series 4900 and an early series yoke couldn't be matched without causing foldover that couldn't be adjusted out. I spent an inordinate amount of time trying to fix it when there was nothing wrong with it. it just was mismatched. Once I figured that out, I was able to do what I needed to do to get a good picture...
 
I had just recapped a 4906 model and changed out the horizontal coil with a new one. When I fired it up and I adjusted the vertical damper the screen would split in half. It wasn't until I moved the center screen jumper to the right leg that this went away and v damp adjusts fine now. If you look on chassis just to right of vdamp pot you'll see a wire color of mine was yellow and 3 posts theses are your jumpers. Do the switch with the monitor off.
 
is this what you guys are talking about?

SSPX1042.jpg


nothing i can do to fix it? its a k4903
 
Reduce your vert size for now, and it should fix itself.

Has this chassis been capped? had the vert section reflowed? vert tranisistors replaced? IC replaced?
 
i just capped it. it was doing this before the cap... i went over all the solder traces but i had forgot about this because i havent messed with it in a while and i fired it up and went to adjust it to fit the screen it does that i can shrink it just a little and it goes to normal
 
I had this issue, and I was using an early chassis (no v damp pot) with a later yoke. Put the older yoke on it, and it's fixed.

The picture still folds in and splitscreens when you turn the v size way up, but the pic's so tall then it doesn't matter.

Here's the measurements of the older yoke:
Vertical 180 mH, 70 ohm ESR at 100Hz
Horizontal 2.3 mH 3.6 ohm ESR at 10kHz

I'm guessing the newer yoke has a lower inductance, so the picture is shorter. Hence needing to adjust the size way up, and hitting the foldover bug.
 
Here's the service bulletin on the yoke compatibility. There's usually a sticker on the yoke.
 

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