Astro City MS8 screen geometry issue

[geekmiki]

Hi,

Again another problem with my MS8... Once I solve a problem another one appears

So here we go, this time it's a geometry problem that has appeared lately:
Basically the lower part of the screen gets compressed which gives an ugly font appearance to the part of the screen where you usually have "credits" or "level" on MVS carts...

I have tried to play with the V-LIN pot on the chassis, but I just can't seem to get a satisfactory result... Which means I basically need to readjust that for every system I play (mainly CPS2 and MVS) and that is a real PITA!

Any ideas on how I could solve this?

Here are some pictures by the way:




Thanks for your help!

[cools]

Any ideas on how I could solve this?

Lock the PCB door and throw away the key

[geekmiki]

Hi,

Here's an update about the problem:
I did a long session of geometry config yesterday to try to get a decent result again... But there is obviously nothing to do. It's like a 3cm strip at the bottom of the screen that is squeezed. It's not dependent of the vertical size or vertical position of the image. As soon as I get into that area, that part of the image is squeezed.

Now the questions are:
-Can this be fixed?
-Is it cold solder joint related?
-Cap related?
-Something else?

Thanks for you help!

[AeroCityMayor]

V.LIN.

HTH

[Rossyra]

Does that chassis have a jumper to select between N and W?

[Rossyra]

My Sanwa PM1745 had the same compression at the bottom with one of the N and W jumper settings.

Can't remember which though.

[geekmiki]

First of all, thanks for your help.

I did try a couple of days ago to switch from narrow to wide... But in wide mode it's mission impossible to get something decent. The image is completely strecthed and it's almost impossible to get a decent aspect ratio, geometry, etc.

After a lot of reading, I found similar symptoms, which led to me believe that it might be related to the vertical deflection circuit... Do you think it could be possible or am I completely wrong?

[grantspain]

yep start of vert deflection failure-probably just a few caps

[geekmiki]

Hi again,

I changed the caps mentioned on the wiki for vertical deflection, with no luck. The problem is still there...

While I kept on reading and reading through the forum, I just thought of several things that could be causing this:
-I have a MS9 tube on a MS8 FSG chassis... could it be related? Even though I only read about width problems in that specific case...
-Could it be that the B+ is not correctly set?

Thanks again for your help... I really hope I'll manage to sort this, it's driving me crazy!

[geekmiki]

Again another update on my problem... This time I think I have found something.

I got in touch with Craig@Giz10p and he told me that the problem came from the fact I was mixing an MS8 chassis with an MS9 tube, because of the differences in scan coil sizes.
Not that I didn't believe what Craig said, he is the specialist, but his answer seemed a bit strange, since I know there are other people running the same setup with no major problems (or at least not as visible as mine). Also, one important thing: this compression problem was NOT present a month ago.

I did another round of testing and here is what I discovered today:

-If I adjust the V.sync pot to have a perfectly stable picture, the bottom is compressed as you already know:



-I started playing with the V.sync pot and I manged to get a "perfect" size at the bottom... BUT as soon as I get my fingers off the pot, the picture starts scrolling (it is really to the millimeter). Also when in the "perfect" position I get diagonal lines at the top of the screen:



I don't know if it's good or bad, but I have the feeling that I came a step further and that this actually could be fixable... It is obviously v.sync related.
Any opinions?

[grantspain]

could be some caps on the sync circuit,any cap in close proximity to a heat source will probably be low esr and worth changing-alot of times a cap you least suspect of being a possible problem will be exactly that fault(hanty polo vert hold circuit is a common one)
the diagonal lines are to do with the screen volts too high-could be down also to the 10uf 250v cap on the neck card

basically check all your caps with an esr meter-long job but worthwhile

[Ordyne]

You can swap the MS8/9 chassis over between tubes but the results are not perfect. The scan coil is different and these chassis were made for a specific coil, they aren't like a 666 or M3129 chassis where they have a wide envelope for DY sepcs.

[geekmiki]

But doesn't the fact that I manage to get a full size picture when playing with the vsync pot mean that I still have a chance to solve my problem by doing a cap kit (or something else)?

The problem wasn't there a month ago and the screen was more than fine imho... And that leads me to think that it can be solved in a way or another. Even though I am still a noob regarding arcade monitors, I think there's some logic in thinking that if it did work in a satisfactory way for me before, it should be fixable to be at least the way it was.

Anyway, I'll finish the capkit this week end and let's see where that takes me.

Thank again to everyone for your patience and help!