Hantarex Polo 20" cannot lock vertical frequency

[chris2922]

The hantarex polo 20" in my old silverline Jamma cab has always been reliable but a few weeks ago the line output transformer failed and I had to replace it. It's back to normal now and has a great picture when I plug a Jamma board into it.

I've been working on a PC - a Dell optiplex 780 running crt-emudriver with a Radeon x600 connected to a Jpac.

I don't understand why, but the image the PC produces will not lock on the vertical axis. It keeps rolling.

I have a small potentiometer board for making positional and frequency changes but I can't get it to set the vertical frequency. It feels like I cannot make a fine enough adjustment.  Even if I could lock the 640x480i desktop resolution it wouldn't help, because the Arcade OSD testing software shows me that different resolutions cause the vertical frequency to be more or less out of sync.  Some even cause the direction it rolls to reverse.

So I'm at a bit of a loss.  Someone has suggested a cap kit will help but I don't understand why.

It's fine with a Jamma Board plugged in.  I can find vertical lock with a lot of tolerance, and it's about in the middle of the pots turning circle as id expect.  With the PC connected its towards the end that it settles, and it just won't lock.  Its quite a imprecise solution and i'm not used to having to make such fine adjustments with it.  Don't think i'm being heavy handed either - i don't think its possible to find the sweet spot.  And even then it would unlock when the resolution changes.

Its just so strange!  I have about 10 jamma boards and changing between them has never meant i needed to change this setting.

Any advice at this point would be greatly appreciated.