I'l raise this topic again because I am facing a similar issue.
A friend picked up a D29C051 monitor as well to replace the current one in his Atomiswave cab.
We tried some things meanwhile to get 15kHz running without any success (anyhow it will not work over Jamma).
Some information what we have tried and figured out till yet:
- Toshiba is running with 15kHz if we connect a Naomi board directly to the monitor via VGA.
- tried to get a CPS2 board running via VGA cable over Jamma harness, not working (The CPS2 board is running with the other monitor, so there is not issue)
- it seems that the v sync has an additional connector in the bottom of the cab to split it out from the VGA. but there was no change with or without the v sync on the 15k/25k header which confuses me a lot.
please find enclosed some picture (due to any reasons I have trouble t include single photos into the board....

[url]https://www.dropbox.com/sh/violw0q7nilr ... a?dl=0/url]
Some ideas what's going wrong here? From my perspective it seems that my friend and myself have overlooked something.
We would really appreciate some support.