#27550
Harrumph
Participant

I think Marqs have given you most of the info already but just to add to it (and maybe clarify).

There are a bunch of dotclocks listed here (including various arcade boards):
https://pineight.com/mw/index.php?title=Dot_clock_rates

Using the formula Marqs posted ( dots_per_line = dotclk/(lines*refresh_rate) ), you can easily punch in a number from the table with the total lines and v.refresh you get from the OSSC display (since you own the PCBs).
You can probably narrow down the plausible dot clock already from following the lineage of various manufacturers (i.e. some dotclocks are more likely used than others).

The other way he mentioned ( /0.75) is perfectly fine as a rule of thumb, but clearly MVS is an exception (and there are many others). If you check other boards using the same 6 MHz dotclock in the link, you see that Konamis GX board does follows the rule of thumb: 288/0.75 = 384.

One additional way (as Marqs hinted at) would be to use a scope and check the total vs active video line length, and then apply the ratio to the known horizontal resolution which can surely be checked in MAME documentation (i.e. the “0.75” term gets replaced by the ratio of the active video length divided by total line length (as measured in microseconds)).