You actually have two different adjustments on the matsushita chassis. One is a knob on your neckboard(contrast) and the other is on the board where your video connector is hooked up, this will be your brightness pot. The one on the flyback is focus only!
Is the brightnesspot a small white dial with a screwdriver slot in it?
There are two white dials for horiz width Hert. To the left or below them is a small white dial with a screwdriver slot with a fuse right behind it. I get no result when I turn it.
Thanks for all of the help identifying the monitor - that lead to an old post which linked to the manual where I was able to identify the brightness pot which you guys identified as the problem.