[Techtalk] Very basic tty question

[David Sumbler]

On Tue, 2019-10-22 at 18:12 +0100, David Sumbler wrote:
> My principal monitor has a resolution of 1920x1080 pixels, but when I
> use a console it only occupies the left two-thirds (roughly) of the
> screen, and not quite the whole height either.  The rest of the
> screen is filled with stripes.  Measuring the usable area I'm
> guessing that the console is probably occupying 1280x1024 pixels.
> 
> Is there a way of getting the console to occupy the whole of the
> screen?  If not, can I get the unused areas of the screen to show
> black, rather than multi-coloured stripes?

Having written that earlier, it suddenly occurred to me what might be
the cause of the problem.  I don't know why it hasn't occurred to me
before.

I have a second monitor, which is normally switched off.  (I use it
occasionally when editing files in Lilypond, which produces printed
musical scores.)  The resolution of the second monitor is 1280x1024
pixels.  So it looks as if the console is using the dimensions of the
smaller screen.

I thought I could check this by rebooting the computer with the second
monitor disconnected.  I did this and, yes, the console filled the
whole of the 1920x1080 pixels.

Having now reconnected the small monitor, and rebooted several times, I
find that the coloured lines are no longer appearing on the large
screen.  No idea why.  However sometimes after rebooting I have had the
large screen being used in full (with the image on the small screen
being cut off and incomplete), but other times I just get a 1280x1024
console on both screens.  I can't figure out why it is sometimes one,
and sometimes the other.

I need a way of telling the computer to ignore the smaller screen
completely when formatting the console, and always to use 1920x1080. 
How can this be done?

David