"Shaun Guth" wrote in message news
My research online seems to suggest that I should be checking the
horizontal output transistor first, but I'm unsure of what it looks like
or where I should connect my probes.

I have just completed a repair of 19" PS790 with similar symptoms
and I have fixed it with replacing this transistor with $6 NTE2958
bought at www.mouser.com - it is working as it was before...

Transistor will be located near the high voltage transformer area...
On a radiator, probably together with some diodes...
Both transistor and diodes will be in the TO220 case - transisor
will be in the isolated TO220 (without metal back plate).
This is power MOSFET transistor so expect it to behave like
mosfet when checking it... Mine had shorted channel.

In my case it was required to adjust tube voltages on the back
of the high-voltage transformer - needed to fixed focus...

Also, because of the shorted transistor, there was one burned
resistor in the +79V branch filter. It was serial with a small coil
and the electrolitic capacitor (the one for 100V). I just replaced
it with a small short, since I do not have the correct sub-ohm value.
I do not see anything wrong on the picture coming from this change.

Of course, your monitor will be a little different, so you have to
work with it on your own to find out...