Cool thanks. I couldn't make out the settings but that is ok. Interesting behavior!!
Take a look at that PDF and just kind of skim through it and get the gist. Their approach is--diagnose from the front panel. That's what we'll try and do.
So, step one, diagnose the faulty section. The o-scope is made up of several main sections. The power supply of course provides various voltage to all the other sections. The vertical section controls the position of the beam on the vertical axis. At present I'd put my money on a problem with the vertical section. The horizontal section is what sweeps the beam from left to right. It also is responsible for triggering when to sweep, blanking the beam when sweeping right to left to start another sweep.
You seem to be having a problem getting the beam to show up below a certain point on the screen. The vertical position adjust is part of the vertical section circuitry. So maybe that has something to do with it. In the PDF take a look at the chapter called "Troubleshooting the Vertical Section"
Let's see if we can isolate down to the block, circuit and then component levels. I am kind of winging it here.
First off, does this behavior only happen on channel A? Or does this also happen on channel B?
Second, when you set Channel A coupling to AC and adjust the position knob, what happens? What about when you set coupling to DC?
Third, when you set Channel B coupling to AC and adjust the position knob, what happens? What about when you set coupling to DC?
Michael