We need to look (initially) at the vertical drive side of the circuitry. The first thing we need to do is to measure the regulated 12V supply on the monitor.
Next, does the vertical height control have much effect?
There are dc voltage measurements and expected oscilloscope traces on certain points as we transverse the vertical drive circuitry. These need to be inspected (from the input signal to the deflection coils) to identify where the faulty component(s) is/are. The oscilloscope traces are located in separate image files identified on the schematic by a circled number. http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/index.html.
Before we start, however, we just need to make sure that the schematic I have identified is the correct one to your actual monitor.
I agree with Dave, the LOPT is fine, the problem is in the vertical deflection system.
The vertical deflection is both low in amplitude, distorted (hence the fold over and the bright area) but there is something else. It is offset downwards. This suggests that there is also (assuming the yoke is ok) possibly a moderate DC current via the vertical scan coils. Most likely capacitor C652 is failing. It could be due to other causes though and as Dave points out, check the power supply, then waveforms and the height potentiometer.
As you can see the vertical output amplifier has a similar architecture to a standard Audio output stage, complete with the Bootstrap capacitor C651. Also, it has both current and voltage feedback applied to the driver transistor Q603. These amplifiers are very easy to test & repair. It does normally require the scope.
Also, notice that with these sorts of output stages, the DC voltage expected at the junction of the two emitter resistors, is usually 1/2 the supply rail voltage. So if the supply voltage is 12V, you'd expect to see 6V there, as shown on TP2. This is a quick place to check, with a simple meter, if the DC conditions of the output amplifier are "normal"
Here is something that might help. I was playing around with the PET's vertical scan circuitry in the Spice simulator (still waiting to get my own PET) and found something interesting about it.
A good vertical scan relies on the voltage across the "Sawtooth capacitor" , C611 in the PET VDU, being a fairly linear ramp. So often, as it is in the PET VDU, the capacitor is charged by a current source.
There is an additional capacitor too which disables the current source when the sawtooth capacitor is being reset (discharged) C602, but these numbers might be schematic version specific. But in this particular vertical circuit, that feedback causes a very interesting effect when (and if) the sawtooth capacitor loses capacitance.
I have attached an image of what happens when the sawtooth capacitor drops in value from 10uF to around 1.5uF or below, it results in a very interesting effect of a twin amplitude scan, where there is an initial aborted scan and a secondary higher amplitude one. This result would pretty match what I see on the screen of your VDU, so this would be the first capacitor to check.
Wow!!! I have to be honest and I supposed I had not chances with my PET... I left the bone...
Today I have decided to have a look anyway and... you are giving me new hopes (in the past I had the possibility to receive emails when new messages were sent to a 3D... I have to check if I can set this option again...).
At the moment I have two issues: the minor one is that the PET is not with me... the major (much much bigger) is that I am not a technician so I only follow some of what you explained me... but you were clear enough and I think I can do something.
And yes... I have an oscilloscope in my house so I can test the mobo better.
This weekend I hope I can meet with my friend asking him to weld the LOPT again and then I can start with some tests (also with his helps!)...