Tektronics THS720 on screen measurements

[distefanom]

Hi guys!
I have a THS720 0scilloscope with a strange quirk.
Almost Sure is because of my ignorance. The problem is this.
When I activate an on screen voltage measurement  (mean, RMS or any other), if the vertical scale puts the track off-screen, the shown voltage indicators are as well the track is shown in a wrong position.
See the enclosed images.
I'm testing a 19Vdc signal at the CH1 probe. I set-up a vertical voltage measurement @ 5 V/div.
In the second I selected 2 V/div. with the very same voltage input.
As you can read the measuremens shown are wrong as well the track position in the second image.
Why is that?
Can anyone explain to me why this happens?

[shakalnokturn]

This isn't the most appropriate section of the forum for the question.

To me this looks like a defect in CH1 input attenuator, does the scope behave the same if CH2 is used for the same measurements?

[distefanom]

Yes, it is the same on both channels.
Sorry if this isn't the appropriate section.
Please tell me where I should put this thread, or if any admin can do this for me, I'll appreciate it. Thanks
Mario

[mk_]

Yes, it is the same on both channels.

Schematic attached... maybe helpful for deeper diagnosis

good luck

[TERRA Operative]

Check your channel marker, the black arrow/tag labelled '1' on the left hand side of the screen, that's your zero volt reference point for that channel, you have it offset down from the centerline on the graticule.
If you count up from the channel marker, your voltages match up.

Any other discrepancy could maybe be explained by mismatched probes, a scope that need a calibration/adjustment (Looking at the service manual, it doesn't seem toooo difficult on these units) or the source device not outputting the expected voltage.

Do you have another voltmeter you can use to compare?

[HighVoltage]

What probe are you using?