Tektronix T912

[cathoderay57]

I tried to use my Tek T912 in X-Y mode to do a phase shift measurement on a Quad II amplifier, applying the input signal to the amp input and to the scope Y input, and the amp output (using a dummy load) into the scope X connector. This should have displayed an ellipse (Lissajou figure) whose dimensions I could use to estimate phase shift. However, the best I could do was to select the slowest timebase (0.5s), trigger mode to "Auto". Then I had to select or deselect the slope button which gave a single slow sweep of the ellipse across the screen. I was unable to lock the ellipse to enable measurement. With Trigger source set to "Int" and mode to "Auto" while applying a signal just to the Y input enables sine and square waves to be displayed and locked normally. Is the inability to display the stationary ellipse by design or is there likely to be a fault on the scope? I'm thinking that normally for Lissajou figures the timebase needs to be switched off completely which isn't possible from the front panel of this instrument. Jerry

[Trigon.]

The trigger source switch has an "XY" position at the bottom. Are you saying the timebase still runs in this position? (It shouldn't!)

Cheers

[cathoderay57]

Looks like there's a fault then. If I turn the scope on with no signals connected to X or Y input, I get, as you'd expect, a horizontal flat line using Trigger Source set to Int, Line, X, or X/10 with Trigger Mode set to Auto. When I move the Trigger Source switch to X-Y the screen goes blank. If I then press the slope switch I get one horizontal line flash up starting part way from the left edge of screen extending fully to the right edge. Then the screen goes blank until I press the slope switch again, and the partial line repeats once. Any clues as to where I should be looking? I have got a copy of the manual. Cheers, Jerry

[cathoderay57]

Oh dear. I'm embarrassed to admit it is me being a dipstick . The brightness control in X-Y mode is set considerably lower than the level for normal timebase display (that's my excuse). All I needed to do was turn up the brightness to reveal the stationary spot near centre screen in X-Y mode.. Jerry

[trobbins]

Brightness, and especially persistence become important for very low frequency assessment of gain phase.

[RichardGM]

Originally Posted by cathoderay57

Oh dear. I'm embarrassed to admit it is me being a dipstick . The brightness control in X-Y mode is set considerably lower than the level for normal timebase display (that's my excuse). All I needed to do was turn up the brightness to reveal the stationary spot near centre screen in X-Y mode.. Jerry

Lower brightness perhaps by design, to minimise the risk of burning the screen?

[John_BS]

@ trobbins

Brilliant comment!!