Hello there!
I just posted this question over at EVVblog.com today, but just learned about this place, so I'm putting it up here as well:
I recently bought a used (obviously) Tektronix 2215A which has a problem that I would like to repair. Fortunately, I have access to another 2215A which is working fine, so I can compare behaviours between the two.
In a nutshell, the problem seems to be that when the trace is "freerunning"/not triggering, the horisontal extent of the trace is compressed to about half the width of the display. When triggering on a normal input signal, the horisontal extent is fine and covers the entire width.
I went through the "baseline trace" procedure in the service manual on both scopes. On the problematic scope, the baseline trace is short. On the working scope, it covers the entire width. With a normal sinusoid input and using "PP auto" triggering, both scopes show a nice trace that covers the entire screen width.
I have a few other observations, which might be caused by the same underlying problem on the problematic scope:
1) When selecting "A trigger source: line", I get no trace. On the working 2215A with a sinusoid input from my cellphone, it shows an "unsynchronized" trace, is it should since the input is not synchronized to the line voltage.
2) Pushing "single sweep" while in "PP auto" trigger mode doesn't always result in a trace. It seems very unreliable. On the working 2215A, it always shows one trace.
3) Sometimes the trigger LED flickers when moving the "A intensity" knob.
4) Say I have a full trace covering the entire screen width with a sinusoid input and "PP auto" triggering. If I then turn the "Volts/DIV" until the trace becomes very flat, it will stop triggering and then the trace width suddenly reduces to about half the screen width. So when the triggering is not happening, the width is reduced, as mentioned. This should not happen, of course, and does not happen on the working 2215A that I'm comparing to.
5) On more thing: Say I have e.g. the baseline trace going (which occupies half the screen width) and set the "TIME/DIV" knob so that the trace moves very slowly. If I use the "single sweep" trigger button, the first sweep starts all the way from the left of the screen and travels across the entire screen, but every subsequent trace is "compressed".
So it seems to me that after a trigger event, the trace will go across the entire width, but when there has not been a trigger event, the trace is compressed in the horizontal direction...
So my conclusion is that it has to have something to do with the part of the timebase circuitry that controls the horizontal amplifier. The input to the horizontal amplifier seems to be wrong whenever there has not been a trigger event. When there has been a trigger event, everything seems fine. The vertical behaviour of the trace is working fine.
Any ideas?
The only thing I have done so far inside the scope is to check the power supply voltage test point on the main board as specified in the service manual, and the values were perfect. The 100 VDC test point measured 100.0 VDC on my DMM...
For reference, here is the post at evvblog.com:
Best regards,
Torquil S?rensen, Norway
