Tektronix 2213A and BOTH/ADD Vertical Mode

[torquil]

Hi!

When the vertical position knobs of both channels on my Tektronix 2213A are set so that the flat grounded input traces are placed on the centre horizontal graticule line, the trace moves about a half major vertical division upwards if I activate the BOTH/ADD vertical mode.

I would expect that the trace with BOTH/ADD activated should also be at 0V if both Ch1 and Ch2 traces are initially set at 0V. This happens independently of the Volts/DIV setting.

Any ideas?

[Smoky]

I'll stick my neck out here because this might be similar to the same issue I had with a Sencore SC61 Waveform Analyzer.

In the calibration instructions, both channels were adjusted individually to the center graticule line.

Next, the A+B button would be depressed.

If the traces didn't remain together on the center graticule line, a variable resistor needed to be adjusted on the Vertical Amplifier Board.

Maybe it's the same for a Tektronix scope 

[David Hess]

The 2232 I regularly used is off by about 3/4 division in ADD mode.

The 2235 series which includes the 2213A does not include adjustments for centering the CRT beam besides the position controls.  The only balance controls are for variable/invert (same thing in this design) and the 2mV/div range.

In principle the CRT amplifier would need a zero adjustment to solve this.  I think it could be added with a resistive divider at the base of Q285 which is part of the sweep separation circuit.  The right high value resistor from that point to either +8.6 or -8.6 volts could center the CRT.  Testing needs to be done with a differential voltage of zero volts at the collectors of Q206 and Q207.

[torquil]

Thanks! As long as I know that nothing is wrong I will not try to do anything. The most important thing for me is the possibility of subtracting the two channels, and a spurious DC component is not really a problem.

I think the last "problem" on this scope now is that the vertical trigger point in p-p-auto mode does not really reach all the way to the top or the bottom of the waveforms when twisting the trigger level knob. I will have to take a look in the adjustments section related to triggering in the service manual.

[David Hess]

I think the last "problem" on this scope now is that the vertical trigger point in p-p-auto mode does not really reach all the way to the top or the bottom of the waveforms when twisting the trigger level knob. I will have to take a look in the adjustments section related to triggering in the service manual.

There are adjustments for the peak-to-peak positive and negative levels but be sure to do all of the trigger level adjustments up to that point.

When adjusted properly, peak-to-peak triggering should reach almost to the extents of the waveform.  My 2232 is actually way off for some reason.

[torquil]

I did the trigger-related adjustments and now it is better. However, I made a rookie mistake when checking PSU ripple by using the AC setting on my Uni-T 61D DMM. The ripple frequency would be far above the upper limit frequency of my DMM in the AC setting since it is a SMPS, so I will go back and check with an oscilloscope. I should have realized when I got a ripple voltage of 0.00 mV AC...