Tektronix 2213A 'ADD'

[Coll]

I have noticed an odd quirk that when I am using the add function on the vertical selection when both channels should be at zero I am getting a positive deflection. I would expect it to be 0 and this deflection also affects measurements. Is there a way to adjust this so it adds properly. I am not finding anything specific in the manual for making this adjustment or how I may be able to remedy this issue.

[torquil]

Check the answer by David Hess in this thread : https://www.eevblog.com/forum/repair/tektronix-2213a-and-bothadd-vertical-mode/msg2902610/#msg2902610

[David Hess]

I remember that discussion.

Unlike many other Tektronix oscilloscopes, the CRT amplifier on the 2235 series, which includes your 2213A, has no vertical zero adjustment so when each channel is individually adjusted to to show zero on the CRT, there is a small offset and in ADD mode, those get added together causing a vertical shift.  So what looks like zero on the CRT is actually off by a small amount.

In that discussion thread, I describe how the CRT amplifier can be modified to zero the CRT removing the shift in ADD mode.

[Coll]

Thank you to you both for this information I will take a look at it and try to go from there.

I remember that discussion.

Unlike many other Tektronix oscilloscopes, the CRT amplifier on the 2235 series, which includes your 2213A, has no vertical zero adjustment so when each channel is individually adjusted to to show zero on the CRT, there is a small offset and in ADD mode, those get added together causing a vertical shift.  So what looks like zero on the CRT is actually off by a small amount.

In that discussion thread, I describe how the CRT amplifier can be modified to zero the CRT removing the shift in ADD mode.

For this modification does this affect anything else or just the ADD mode? I haven't taken a look at the schematic yet for this change I will soon but if I consider doing this what else might it affect?

[David Hess]

For this modification does this affect anything else or just the ADD mode? I haven't taken a look at the schematic yet for this change I will soon but if I consider doing this what else might it affect?

The change would shift the zero position of the vertical position controls by the same amount but probably closer to true zero.  That would be the only other effect.

Like I said, other Tektronix oscilloscopes did have an adjustment for the CRT amplifier zero which depends on the tolerance of the CRT.  Usually having the vertical position shift slightly in ADD mode is not a problem unless the oscilloscope has a readout, and in that case like with the 2232, the readout board has a vertical zero adjustment, but still not the CRT amplifier.