Problematic Tek 7A26

I have a 7A26 vertical amp unit that is misbehaving. The symptoms are as follows:

(1) The amplitude of a given signal does not remain constant when trace is scrolled from top to bottom on CRT.
(2) Both channels behave as described here.
(3) Occurs with low frequency traces (10 KHz) AND high frequency traces (10 MHz).
(4) A signal that shows 3 div is positioned with the upper peaks at the topmost graticule line.
(5) Its position is lowered using the POSITION pot to the next lowest graticule line and still shows 3 div.
(6) Its position is lowered using the POSITION pot to the next lowest graticule line it shows 2.6 div.
(7) Its position is lowered using the POSITION pot to the next lowest graticule line it shows 3 div.
(8) Its position is lowered using the POSITION pot to the next lowest graticule line it shows 3 div.
(9) Its position is lowered using the POSITION pot to the next lowest graticule line it shows 3 div.

The rapid motion sequence of the positioning is that the trace size diminishes in the upper portion (at approximately the second graticule line down) briefly and then regains its amplitude.

The pictures of this sequence are shown below:

You know me, Al..

..is, of course, a title stolen from Ring lardner's incomparable baseball about a rookie pitcher trying to catch on in the majors in the years before World War I. Of course, this post is for my good friend, W6AAX, who is named Al.

Basically, the crystal checker outputs the oscillator sine wave through the BNC but it also have provisions to an output meter in the form of a plus and minus set of terminals. The voltage (energy level of the crystal) can be measured by using a DMM connected to these terminals. These and some more aspects of the checker operation are depicted in the pictures below.

Note that the meter output terminals are just leftover component leads suitable for alligator clips.

Here are the clip leads in place.

Here's the unit under test,

..and the frequency and voltage output..

..and, finally, the scope output.