Jerry,

I did this on a 7934 and a 7854 (I don't have a 7904A). They both worked
identically and your 7904A should work the same as these two did.

First: I am very suspicious of this sentence: "As soon as I push the X-Y
button on the scope the screen goes blank except for the 7A26 mV readout".
You must have the vertical plugin set to a Volts/Div setting or else the
waveform will be moving way off the screen to the left and right.

Second, Do this test: set both channels of the amplifier to ground and press
the BeamFinder. It should locate the beam somewhere and you should be able
to bring it back on screen with the two position controls of the 7A26.

Third, just to be safe lower the frequency of the signal to 100KHz. Most of
the 7000 mainframes have very limited horizontal amplification capability
and I am not sure what the 7904 is capable of. It will be one of the
specifications listed in the very front of the service manual however.

Fourth: When the 7B80 is in Time Base mode (Display Mode push button is IN)
the On-Screen Readout shows the Time Base speed but when I pop the Display
Mode push button on the 7B80 to the OUT position there is no time base
anymore so that portion of the readout goes blank. But the Vertical
Amplifier on-Screen Readout remains in either setting of the Display Mode
push button.

Fifth: make sure the A Trigger Source and B Trigger Source push buttons on
either side of the Horizontal Mode switches are set to VERT MODE just to be
safe. This is necessary since the 7B80 gets its horizontal signal from the
trigger path and the Trigger Source pushbuttons determine that path as well
as the 7A26 Trigger Source switch.

FINALLY, I did check the A and the B lights and in every combination of
switch settings I could think of the A light never went out or switched to
the B light. This is why I think you should try what Albert suggested. It
sounds like he has a 7904A in which case he would know what this switch
does.

Dennis