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Do you have a lot of Overshoot or Undershoot on the waveforms you are
supplying it? On the other scope, what is the Rise and Fall Time? If it is too
fast, since the Impedance is High, it could cause reflections. It says the
Rise/Fall time is less than 10ns.

You are likely been bit by a Signal Integrity issue. The Signal Generator edge
is too fast. With an edge as fast as that is, it needs to be terminated into
the cable Impedance. This isn't possible in your case because the Oscilloscope
Calibration routine expects certain values to be met.

As a test you could use a 50 Ohm in-line Terminator, connect that to the
Oscilloscope Input. Set the Generator for twice the Voltage level (only below
2V to protect your load and Generator), start the test and see if the problem
goes away (as a test only). If it works it is likely caused by Undershoot or
Undershoot. If not, the problem might be somewhere else in the Signal Chain.

1) The board layout is a nightmare. Component placement is scattered, and 90%
of the reference designators are missing in the silkscreen. So locating part
positions
is so slow and tedious, that by the time I locat the part, I've forgotten why
I was looking for it.

2) It's a complex board, conceptually. Easily one of the most challenging that
I've dealt with.

Thank you so much Mark.
I have replaced a few capacitors that I can tell or measure to be bad. I have not gone systematically, because of the limited supply. I have order a bunch of more generic ones to replace some other capacitors as needed. I am also ordering a desoldering gun. Without it, it is very hard to put the new capacitor back on, because of the remaining solder.

Thanks again - Luca

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Anyways, the problem I’m facing concerns the horizontal amplifier. For
starters, the trace is always moving a bit right and left. Also, it is just
impossible to adjust the timing. It’s completely off even with the X1 and
X10 all the way CCW. It’s still way to wide. I replaced what I found to be
out of spec, but there is still no change.
I haven’t measured the 30K and 18K 8W precision resistors though…

Here’s a short video of a 1KHz from a Type 184. You can clearly see the
trace moving right and left, as well as the timing being off, and I can’t
adjust it any further.

Any help would be appreciated.

Very interesting. I was unaware of that page. It gives me some hope.

In putting in this effort in to a 576, retrofitting it for an LCD display rather than a CRT. A requirement for this project in my mind will be to delete the need for the HV transformer.

I think in the rigol world last time I tried it the best
resolution I got on the input is 10's of mV steps. So you would be able to
get close but there might not be enough adjustment on the digital scope. At
higher price points this is less of an issue of course.

In a hypothetical. If I aligned a 576 with the built in tube. Then
took a scope and tapped the vert and Horz signals for X-Y. I would want to
do a full realignment of the circuits to the new graticule.

At present there is not anything that can reliably capture the wave form for storage out of a 576. However a scope in X-Y mode tapping the vert and horizontal will do the trick. You will not be able to make quantitative measurement however due to the scope graticule not being an exact match for the tube graticule.
I have some information out there on rebuild and restoration of these units.
I would like the solve the tube issue permanently however this is currently above my skill level, but I am rapidly gain ground on it. I am thinking about tapping the preamp signal running that through an ADC Then possibly driving an LCD replacement screen as well as a digitized output for a 576 and 577.