I expect the fault to be in the electronics, not in the tube. As you don't report any geometry issues, just decreasing brightness, I don't expect the problem to be caused by the central power supply on the monitor, but on some circuit that is directly related to the beam current control. The issue might be related to a local voltage supply problem on the cathode drive amplifiers (often located on the neck board), or the beam current limiter inadvertently kicks in. CRT Monitors (especially the bigger/higher end ones) typically monitor the current supplied on the high-voltage beam supply, and if that current gets excessively high, brightness is reduced to prevent overloading the electronics and to ensure a maximum level of X-Ray generation. CRTs are typically built in a way that no signicant amount of X-Rays leaves the tube, but shooting electons onto the metal shadow mask is a process that inherently produces X-Rays, which are contained inside the tube by an adaequate amount of damping. Higher beam current means the generation of more X-Rays, possibly exceeding the amount for which the tube is certified to provide sufficient damping.

Both the local cathode drive supply as well the beam current limiter might rely on failed electrolytics, so a recap might solve your issue.

Thanks for the info, that’s useful. Here’s some more context. When I got the CRT it was dim. Dim enough that it was difficult to see in a bright room. I figured out how to get the Sony DAS software to talk to the monitor via serial and applied a higher G2 value until the brightness looked normal, as I read in several places this was the one to adjust (still unsure what that means exactly and how it relates to the “screen” pot you usually find on a normal CRT that doesn’t have this absurd proprietary microcontroller-based calibration process). After this it got brighter under most circumstances but began exhibiting the symptom I described in the OP.

Geometry, convergence, etc. overall is acceptable. Not noticing any breathing yet.

keenmaster486 wrote on 2024-07-27, 23:34:

Thanks for the info, that’s useful. Here’s some more context. When I got the CRT it was dim. Dim enough that it was difficult to see in a bright room.

With that context information, I have to adjust my judgement of the situation. While I still blame the beam current limiting circuit for the symptom, the overall low brightness might be a symptom of a tube that's at the end of its useful life.

keenmaster486 wrote on 2024-07-27, 23:34:

I figured out how to get the Sony DAS software to talk to the monitor via serial and applied a higher G2 value until the brightness looked normal, [...] how it relates to the “screen” pot you usually find on a normal CRT

G2 and "screen" are two alternate name for the same setting. Some flyback transformers have that pot labelled as Screen/G2. The name "G2" refers to the second grid, starting counting at the anode. This grid is used to control the overall beam current by "screening" (hence the functional name) a fraction of the electrons from leaving the neck part of the tube.

keenmaster486 wrote on 2024-07-27, 23:34:

After this it got brighter under most circumstances but began exhibiting the symptom I described in the OP.

So you basically increased the beam currrent to make a dim tube look brighter, and now you get a symptom that looks like some protection circuit kicks in, because the beam current is too high. I no longer think that worn out electrolytics are the root cause, but the whole electronics section operates "as specified" by Sony. So I now suspect you either need a way to adjust the the beam current limit (another parameter in DAS?), defeat that circuit, or you reduce the brightness to an amount that is still considered "OK" by the electronics. Defeating the beam current limiting circuit (which is a safety devce!) is something you should only do if you can judge all the consequences. I am not qualified for that judgement, so this post is telling you about the possibility, but not a recommendation or endorsement to do so. I do no longer expect that recapping has a high chance of improving the situation.