^^^^ This!
Why mess with it? Scopes are an 'about so much' kind of instrument. A little + or - doesn't mean much. I've been using scopes for a very long time and I have never removed the cover from my Tek 485. It still shows traces, they pretty much look like what I expect and the Time/Div seems pretty close. I don't even have the service manual because I know I don't have the tools or the capability to service it. It works! No need for me to break it...