The first step in fixig anything is the inspection step.Looks like you;ve done that, Look for missing items, evidenceof burning and corrosion etc.
2nd step. Check the power supplies. Unfortunately you don't have a scope to check the scope.
On page p0037, fig 5
With power off and caps discharged (off for a few hours at least) or check voltage across C101A & B first and make sure it's zero.
Check R101 in circuit and R across C101A and 101B. The latter should not be steady. You can also try to reverse the leads on A and B On the old analog meters, you should generally see a kick to a low value and the charge when you do that.
Locate A1 and A2 which are both common. Didn't check to see if they connect to common of the the probe input. It probably does. You can verify wirh an ohms check.
Since your having an effect with brightness, let's check with signal applied and no brightness and max brightness and/or distortion.
Ideally we are interested in the DC value and if there is significant ripple. No scope,so see if the voltmeter on the AC scale will pick up anything significant
We want to look at C101A/A9, C101B, A5, A4, A8 and A6 relative to A1 or A2 in DCV, ACV under min and max brightness.
I'll print the pages and see what else I can come up with.