Volvo RWD dash lighting, especially of that vintage, was notably not bright, presumably some kind of engineered intention so as not to affect night vision. I do recall them getting worse with age, presumably from rheostat contact wear/oxidation from repeated temperature change more so than actual use. Have you tried a good contact cleaner? Some people just end up bypassing them. There were a number of slightly different types used. In one I had, I think was a 140, I recall being able to file/bend/adjust something to get the contact wiper inside to go a little further that made a slight improvement. New replacements are available, probably better than going for used. I don't recall anything in the way of an electronic component on them, perhaps just a heat sink that shouldn't change with age.

I don't recall that this applies to 240s, but in the 700/900s the instrument cluster bulbs are spec'd as 5W, but often 3W is what's commonly available and used for replacement (3W is spec'd for the door markers). Although 3W is acceptable at night on dark roads, I'm now going back to 5W when I can find them as I find them better on well lit city roads. If you do a little research, you may be able to find a slightly higher wattage using the same bulb base in your 240. You should also pull a few bulbs to see if they've darkened/blackened from heat over the years. Then of course there's our aging eyes.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now