Yup. Check the simplest stuff first. The first pin I bought, way back when, was (still is) a High Speed. The seller explained there were some problems but he didn't feel like working on it (he had a whole route of other pins to deal with). One of the problems was that neither of the blue flashers in the middle of the playfield worked. He suggested I look at things like the transistors, connections, etc.

Well, I did all that, didn't find a single thing wrong. Took forever too, in spite of a modicum of experience with electrical/electronic stuff, because I was brand new to pinball.

Eventually, I took a look at the bulbs. Sure enough, half the bulbs in the flashers were burned out. Turns out, there are pairs of bulbs in series, so when one goes, the other won't light up any more. Have that happen a couple of times, now all your blue is gone!

Sigh. Reminiscent of the time I spent hours trying to figure out why my car ran so rough for 10-15 minutes, came up with all kinds of cockamamie theories based on hydraulic lifters or thermal expansion of rings or whatever. Finally, just replaced the spark plugs, and all was well again.

I always check the most basic, easiest to deal with things first now.