First reaction to this is to think "Bad Contacts". These would be to do with the flash hot-foot. For the sake of removing 4 little screws, you could make a start by taking off the hot-foot and checking that each wire is still intact. It could be something as simple and silly as a wire coming adrift inside that foot. Broadly speaking, unless all circuits and sub-circuits are in prime condition, flash units will tend to revert to 1/1 firing. They may also revert to 50mm zoom setting on half-shutter.
From there on in, it starts to get a bit more awkward to analyse. Could be any one of a number of things. It could be any two of a number of things. It could be any three of a number of things. Etc., etc., ad nauseam. It's uneconomic to check each and every component so the norm (at repair centres) is to just replace everything whether it needs replacement or not.
I think the only other thing I would do, bearing in mind that it's an old flash anyway, would be to open up the head. If memory serves correctly, there's a mini-pcb in the head with a capacitor onboard. Visually check this cap to see that it's intact, no browning, no popped end-caps, etc. After these checks, bearing in mind the cost of repair, I'd dump it in the trash and by a new replacement. As my Metz units reach life-expired status through old age, I am replacing them progressively with new Yongnuos - it's cheaper to buy a new Chinese one than to have an old German one repaired. The same doubtless pertains with Canon flashes. I certainly wouldn't go poking around the main body of the flash as without test instruments there's no way of knowing what's OK and what's suspect.
Usual warnings about capacitors apply, but you know that already.