I want to be able to test and be sure, I really don’t want to be changing parts ‘just incase’.
Struggling to find the transistor on the board as it is, new one on the way if I need it anyway.
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I sympathise with that completely but you have to factor in your time's value. There are some components that are just awkward as hell to test in circuit, and some that are awkward to test at all - like crystal oscillators.
The problem is that in-circuit you can only really compare multimeter results to other transistors, and you'd need to make a judgement as to what measurements are material, and which are just normal variations.
https://vetco.net/blog/test-a-transistor-with-a-multimeter/2017-05-04-12-25-37-07 You would have to take deviations from that guide in the context that you'd (I'd presume) be testing in circuit. So you'd have to compare to, ideally, the other flipper's transistor, or other transistors if you know they're electrically nearly identical to all on the flipper coils.
This is all made much worse by the fact that it's intermittent. You could do all of the above while cold, and find absolutely nothing wrong with the transistors. And you don't want to be trying to measure transistor legs in-circuit while the thing is powered on. I do some daft things but that's a trick even I wouldn't attempt - far too much chance of shorting stuff out.
The only other thing to really nail down whether it's in the flipper physical mechanism or it's going to be one of the transistors, is to physically swap the mechanisms between the flippers and seeing if the issue follows the mechanism or you still have the issue right sided.
That's an enormous bloody faff by my measure but if you're dead set on not replacing without a test, there's your test.
EDIT: Though I do have to ask - why the hesitancy to replace old-for-new if you're ordering parts in advance of knowing whether you need them...?