Well, sorry for making you put all that up. I would've provided them and linked to the datasheets I dug up today if you hadn't.
Anyways, the aforementioned Fairchild part was a
Fairchild Semiconductor KSA1381E, last letter specifying hFE class E, putting it at 100~200 according to the datasheet. It originally was a 2SA2912S, in the position of TR376. So it turns out, though, it wasn't alone. The pair, TR376 and TR375 were KSA1381s, leading me to think it might be fine to just buy a pair of new KSA1381E from Mouser.
In any case, I was more hoping to replace these with in-production substitutes where at all possible; the replacements, could total more than I paid to buy it, and put me instead in the market for a new scope. The unit cost isn't that bad with e-packet free shipping, but I was thinking of replacing all ten transistors in the vertical section; just in case the failure might propagated or made any go marginal/intermittent; these are old parts, failure modes do start to get weird at this age. I might as well swap them all while I have it open, right?
The
2N3904 seems a close enough match to a 2SC641K (as installed, hFE class C, 80~160@Vce=0.5V and Ic=1mA, while the part here has a min hFE of 70 at Vce=1V and Ic=1mA).
It is worth noting that the schematic is actually wrong; specifically just for the parts specified. The second pair of 2901s probably got too much power pushed through them in development, so they replaced them with the second pair of TO-126 parts. Checking the helpfully included BOM (pg 40), the transistors are as specified, with links leading to the alldatasheet page:
TR331/322:
2SC641KTR365/366:
2SC2901TR371/372:
2SC2912STR375/376:
2SA1210STR377/378:
2SA781KFor the pair of 2SC2901s, they're installed for class L, 40-120 (Is there a standard for these or are they just part specific?/manufacturer's whim?), and it looks like the AliExpress you sent is for L anyways.
The 2SA2912Ses were replaced with KSA1381Es, as mentioned. They might be suitable for just buying a new pair and sticking those in. I'm still considering options for the other transistors, but I'm not pulling the trigger on any order until I'm sure what I'm buying from Mouser and what I'm buying from AliExpress.