The transistor (marked D545 F8H) in this HP-33E turned out to be an NPN transistor with a hFE of approx. 190. I used a 2N3904 as a replacement. One caveat: the original transistors pins are ECB, whereas the replacement's pins are EBC; in other words, the collector and the base must be swapped. Due to the way the transistor is soldered in, this is actually fairly easy to do.
Bad transistor in HP-33E DC-DC converter circuit
