I just did a redesign of these myself - for the chopper I used a 1N5817 for the diode and BC109Cs for the transistors because I already had a drawer full - although 2N3904s would work just fine in that application.? I used two 1N4151s for the input limiting, a NZX18B for the zener, a 2N3971 for the FET and a 2N6428 for the output.? Everything seems to work at least as well? as the original but I have not taken the time to really dig in and to more detailed testing. The response is a couple of percent high in midscale but I don't know if that is from the converter or is something from the input stage (which is all original.) I'll swap boards around with other units over the holidays and see if I can track that down - may be well be something related to the FET but we'll see...?
On a somewhat related note, I just made new versions of the boards for the 5087A - and relearned yet again that transistor footprints are **NOT** to be taken as a given. Sigh. At least boards are cheap now-a-days...
Hal
Harold Foster