Unfortunately,
some manufacturers of test equipment (i.e. "BK Precision" - &%$#@!) will not give you the schematics because they are afraid someone will use it to copy them. This is ABSURD, of course, because anyone copying their equipment is going to copy the PCB layout, the case design, the placement of cable strain reliefs, and they're going to measure the diameter of the steel tube in the soldering iron, check the type of plastic in the heat-resistant handle, etc. The schematic would be - at best - an afterthought. Preventing legit users from having a schematic to fix their own equipment is nothing but worthless paranoia.
Having said that - did you try the document portal on the Hakko site?
https://doc.hakko.com/?l=en
You have to register, but there might be something on there that is useful. Hope you have better luck than I have had with BK Precision.