I remember a video he made repairing a very ordinary, mid 90s, cost-reduced Sony Walkman, where he very carefully spent hours reproducing a tiny bit of metal that had worn out in the internal mechanism. Rescuing something that (even when bought) was mostly e-waste.

Edit: This one: https://www.youtube.com/watch?v=rxkuOzvHToc

Looks like he made his own service manual for this piece of gear and it has the manufacturer's logo on it without their consent.

Which he prepared for the client in decent presentation quality.

One of the most admirable things to do, above and beyond most repair professionals, looks like Mark really is a cut above and it shows.

The top instrument companies have always recognized the advantage of partnering with the rare individual who can service their complex and unique products, and have been the most willing to provide schematics and discounted parts in order to make as many into authorized service centers as possible. To enable field calibration and service, or bench work without having to send their own people or ship the unit back to the main repair depot.

The lesser outfits, not so much.

If you've got money-making instruments to sell, you really don't want to work against someone who has the talent to fix defects without even having any factory documentation. That's hard to come by, they could be your best ally. Imagine what could happen with full factory support.

And Mark prepared his own documentation! How much more respectable can you get?

Posting it on Youtube is the only real mistake, unfortunate but true.

Obviously, Youtube is not a respectable enough place, oh well, who knew?

From the commentary it does look like the circuits are not more innovative than the "generic" guidelines published by the component manufacturers to encourage engineers to adopt their semiconductors for various intended purposes.

When these analog devices were first emerging, some of these data sheets were widely published back when some of the example circuits were still under patent. There was every expectation that if you copied one of them, you would have to license it before you could legitimately include it with your own product. For these preamp components, patents have all expired now so that's not a consideration any more. However it's possible that somebody 30 or 40 years ago might have drawn up a PCB of a completely generic circuit that exactly conforms to an example public-domain schematic, no longer under patent by decades, but that pattern on the PCB could easily still be under copyright for decades to come.

You create your own original artwork, you own it, even if the circuit is exactly the same.

Thus I would say the patterns on the PCBs are only legitimate to reproduce in much less than their entirety, like passages from a book. That could be a pitfall, but I don't think more than a few relevant excerpts were casually shown in the video.