Greetings
Aside from the obvious, what is the difference between an older D6C with a low serial number (123665) and a newer one with a higher serial number (560786)? Is there any real difference in performance, durability or quality? I once read that the main PC board was changed for some reason in the newer models (perhaps for better manufacturing efficiency?).
Thank you in advance for considering this question.

Hi
Yes, the PC board has undergone some change as you already know, with most of the components changed to surface-mount SMD components. I guess Sony did this when technology for SMD soldering became more economically viable (for traditional components, I guess you needed a person to sit down, insert the compnents and solder every single joint but for SMD, all that was to be done was to add solder blobs to the joins, stick on the components and then put the whole board into an oven, and so this can be done by machine. I believe this reduced the need for an extra board for DC-DC but I'm not sure - I have the old one.

In terms of performance, SMD or not, components should offer identical performance. However, Sony made one good modification in the later SMD revision in that I think they modified the servo pwm circuit, in particular so it doesn't get damaged by reverse-polarity as easily. Otherwise, from what I have heard nothing else has changed. Also, I guess the machine is more tidy inside, easier to service and somewhat lighter?

Maybe someone else with the newer unit and service manual could tell you more.