Does anyone know a a site or sites dedicated to diagnosing and repairing Mac computers and components.
You won't find it as they don't exist. Apple do not make schematics available, nor custom IC's. Even if they did, the test equipment requirements would make repairs economically not viable unless there was a high volume of repairs.
In the 1990's I was a workshop supervisor working for a company component level repairing manufactures failed boards, our customer base was all of Europe. The biggest customer was Radius, who rebadged monitors, but manufactured typically video cards and accelerator boards for the Apple Mac. Two of us were trained by Radius in San Jose, was a great trip to the USA. We then trained the 15 technicians in our workshop when back in the UK. Although many boards were simply repaired with the use of a scope, Mac and surface mount rework station, for others we developed programs for our ATE equipment, thus reducing repair times and costs. The high volumes of repairs made this viable. This all related to nubus Macs, now long gone.
Current Apple products are many times more complex, and without manufactures support and high volumes, cannot be component level repaired. There are a few exceptions, there are repair companies who set themselves up to replace the Nvidia GPU in older MacBook Pro's, and anyone with soldering experience can replace defective electrolytic capacitors in early G5 iMacs. I myself will repair some Powermac G5 power supplies as they are no longer available from Apple. Generally, component level repair of recent Apple products isn't viable.