Where to find service manuals and upgrading firmware

I am a home enthusiast starting to dip my toes into repairing hi-fi / AV equipment (due in part to a couple of items I have purchased on eBay that were damaged in transit or are past their best).I have a reasonable knowledge of electronics and computers, having studied these to just below university level.

Can anyone recommend good places to find service manuals and service bulletins (to help address known issues)?

I am trying to troubleshoot a poorly Arcam DV89 DVD player that won’t power on and I can only find a DV88 service manual (which looks similar but don’t want to risk this assumption too far!).

Also, does anyone have any tips on updating firmware for equipment that pre-dates updates over wi-fi / USB? The manual refers to an upgrade CD or DVD, but I know some other kit (e.g. an Arcam AVR300) has an RS232 port, but I would need a particular piece of software to apply a patch.

Many thanks in advance,

One good place to start is at a site called "Hi Fi Engine".  In the top menu you will see headings for "Database" and "Library".  Look up the brand and equipment you are interested in, and they may have it.  I also have at times found service manuals just by typing the brand and model into the Google Search Engine, and then exploring what sites come up.

Hope this helps.  
Many thanks for the suggestion. I did try Google first, but could only find a DV88 that way!