And electronic troubleshooting skills. For this I looked at the first similar service manual at http://sportsbil.com/sony/STR/STR-D/STR-DB840_DB940.pdf and the "protector" signals.
For me to work this I'd still need the service manual for your DH550 plus the usual Oscilloscope to check the protector signals and find which has tripped.
Don't have those items? Replace it for much less. https://www.amazon.com/Sony-STRDH550-5-2-Channel-Receiver/dp/B00J30GXW2 is about 250USD and Oscilloscopes run a few thousand for a good model.
All recommendations followed. Unable to get past PROTECTOR fault. Is there a point where you just rename it "trash?" It keeps thinking it's conected to a Sony BD player, but it's not.