When you take the top off, make sure that both you and the receiver are grounded. You will need to purchase a grounding strap. ICs are easily ruined by static charges.
In terms of your volume thinking, you are wedded to old analog thinking with variable resistance potentiometers. Volume controls have been digital for some time now. Sourcing analog pots for vintage repair has become a huge problem and limiting factor.
Later I will see if I can find an online service manual.
Sorry, no service manual is available, and many seem to be looking for one. So, without one, I can't help you any further.
There needs to be a change in the law. No one should be allowed to bring anything to market until a full service manual is published.
Receivers generally do not have cards, but preamp board, mother board, HDMI board, power supply/regulator board and amp boards.
So the one you dropped possibly has a broken board, or a component that has come away from the board. Even the latter will be a problem because of robotic surface mount installation.
Without instruments to find the board at fault, you will be lucky to make progress, unless you can see visual damage.
I suspect your refurb has an intermittent fault and was classified as open box and not touched. It likely left the factory with that fault.
There is another wrinkle, in that changing a broken board for a working board may not solve your problem as the board change may need initiation via firmware, that can only be done at a service center.
I ran into that servicing a Panasonic BD player once, but I persuaded them give me a connection to the firmware I needed. That was a can of worms, I wish I hadn't opened for a while. Things are just not like they used to be, and its a disgrace, as it leads to greater pollution from discarded electronic junk.