I have received 4 of the used Avalon 741,s and not impressed 
Two of them look like they where dropped from a great height with warped cases,
one the fan is running at maximum constantly.
The last one is in fairly reasonable condition.
They all where full of dust and insects and have a greasy film inside that smells like stale grease.
Looks like you get what you pay for with these units.
I will post pictures later as they say a picture is worth a thousand words
Two of them look like they where dropped from a great height with warped cases,
one the fan is running at maximum constantly.
The last one is in fairly reasonable condition.
They all where full of dust and insects and have a greasy film inside that smells like stale grease.
Looks like you get what you pay for with these units.
I will post pictures later as they say a picture is worth a thousand words
As stated in the describtion, all miners are cleaned with air and and a cleaning rag. There can be leftovers of dust sometimes. After cleaning all miners are tested if they are hashing. Optical issues can come from transport.
cherrs