Based on the information provided, I'd recommend the following first steps:
Clean the monitor jack on the 800XL using contact cleaner.
Inspect the monitor jack while inserting and removing the cable several times. It should slide in and out smoothly and the pins remain clean. Also the monitor jack itself should not flex or move appreciably.
If there is any ...
The R38 potentiometer it's has to be replaced.
Edit: the potentiometer was replaced (and also a "hack" on the motherboard to enable the S-Video signal), here is the sample:
Assuming what you want to do is recreate disks from disk images stored on your PC (or accessible from your PC), there are a few ways to go about this, all of which involve extra hardware in one form or another.
Connecting your Atari to your PC
A number of devices allow your PC to act as a “server” for the Atari; in most cases, this is used to provide virtual ...
According to this forum site, you can construct (or purchase) a cable to convert from USB (Windows) to SIO (Atari) which will allow you to run files inhabiting the windows computer. The fourteen minute video covers booting the atari (Option held while powering up) from a file mounted to the application running on the windows computer.
The video in the forum ...