I purchased this 520ST as untested for parts. The case was in bad shape with all screws missing and instead there was electrical tape holding the system together. I was able to clean up all the tape and glue on the case. I also found that the front screws had been over tightened so that there are dimples on the top of the case.
I replaced the motherboard with a new unused one purchased from an Atari dealer in CA because I found that there was a failed modification to the RF Modulator circuits. After removing much of the modification I found that I could only get Black and White from the RF Modulator. I tried replacing the MMU, the Shifter, and the Glue chips to no avail.The TOS on the new motherboard was replaced with Rainbow TOS Roms. The serial number on the system matches the serial number on the original box.