I have a Commodore 1084-D with RGB input that I wanted to use with my Apple IIGS's monitor output. The 1084 series is regarded as an excellent monitor for vintage computing and gaming but there are some peculiar variations between production runs of these classic CRTs that can wreak havoc on compatibility.
Here's my quick story and a fix. I'm posting in hopes that someone else won't have to spend as long as I did seeking out a solution.
According to the label on the back, the particular 1084-D that I have was manufactured in June 1989. A few posts online describe the creation of cables to connect the IIGS monitor port to the DB9 connector on the 1084. When I did this, it was clear that the sync signal was not being properly processed by the 1084. Some searching led me to some clues that some early 1084 monitors did not accept composite sync. The IIGS only outputs composite sync, unfortunatly. There is some analog video processing that might be able to solve this in a breakout box but I wanted a simpler solution. Fortunatly, I found this 15-year old post in an Atari forum which descibes a simple workaround!
In case this link ever stops working, here is a simple summary and photo from my implementation:
Disasemble the monitor (taking precautions for the high voltage CRT). Solder a jumper across pins 6 and 10 on the chip labeled I202 (a 74L506). Create a cable that connects composite sync from the IIGS to both pins 7 and 9 on the 1084's 9-pin connector.
See photos of the PCB and standard IIGS to 1084 pinout information.