Thefirst monitor I owned was the same model you have. I bought mine in 1988 for a Coleco Adam Computer, I later used that monitor with a Commodore Amiga 1000.
Yours should say "80" in the monitor name on its front side. The "80" means it is capable of displaying 80 columns of text per line. Some Apple IIe computers came with 80 column text cards and some other computers (such as the Coleco Adam and Radio Shack Tandy computers) also had video which could display readable text at 80 columns/characters per line on a suitable composite video monitor.
The Commodore Amiga line of computers include stereo audio using left and right RCA jacks, the Commodore 64/64C/128/128D had only mono audio.
It is currently the standard to use Red for Audio Right, White for Audio Left, and Yellow for Color Composite Video on DVD players, VCRs, TVs, etc., but it also true that the Commodore monitors with the separated video outs with mono sound output used the colors differently as previdously mentioned by another poster.
Did you try adjusting the picture using the knobs (if any) under the front flap (if any)?
I have a "Magnavox Professional RGB Monitor 80 [Model Number 8CM515 0741]" which I have listed for sale on Craig's List at https://accounts.craigslist.org/post/shwpst?pii=813385419&db=lv . It has color composite video, mono audio, TTL RGB, and lin. RGB. One of the RGB jacks is for digital RGB (probably CGA and possibly the kind used by the Commodore 128D) and the other is for analog RGB (which is compatible with the Commodore Amiga). It works fine, except the colors are not quite right. The resolution, sync, and audio are working fine. The RGB connectors on that monitor are round.
Yours should say "80" in the monitor name on its front side. The "80" means it is capable of displaying 80 columns of text per line. Some Apple IIe computers came with 80 column text cards and some other computers (such as the Coleco Adam and Radio Shack Tandy computers) also had video which could display readable text at 80 columns/characters per line on a suitable composite video monitor.
The Commodore Amiga line of computers include stereo audio using left and right RCA jacks, the Commodore 64/64C/128/128D had only mono audio.
It is currently the standard to use Red for Audio Right, White for Audio Left, and Yellow for Color Composite Video on DVD players, VCRs, TVs, etc., but it also true that the Commodore monitors with the separated video outs with mono sound output used the colors differently as previdously mentioned by another poster.
Did you try adjusting the picture using the knobs (if any) under the front flap (if any)?
I have a "Magnavox Professional RGB Monitor 80 [Model Number 8CM515 0741]" which I have listed for sale on Craig's List at https://accounts.craigslist.org/post/shwpst?pii=813385419&db=lv . It has color composite video, mono audio, TTL RGB, and lin. RGB. One of the RGB jacks is for digital RGB (probably CGA and possibly the kind used by the Commodore 128D) and the other is for analog RGB (which is compatible with the Commodore Amiga). It works fine, except the colors are not quite right. The resolution, sync, and audio are working fine. The RGB connectors on that monitor are round.