I have been experimenting on an old VCR that was producing a green screen. I gave it a good clean following TGrant's guide with the soggy paper. There was a bit of muck from the heads and drum.
I ran a tape through it and it looked pretty good, so I tried another tape, and after a minute or so, the screen went green.
I saw on Tom's guide that running a tape in fast forward will reveal dirty heads (snow on the cue lines). This VCR sort of does that, but also jumps and rolls as well.
The green screen (which I assume is no signal) occurs with or without an ES15 in the workflow. Also note the jumping OSD.
The attached capture is of Fast Forward, then Play.
The heads themselves seem to produce a good picture; could it be something else?