Hi, this is the neck board of well gardner 19k4915:
you have tryed to adjust trimmers shown below in the photo ?
While right trimmers there are used for gain red and green...
If yu have tryed to adjust it but not have red color you must check if there are some broken solder, if is all ok the problem could be in the broken red transistor (these are highlighted in the photo), put the red transistor instead of blu (or green) transistor, if that's the problem you have red ok but blu (or green) missing