Hi again = I still have no cct diagrams for the Avant 28 but it sounds like you may have a LINE OUTPUT stage problem (hence the whistle noise). Unless you are used to working on LIVE tv ccts, where there are some dangerous voltages, I do not recommend you do this.
You could look for the LINE OUTPUT transistor and check it for defect. Unsolder it from the cct, if necessary. Look for dry joints (if you are familiar with these) and re-solder any obvious ones. If you are experienced enough and able (and I stress this) remove the line output transistor and connect a 240v light bulb in its place and see if the bulb lights when you switch the TV on. As I say, do not attempt to work on the tv LIVE if you do not have the experience!!!
If the bulb does not light, you probably have a PSU problem, so look around that section for obvious problems. Are you familiar with any of this circuitry? Anyway, see what you can find out and let us know. Good luck just now. = Brian (Northumberland)