Hi,
chances are good that you will not find anything in print, so, it is time to break out the tone generator (or screwdriver) and multimeter, and start applying theory. Although, depending on age, you might want to test the caps first. In practice, as opposed to theory, I have seen plenty of caps failing when theory suggested that they should not be failing. Even the electrolytics from the seventies can go bad. I threw a few caps into a nice vintage calculator, and it has worked fine ever since- before that time I had a window of 2 minutes before it would fail.
:My wife's old Lloyds 7S43B portable transistor radio needs some help. Where can I find service info? Sams doesn't even list it. Thanks. Denis