Ooh, I have one of those in my warehouse as I couldn't bear to just pitch it in the dumpster.

You can take a butt set and listen to the A battery for external noise. A lot of times, when the power supply starts to go South, you'll hear the lack of filtering in the battery supply.

Probaby your best bet, is to test the system without any amps connected. Either make a pigtail for a single station, or just test the battery for noise with the cables unplugged. Then, it's just a matter of isolating all the stations until you find the bad one, if the KL is not bad.

A small sum of ca$h will pry this unit from my hands if you need to replace it.

Carl