Tip 3 - There's a bug in the machine

When I began tracing suspected BFO leakage problem with my RAY-152, I threw the symptom out on various mailing lists to see if anyone else observed the condition.

Surprise...they ALL do it. With the display backlight turned on. It's not BFO leakage. It's an audio tone from the DC-DC converter circuit used in the backlight, for generating voltage required for the EL panels. Somehow this is getting coupled into the Mic Amp circuit.

I tried all the usual fixes (electrolytic cap replacement, bypassing, filtering) and didn't make much headway. Someone from the PNW had a friend who knew how to fix this problem but unfortunately the gent went SK before passing on the knowledge. I may try contacting Raymarine for any Service Bulletins they may be aware of, -but for now, turning the backlight off addresses it.