Reply to "RW Transformer - Whistle dropping out when whistle button fully depressed"

I'd find it hard to think the switch is the problem. As soon as the switch is half way down, it's connecting the diode ( via point A) into the circuit and that works.  The main thing that happens when the button is fully depressed is that it adds the balancing resister (9) when point C is connected.  The diode is dropping out - why?  I have tried pushing on the arm of the copper slider (switch) overlying point A to see if it was losing contact but it doesn't seem to make any difference.  In other words, the extra pressure doesn't suddenly make the copper slider connect to point A and add the DC back, so I don't think it's a worn slider/switch that's the cause.