RW Transformer - Whistle dropping out when whistle button fully depressed

From what you said, it sounds like the problem must be in the whistle switch. Those switches have a reputation on needing work or replacement in time. The timing of the switch is important and with use and arcing they can fail.

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.

@R Lamparter posted:

... When the button is half depressed, there is 7.1v DC but all DC voltage drops out when the whistle button is fully depressed.  There should be 0.5v of DC when the button is fully depressed to keep the whistle relay engaged and it's not there

This has to be tested with the rated load. Without a load, yes, all of the current will go around / skip the rectifier.

Well, you learn something new every day.  I tried it with the whistle tender on the track and it works as designed.  The DC voltage is about 0.7v to 0.8v with the button fully depressed, not the zero it had been without a load.  Thanks.

I was reviewing this again.  I had forgotten that the whistle tender's relay wouldn't stay latched with the button fully depressed which is why I started the thread.  Now it's staying latched and sounding with the button fully depressed.  I don't know what changed, but it's working now.  I tried three or four other whistle tenders and the transformer has no problem making any of them sound.