"Quirky postwar whistle"

[lionelsoni]

The March, 2020, CTT has a question about a toublesome whistle involving a 239 Scout locomotive, 234W tender, and a Powermax power supply.  One complaint is that the whistle does not blow at higher track voltages.  The answer gives reasons why the whistle relay might operate at higher voltage, but not why it might not operate.

The whistle relay is specified to operate at a DC voltage of 1.5 volts or more.  It should not be hard to verify whether the Powermax is putting out at least this much DC over the upper part of its range simply by connecting a DC analog voltmeter.  I would do this test with a typical load on the Powermax.

The whistle relay is incorrectly described as a "shaded-pole" relay but is actually a "slugged" relay, which has a thick copper "slug" around its pole to make it insensitive to AC while not affecting its DC operation.  In this case, Lionel specified that the relay had to withstand 28 volts at 60 hertz without operating.

A shaded-pole relay is an AC relay with an auxiliary pole whose magnetic flux is phase-shifted to insure that the total force on the armature never passes through zero, preventing the sort of buzzing that we hear from e-units.

[ADCX Rob]

lionelsoni

...239 Scout locomotive, 234W tender, and a Powermax power supply...

Since the PowerMax is only 30 watts - 2 amps - one reason it might not work is it is overloading and going into "foldback" mode.

However, the PowerMax/CW family of transformers does have a smaller & smaller DC offset as the throttle approches full output, which is easy to do when taxing these little power supplies with postwar type loads.

[lionelsoni]

I agree with your diagnosis, Rob.  The way they create the DC is clever but has its limitations:  The amount of DC that you can get by unbalancing the half-cycles' turnon times decreases rapidly to zero as the average approaches the beginning of the half-cycle.  Of course it would have been possible to compensate for this in the design by increasing the time difference, but even then, when the earlier turnon reaches the start of the half-cycle, the game is up.