Williams SD 90 Horn Bell Question

[KRM]

Quick question on my  Williams SD 90 Part # 21805

 The horn will only work now and then off the control of my ZW.

So I tested it off my CW-80 and it works fine every time. Any ideas why? The ZW has had rectifiers  diodes install in it and all of my Lionel units work great?

[lionelsoni]

Postwar transformers and the CW-80 generate the DC voltage component that tells the locomotive to run the whistle in entirely different ways.  The transformers switch a 5-volt winding and a diode into series with the normal output voltage, then shunt the diode with a resistor.  This produces a DC component whose amplitude increases with the current being drawn by the train.  A modern locomotive might not draw enough current to produce the necessary amount of DC.  The CW-80 on the other hand varies the times that its phase-control triacs turn on the voltage every half-cycle, earlier on the positive half-cycles and later on the negative half-cycles for the whistle signal.  The amplitude of the resulting DC voltage component therefore has nothing to do with the amount of current drawn (unless the current is extremely low, because of a resistor-capacitor network shunting the triacs).

Another consideration is that the modern silicon diodes usually used to replace the original copper-oxide rectifiers in postwar transformers have a substantially higher forward voltage, which slightly reduces the DC component in the whistle signal.

[KRM]

Thanks Bob,

 I meant to say that my ZW has had the diodes installed in it. Sorry for the confusion.

[lionelsoni]

No confusion.  They can all be called either diodes or rectifiers.  "Rectifier" is the older term.  "Diode" originated with vacuum tubes:  Diodes had 2 elements (cathode and plate); triodes had 3 elements (cathode, grid, and plate); tetrodes had 4...; and so on.  When tubes went out of fashion, the replacements for vacuum-tube-rectifier diodes, that is, semiconductor rectifiers, inherited the name "diode".  On electrical schematics, you can now use the older abbreviation "CR" (crystal rectifier) or the newer "D" in a reference designator for the same component.

[KRM]

Okay Bob, So I guess I would ask if there a cure to my problem with the ZW?? 

[lionelsoni]

You might be able to get it to work by not moving the control all the way.  When you first move it, you should get the extra 5 volts and the diode, which is the point of the strongest DC component and meant to operate the whistle relay.  If you move the control all the way, that puts in the shunt and reduces the DC component, since the relay needed less holding current after it operated.

It should be possible to remove the 1.5-ohm shunt resistor from the whistle-control switch if you want to do that; but that might cause older trains to slow down when you blow the whistle, since it was intended to supply extra current for the whistle motor.  It's also possible that you could put in a compromise, higher resistance that would be satisfactory for both cases.

In any case, try moving the control just halfway, to see whether that works any better.

[KRM]

Got it Bob!

 Here is what I found. If I move the whistle control half way the SD90 horn will only chirp. If I move it forward slow it will reverse the unit. If I push it all the way forward to the end and hold it nothing will happen. If I push it all the way forward to the end and hold it for about one second then release it it will function every time once released.

Thanks for your help!  

[rtraincollector]

Kevin I have found that Williams sound system is a creature of its own nature lol I have a old K-line Whistle/bell relay or what ever you want to call

[Ribdoner]

I run a Z 250 so I had to use a separate whistle control. I used to lionel 8251, one to control the horn, and one to control the bell. They both work great.

[phillyreading]

I have the older ZW with all original parts from the 1960's. To access the Williams True Blast 2 horn I have to hold the whistle control lever about two thirds the way toward full open and keep it there for three to four seconds and the Williams horn will sound off. However I have no problem when using an MTH Z-1000 transformer with both whistle and horn buttons.