No this is not going to change your old ZW to the new $650 version but it does make a big difference with whistle/horn activation and operation, especially so, with the electronic versions. This has probably been discussed here many times but my search did not reveal any procedure.
On eBay I located a guy that sells a replacement kit consisting of two solid state diodes, mounting nuts and washers for $10 shipped. The diodes are stud mounted units rated for 16 AMPS@500V and the cathode ( - end) is attached to the pin (solder connection) while the anode is on the mounting screw. You must use diodes with this orientation as if you use the wrong ones they won't work properly.
You remove the 4 screws that hold the cover. Inside on each end you will see the old copper rectifier discs on brackets held by slip nuts. You use a small screwdriver to separate the ears of the slip nut and it easily comes off releasing the rectifier disc, the holding rivet and washer behind it. You can try to fish these out with a bent piece of stiff wire now but I leave it for later.
Now comes the tricky part of getting the nut behind the bracket so that the new diode can be screwed in. Here's my trick. The night before I glue the two nuts to the 1" washers, being careful to center in the washer and not get any glue on the threads. I use Crazy glue gel. The next day (or a few hours later) you have a nut on washer assembly. Now to get those nuts behind the bracket to allow the diodes to be screwed into place. You can use a small head magnetic "pick up" tool to do this or you can use my method where I twist a loop of stiff wire around the nut to hold it firm to the wire. Now using the wire, I fish the nut/washer behind the bracket and screw the diode in by hand. It may take a few tries but it can be done. Then tighten the diode in place using a small wrench (firm not super tight) solder the wire back on and you are ready to enjoy better horn, whistle control and operation. Before the electronic horns were someone difficult to activate. Now they activate easily and they also sound much better. Finally of you have not done it before use a piece of stiff wire to sweep out the old mounting rivets and washers. If the transformer is on the work bench you can just turn it over and shake them out. Best $10 upgrade for the old ZW that I know of. Sixty years old and still going strong. They don't build heavy duty stuff like that anymore.
Note: Some have done the diode replacement by disassembling the transformer and removing the brackets. While its easier to replace the diodes this way, there are paper insulators inside that must not be damaged and if everything is not reassembled exactly as it was before you can have a short and possibly burn out the transformer. Experts care to add anything?
