Lionel ZW Whistle Problem

Hello,
I bought an old ZW off ebay and have cleaned it up. It works nicely except for the whistle on the left hand side. I cleaned the contacts
and re did a couple of flaky looking solder joints but the whistle still doesn't work on the left hand side.
I'm thinking it's the rectifier disk. Any corroboration or suggestions?
Thanks and Happy Holidays!
Paul
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You're lucky to have a ZW - it's a very good transformer, but better, with two whistle circuits, it gives you a reference for comparison. If you don't already have one, get a small digital volt-ohm meter (VOM); I found one a few years ago for under $20 at Radio Shack. Take a resistance reading across the rectifier on the good side of the ZW. Don't touch the copper disk, but rather put one probe on the metal plate it's attached to, and the other on the rivet with the speednut on it. Then reverse the two probes and take a second reading. The two should be quite different if the rectifier is working. Now do the same on the left side rectifier. If the two pairs of readings are about the same, the left rectifier is OK. If not, you may have a faulty rectifier. While not as efficient as modern silicon diodes, the copper oxide rectifiers are pretty durable. They can be damaged if someone tries to twist the disk, causing the sharp corners of the speednut to scratch though the oxide layer and bypass the rectifying action. If scratch marks are evident,the rectifier can sometimes be rejuvenated by very carefully lifting the ends of the speednut just a bit with the points of two X-acto blades, and the rotating the rectifier disk so that the speednut will contact unadulterated surface of the oxide layer.
Another test is to use the voltage range of the VOM to look at the transformer output. On an appropriate AC range, increase the throttle to about 16 volts. Then slowly activate the whistle. You should see the voltage increase by about 5 to 6 volts, with no dips in output. Now switch the VOM to a low DC voltage range. With the throttle still at 16 volts, the VOM should read 0, indicating no DC in the AC output. Now again slowly activate the whistle. You should see the VOM jump to a little over 1 volt DC as you move the lever, then settle at a less than 1 volt when the whistle lever is at full travel. This is because Lionel uses the full DC output of the rectifier to initially pick up the whistle control relay in the tender as you move the lever part way, but the at full travel, a resistor is added to the circuit partially bypassing the rectifier disk so it won't overheat, which results in lower DC to the whistle relay, but still enough to hole it in the on position. Again check both sides of the ZW for very similar readings.
The Lionel service manual pages for the ZW are available on line at
http://pictures.olsenstoy.com/library.htm
(although the site is down as I write this). Also search for the general pages on Lionel "multi-control" transformers for a better description than mine above about how the whistle controls work.
Good luck. Geezer
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wrote:

Excellent diagnosis procedure. When I find a bad copper oxide rectifier disk I replace it with stud rectifier...which has purer DC and lower voltage loss. I use 1N1185R or 1N1185, rated 35amp150volt and cost $185 from Hosfelt Electronics via mail order.
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I hope you forgot a period somewhere in that number <VBG>!
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Yes, should be, $1.85. What's a missing decimal point to rich tinplate operators? :)
Ray H.
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On Dec 16, 7:01pm, snipped-for-privacy@earthlink.net> wrote:

I want to thank all of you who took the time to reply. I have run the diagnostics and found a noticeable difference between the working and suspect rectifier. I tried to carefully pry the speed nut loose, but had trouble. I'll ask my son to lend a hand to see if we can rejuvenate the old girl.
If not, I suppose I can order a new one from East Coast or try the stud rectifier idea.
Thanks again and happy holidays!
Paul
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Geezer wrote:

Thats a real nice well written description of a repair.
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