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Discussion Starter #1
Been using a refurbished ZW 275 for a couple years. Just a few hours a week at most. While just running the trains, I heard a loud Pop sound (like an arc) from the ZW and then smoke came out of the right side. Everything still runs except the whistle doesn't sound on POST D. I switched the track to POST A and the whistle sounds but also reverses direction if the whistle control is pressed all the way. I have to admit I never tried the whistle on POST A so that may be unrelated.

I opened it up but there's no obvious damage I can see. Any suggested repair tutorials or trusted repair centers?
 

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Pop, smoke, no whistle points to the rectifier that provides the DC signal for the whistle.

Did it fail when activating the whistle?
 

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Discussion Starter #6
Thanks Pingman and John,
I followed the test procedure at tranz4mr. The D side reads 21v at max throttle and 26v at the whistle position. The A side goes up to 26 then drops to 3v or something once the lever moves past a certain point. In terms of placing an order for one versus two rectifiers, is it the rectifier that's bad on A or something mechanical on the A switch? TIA
 

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Thanks Pingman and John,
I followed the test procedure at tranz4mr. The D side reads 21v at max throttle and 26v at the whistle position. The A side goes up to 26 then drops to 3v or something once the lever moves past a certain point. In terms of placing an order for one versus two rectifiers, is it the rectifier that's bad on A or something mechanical on the A switch? TIA
Try setting the voltmeter to DC. Then activate the whistle. If it reads zero (in conjunction with your earlier measurements) the rectifier has shorted closed. It is letting the 5 V AC whistle winding through without rectification to DC thus adding 5 V AC to the max throttle 21 V AC.

Me,. personally, I'd put in a modern 10 amp 100 V diode. But, I don't own a ZW and I don't know how important it is to keep it "original".
 

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Discussion Starter #10
I'm confused. I see that the power cord and thermal circuit breaker were replaced in the rebuild but I don't see any rectifier discs nor the 1N1190AR Stud Diode. What the heck?

2019-02-10 14.29.50.jpg
 

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Discussion Starter #11 (Edited)
EPILOG: Mystery solved. I heard a rattle when I tilted the unit to move it. The reason I saw no rectifier was because it had blown up. That was the pop and the remaining bits were black and camoflauged inside the case. Anyway, I figured it out by comparing the remaining fragments from the D post to the A post. That's when I saw the can shaped rectifier on that side that was missing. As for the A post whistle stopping when pushed too far, it seems the rebuilder inserted the wire tension spring wrong which allowed the switch to travel past its contacts. I think the case will cure that problem. TBD
ZW Rectifier.jpg
 

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IIRC, you posted a 100 A diode from eBay. I would use a 10 A 100 V diode like the one remaining in your ZW. 10 A should be plenty for a whistle and several locos on the same post. And I'd rather replace a diode than overheat the windings or fry contact points.

And that fat can is one of the connections (it can touch / short) where as the black body of the diode in your pic is insulated. You can find that same case type at Digikey in 100 V 10A (and 12 A / 15 A for that mater) for a buck or two. For small parts like that, Digikey's shipping is very reasonable at ~$3.75. Buy a couple for spares (and impress you friends when their ZW needs one :laugh:)

If you need help finding the diode, let me know.
 

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Discussion Starter #14
Did I say Rectifier? [blush]. Yes. 10A10 Diode. :eek:

The explosion was so violent it bent the top lead back and shorted on the frame (I bent it back in the photo). Thanks for the pointer to digikey but I had already ordered 2 replacement Diodes from mouser.com and since I was paying $8 shipping, threw in a 40amp 600v Stud Rectifier using the part number recommended by Tranz4mr. I got options. :)
 

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Discussion Starter #17
It happened again during a whistle blow only 10 months later. Any idea why these diodes keep blowing up?
 

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It sounds like you blew the rectifier disk on the d handle. This is usually because the whistle switch is worn and gets stuck on, overheating the disk. On the A side the contacts in the whistle switch either need adjusting or need to be replaced.

Kent Schwarz
Website www.tranz4mr.com
Email Tranz4mr at Tranz4mr dot com
 

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Question about rectifier disk

Gunrunnerjohn, you mentioned that the original rectifier disc is a selenium type. I was always under the impression that the disc is a copper oxide type, not selenium. I have never seen them referred to as selenium rectifiers. I am very familiar with the old selenium stacks that were used in old radios, TVs and jukeboxes.
 
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