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Exploding Zeners. Can't figure out why


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Well, I turned it on this morning to prepare to take some more readings, and it blew up once again.  Every diode on the positive rail gone to short condition (including the 24V one), with the 150V zener totally evaporated.  Just like the past 2 times it happened.   I had replaced all parts except for the 2 caps and the rectifier, and put in nylon screws on all heat sinks.  The rectifier on the positive rail does continue to put out the expected 425VDC and appears very stable.  And the bias does put out 580V rock stable.  

I give up!  Why would a PSU work OK one day, then blow up the next?  I never even moved it.  Only issue was that the Positive rail was showing less volts (+310VDC) vs the negative rails which was producing the expected -345VDC.  Ugh.  

Edited by Blueman2
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Having built hundreds of these PSU's over the last 10 years I know they will only act like that when there is a short on the board.  Now what's causing the short can be a number of factors, sloppy soldering, flux or just a sub-par PCB.  It is so often over looked that PCB's aren't the same and some are just crap.

Are the pass transistors sourced from Mouser?  There are some fakes out there which could be a factor... 

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Boy, I really have to give credit to the amazing community of people here.  I have received several PMs giving me great ideas and encouragement to continue with my learning in HV PCB work.  People's generosity in time and information is humbling.  I think I will try starting from scratch with a new batch of parts 100% from Mouser for the positive rail, and see if I can make this work.  If nothing else, this is great practice for my upcoming SRX-Plus build.  

Thanks again everyone for the kindness and support.  

 

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  • 1 year later...

Well, they say time heals all wounds.  Perhaps so, because after letting this project stay idle for almost 2 years, my son and I returned to the project last weekend and were able to build a very nice and stable PSU with the help of gepardCV who graciously provided an extra KGBH PSU board to me.  I believe the issue with my original PSU was that I used non-mouser parts I got on eBay.  When I purchased 100% mouser parts, it worked like a dream.  Well, and maybe my son's soldering skills are more advanced than mine.  But I did go back and unsoldered all the parts from the bad PSU and replaced with Mouser parts all by myself, and it too now works.  

So I learned the hard way to only use Mouser parts and avoid eBay like the devil.  And to use younger hands to do some of the delicate soldering work.  

 

Now on to building a pair of SRX-Plus boards.  

Edited by Blueman2
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Yes, based on some friendly advice here, I now have a DY294 tester.  What a great piece of equipment (though lacking in instructions for a newbie like me).  I just threw the non-mouser parts into the trash, so never bothered to test those.  But I am now doing 100% testing for diodes, caps, etc.  It is nice to be able to match zeners so that both sides of my PSU are well matched.  

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