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Old 5th June 2012, 10:31 AM   #1
diyAudio Member
 
Join Date: Sep 2010
Location: Peppermint Grove Beach, Western Australia
Default Accidental Death of a DoZ...

Hi All,


I thought I was in the finishing stage of testing a DoZ (Death of Zen 15 watt Power Amplifier) build. I'd tested out the power supply, calibrated it for ~34V, powered up the amp modules one by one and spent quite the while setting the bias to about ~1.4 amp. Left it settling for a while, tweaked bias, etc, etc, and finally decided to hook up a load and a source...

An lo, the magic smoke came forth!

The amp was de-powered during the change, and once powered up there was a short period, and then the current went (rather quickly!) from ~1.4A to over 6A. Before I could get to the power switch a fair bit of smoke wafted out from under the PCB.

My question would be - what the hell happened? It seems a bit of an arbitrary fault point that it would fail at the point of load/source being applied; I'm about to tear the unit apart and find what broke, but I'd be curious if the assorted folk at Diyaudio had an idea of what went wrong.

Parts/details
300VA transfo, 30V-0-30V center tapped.
rectified power supply's for each channel based on a lm317 and current boost transistor, set at 34V.
DoZ set to a quiescent current of 1.4A; built using MJL21194 transistors in a rather neat heatsinked enclosure (pic to follow)
Temps had stabilized at about 70 deg C at the hottest point on the heatspreader, and at about 50C on the external heatsink surface.

...Just had a look at the power transistors and they're toast. Purchased from eBay Australia: Buy new & used fashion, electronics & home d?r
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Old 5th June 2012, 10:35 AM   #2
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Join Date: Sep 2010
Location: Peppermint Grove Beach, Western Australia
Default Dodgy Parts?

eBay Feedback Profile for ebuystorehk

Second line is a complaint about counterfeit parts - the exact same components as I bought...
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Old 5th June 2012, 11:42 AM   #3
jaycee is offline jaycee  United Kingdom
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Join Date: Apr 2004
Location: Norwich, UK
Yep. Never buy power transistors from eBay. You are almost 95% guaranteed to get fakes - especially from a chinese seller
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Old 5th June 2012, 11:48 AM   #4
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Join Date: Apr 2010
Location: Coffs Harbour, on the east coast
How did the price compare to say, US Ebay sellers? There are many so it shouldn't be hard to check. When you find cheaper than
bulk prices anywhere, the word "Dodgy" should be flashing inside your eyelids.

Sorry you had to find out this way and there's little comfort in knowing the seller probably doesn't realise how bad his fakes are either.
Why not crack open the failed parts and check the die size compared with pics on the ESP site and old threads here too?
__________________
regards
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Old 5th June 2012, 10:05 PM   #5
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Join Date: Sep 2010
Location: Peppermint Grove Beach, Western Australia
Default I knew the risks...

Oh, pricing was very reasonable for the parts in question, but I was surprised they were dodgy anyway. Perhaps I've just been lucky so far, but this is the first time this has happened to me.

Ah well, such are the joys of ebay.

Reckon I will bust one open and have a look - I'll post an image here.

Cheers all!

Last edited by aspringv; 5th June 2012 at 10:15 PM.
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Old 5th June 2012, 10:21 PM   #6
jaycee is offline jaycee  United Kingdom
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Join Date: Apr 2004
Location: Norwich, UK
Quote:
Originally Posted by Ian Finch View Post
Sorry you had to find out this way and there's little comfort in knowing the seller probably doesn't realise how bad his fakes are either.
Hah. He probably knows full well he's got fake parts.
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Old 7th June 2012, 09:18 AM   #7
diyAudio Member
 
Join Date: Sep 2010
Location: Peppermint Grove Beach, Western Australia
Well, interestingly enough, I've been offered a refund on my parts and an apology from the vendor. I have to say that I'm pleasantly surprised. Full text of the message below.

Dear aspringv,

Hello, my friend
I am sorry to hear that. We are only the retailer,
it's very diffcult for us to test each product.
We have sold many these transistors and received good feedbacks,
so we think the quality is good. We would like to full
refund your money back, then change the supplier.
Is it OK? Hope to hear from you soon.
Best Regards


- ebuystorehk

I'll let you all know if I get said refund...
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Old 9th June 2012, 01:45 AM   #8
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Join Date: Sep 2010
Location: Peppermint Grove Beach, Western Australia
Default Refund Received

And I have to say I'm pretty pleased. That's a level of service I didn't expect, and I'm happy to give the seller the benefit of the doubt over the parts legitimacy.

All in all a good ending. I'm tempting fate though - I've subsequently ordered replacement parts from eBay Australia: Buy new & used fashion, electronics & home d?r

We'll see if these turn out to be legitimate...
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Old 9th June 2012, 04:35 AM   #9
diyAudio Member
 
Join Date: Mar 2011
I had a Le Monstre conk out recently. Fake Toshiba power transistors, bought on eBay. I'm finding it's best to either buy from a big distributor or use modern substitutes for all old designs.
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Old 9th June 2012, 06:12 AM   #10
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Join Date: Sep 2010
Location: Peppermint Grove Beach, Western Australia
It's the mixed blessing of the internet age - absolute convenience (buy an obscure electronics component at 3am, cheap, and delivered to your door?) vs reliability.

I choose to view ebay as a vast flea market, and factor my warranty needs in appropriately

That said, I was very relieved there was an output cap on the DOZ design when the fault occurred!

Last edited by aspringv; 9th June 2012 at 06:18 AM.
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