So what happened here?

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Letitroll98

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So what happened here?
« on: 31 Jul 2020, 03:03 pm »
The plug was pulled without turning the DAC off.  It didn't make any noise or smoke, however when plugged back in and turned on there was no power.  It's an Audio gd DAC from so long ago that I don't recall the model number.  The back label says 240V, but it's been running on US 110V for a decade.  I'm assuming the unplugging was simply the last straw of running it on the wrong voltage.





WC

Re: So what happened here?
« Reply #1 on: 31 Jul 2020, 04:18 pm »
Looks like the transformer case is thermally deformed. May work again with a new transformer? :dunno:

Elizabeth

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Re: So what happened here?
« Reply #2 on: 31 Jul 2020, 06:48 pm »
My bet is on a diode in the power supply has failed. cost you more to pay to track it down than it is worth.
Unless you can do it yourself or have a good buddy

Speedskater

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Re: So what happened here?
« Reply #3 on: 31 Jul 2020, 09:02 pm »
My thoughts:
1] Pulling the plug is no different than switching the unit off (as far as the unit is concerned) but it's hard on the plug's contacts.
2] Over line voltage is a problem, but under voltage almost never is. (there are a very few situations where it might be)
3] Power transformers should have an internal fuse to prevent melt-down.

Letitroll98

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Re: So what happened here?
« Reply #4 on: 31 Jul 2020, 11:35 pm »
Thanks for the replies.  I was hoping to find a blown fuse, no such luck, the power supply doesn't appear to be protected.  I thought the same thing Speedskater, that undervoltage wouldn't cause a meltdown and that pulling the plug wouldn't be different than flipping the switch.  But with my rudimentary electrical skills I was sure I must be missing something.  Perhaps Elizabeth is correct and a diode failed.

JohnR was kind enough to send me this unit many years ago.  He was using it as a desktop DAC-headphone amp and thought it sounded terrible.  He was right, the very powerful headphones sectioned sounded like caca.  But at the time I had only a pair of cheap JVC phones and used it as the DAC in my main system, where the dual Wolfson chips sounded great.  Thanks so much John, I got your money's worth out of it, returning it as a desktop amp at the end.  The sound of the amp seemed to mellow lately and I've been using it as a change of pace from a Sabaj mini.  It will now be employed as a fancy doorstop.

JohnR

Re: So what happened here?
« Reply #5 on: 1 Aug 2020, 04:18 am »
Ah :) And oh no! :( That would be the GD Audio NFB-12 (I think that is the correct model). I did convert it to 120V, which would be a change in the jumpers to the left of the transformer in your photo, IIRC. I imagine I did that correctly as it's lasted all this time... was there any evidence of overheating prior to now? That transformer certainly looks fried...

Letitroll98

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Re: So what happened here?
« Reply #6 on: 1 Aug 2020, 05:09 am »
Yes John, you're correct on both counts.  I believe it was the first iteration of that model, on subsequent models you could change the digital filter being used.  And now I remember you said you flipped the dip switches and I could ignore the 240V sticker.  I was wondering why I had been so negligent as to not notice that it was a 240V unit.  Thanks for jogging this old man's noggin. 

No, no new signs of failure.  No smoke or smell.  It's always ran a bit warm, but nothing untoward.  It was running in a bedroom system that got disassembled when I gifted my son the speakers from it.  I plugged it into a desktop headphone system to compare to my Sabaj Da2.  I kinda liked it as it had lost that horrible glare it used to have.  A touch veiled with much less detail and air than the Sabaj, but it had a lot of dynamic slam and a certain organic flow to it.   Yes, the tranny is toasted, worse than the picture shows.  I wonder if the imminent failure was linked to the headphone amp section suddenly sounding better.  Do you want it back?  ;)

JohnR

Re: So what happened here?
« Reply #7 on: 1 Aug 2020, 06:14 am »
Hah, thanks but no, as I recall it was quite a heavy little thing and the postal service compensated themselves quite well for it! But see PM... ;)

AVnerdguy

Re: So what happened here?
« Reply #8 on: 1 Aug 2020, 02:09 pm »
The distorted cover of the xfmr is typical of equipment made in the Asian countries. The xfmr is more likely made up of recycled steel (cost factor) instead of iron core styles used in older equipment. The steel laminations do not dissipate heat as easily and tend to run hotter and are smaller in size to fit in limited space. A vinyl/plastic cover and potting material are typically used to aesthetically dress up the appearance. I've had many melted goo units for repair and the xfmr is usually fine but cosmetically challenged.

Running a xfmr at a lower voltage will reduce heat so that was a non factor in the failure. Most equipment manufactured now use universal power supplies (wall warts common) that can be configured to work in any country regardless of voltage standards. It reduces cost as they only have to supply a different plug type and move some jumpers or flip some switches.

It is more than likely that a short ckt in the power supply blew the fuse. A lot of digital components use a switching style power supply and the components tend to fail catastrophically when they malfunction. Another typical cause of a fuse opening is voltage surges caused by any number of things happening on the AC service line – lightning strike during a storm, AC motor kicking ON or large noise spike from a heavy current draw device etc.







AVnerdguy

Re: So what happened here?
« Reply #9 on: 1 Aug 2020, 02:19 pm »
Oooops, forgot to copy and paste a sentence. The fuse is not a glass cylinder type you'd expect in older equipment. More likely a device depicted in the photos and will be soldered to the board neat the xfmr. Follow the AC line and you'll find it.

Speedskater

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Re: So what happened here?
« Reply #10 on: 1 Aug 2020, 04:52 pm »
It's good to see 'AVnerdguy' posting in technical threads. He has a ton of useful knowledge.

Letitroll98

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Re: So what happened here?
« Reply #11 on: 1 Aug 2020, 07:34 pm »
Yeah, thanks to all the folks replying, you've been a world of help.  I'll keep the unit to see if I feel like a project some day.  If anyone missed John's reply, who kindly sent me this DAC many years ago, he flipped the dip switches converting to 120V before sending it to me.  I had forgotten that and was wondering how I'd been so negligent to not notice the stamp previously.  Turns out I'm not as stupid as I thought, I simply suffer from CRS disease.

richidoo

Re: So what happened here?
« Reply #12 on: 2 Aug 2020, 01:06 am »
Dan, I emailed a link to your thread to my friend who is a talented audio tech and he replied:

"A couple things, the fact that the sound improved, I wonder if that was thermal runaway in the circuit. It could also have simply been reduced voltage. Obviously, the transformer is cooked. It looks like a Talema toroid. A diode or snubber cap could have shorted. It’s also possible that the transformer simply developed a shorted turn. What’s strange is, where is the fuse???? Transformers are usually really tough."