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Power Supply Help Needed
Power Supply Help Needed
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Old 21st October 2010, 05:29 AM   #1
RickD is offline RickD
diyAudio Member
Join Date: Oct 2010
Default Power Supply Help Needed

Hello. First timer here. I am a novice, so here goes.

I have a Yamaha EMX 5000 powered mixer just bought used for dirt cheap. Upon inspection, I noticed that the SMPS regulated section (separate PCB) was taken apart, soldered badly, and basically destroyed. So I ordered a new one from Yamaha, installed it, and when I turn on the switch, the power light comes on for a second then off. The cooling fan moves a little and then nothing.

There is another PCB where the AC cord connects to. On the board, there are two circular coils, 3 small blue capacitors, 2 solid yellow horizontal items about 1/8 thick 1/2" high and 1" long, and a 15amp 250v fuse. A small transformer is mounted near the board. Leads from the small PCB goes into the transformer and from the transformer plugs into the SMPS PCB.

So my questions are: Is it possible the transformer is bad? No fuses have blown. Don't smell anything. Should I replace the smaller board AND the transformer? I do have a schematic but it does not show what the voltage should be from the transformer to the SMPS. Therefore, I have no clue what is going on.

The schematic does say the SMPS is a regulated +/- 15V supply unit.

Thanks in advance for any replies.
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Old 21st October 2010, 06:48 PM   #2
RickD is offline RickD
diyAudio Member
Join Date: Oct 2010
Well, no responses. Okay, I guess my questions were not worthy.

For those reading this, the simple answer is when a power amp or mixer with an amp built in powers up and shuts down immediately, chances are it is the main amp section of the unit that has some sort of short. The system is simply protecting itself. You will need testing equipment to find out where the short is.

In my case, I went to a service center and they put the unit on a variable AC supply with meters and immediately determined that the main amp was shorting. Turns out, a transistor was bad. The unit is now working, but the tech is testing other components for weakness and / or faults.

Lesson learned.
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Old 22nd October 2010, 09:26 AM   #3
metalsculptor is offline metalsculptor  Australia
diyAudio Member
Join Date: Sep 2010
Location: Melbourne
Originally Posted by RickD View Post
Well, no responses. Okay, I guess my questions were not worthy.
Expecting a response in a day midweek when most people work is a bit much.

The fact the light came on and went out pretty much indicated the supply was probably good, probing the DC link would have been my next step. then powering up the amp from a lab supply.

FWIW transformer failure is rare and they usually fail with shorted turns (lots of heat) large ones blow fuses.
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Old 26th October 2010, 09:39 AM   #4
dtproff is offline dtproff  United States
diyAudio Member
Join Date: Oct 2008
Location: Plano, TX, USA
If I were a betting man I would suspect that something down-stream is shorted and the PSU is hitting its over-current limit and shutting down. Best gues is that the unit failed, someone tried fixing it and couldn't so they stuck you with it. If it was a good deal it may be worth spending time on.

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Old 26th October 2010, 10:23 AM   #5
DF96 is offline DF96  England
diyAudio Member
Join Date: May 2007
If you want to guarantee a quick response to a technical question then you have to have a consultancy contract in place, with a retainer fee. Otherwise it helps to be patient and polite.

Diagnosing faults can be difficult. Diagnosing faults remotely can be impossible. We are clever, but not that clever!
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