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Old 15th January 2010, 05:49 PM   #1
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Default Yamaha mixer EMX312SC

I need some troubleshooting help. The mixer does not power up. I opened it up and found: 12A fuse good, 120VAC at power on switch. Switch checks good. Looking for schematics now-I see where soundmankelvin posted schematics. Any help out there?
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Old 20th January 2010, 02:42 AM   #2
Enzo is offline Enzo  United States
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COnsider the system is not much more than a table lamp. Except there is a transformer instead of a bulb.

The mains current comes up one leg, through a fuse, a switch, maybe a surge limiter, maybe some sort of mains voltage selector, no doubt some circuit board traces, through the transformer primary and back down the other mains leg. ANY of those things can open up.

SO are both ligs of the mains cord intact? Just because the hot side gets 120v to your switch doesn't mean the cold side completes a circuit. Are there multiple posts for various mains voltage configurations? ANy of those come off? Does the switch function. In other words does it go to zero ohms when ON? How about the transformer primary, is it open?


And let us not assume. YOu say the thing doesn;t power up, but what is that based upon? A lack of power indicatior could mean an open internal secondary fuse. SO check teh secondaries, are they making voltage?
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Old 29th January 2010, 02:41 PM   #3
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Quote:
Originally Posted by Enzo View Post
COnsider the system is not much more than a table lamp. Except there is a transformer instead of a bulb.

The mains current comes up one leg, through a fuse, a switch, maybe a surge limiter, maybe some sort of mains voltage selector, no doubt some circuit board traces, through the transformer primary and back down the other mains leg. ANY of those things can open up.

SO are both ligs of the mains cord intact? Just because the hot side gets 120v to your switch doesn't mean the cold side completes a circuit. Are there multiple posts for various mains voltage configurations? ANy of those come off? Does the switch function. In other words does it go to zero ohms when ON? How about the transformer primary, is it open?


And let us not assume. YOu say the thing doesn;t power up, but what is that based upon? A lack of power indicatior could mean an open internal secondary fuse. SO check teh secondaries, are they making voltage?
It appears the power supply is dead. None of the Dc V's read anything.I've got the schematic now. The power supply transformer primary is shorted. The secondary is open-is that correct? There is a relay early in the 120vac input that may be open?? Not sure yet how to test this relay. Thanks for your input.
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Old 30th January 2010, 03:00 AM   #4
Enzo is offline Enzo  United States
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The transformer primary is probably not shorted. If it were shorted, your mains fuses would blow immediately. if you are measuring its resistance, that will naturally be a low value.


Secondaries should not measure open. it is not likely more than one secondary would measure open even if one was.


I made an erroneouos assumption. This is more complex than a table lamp - this unit has a switching power supply.

I am looking at page 86 of hte manual which covers the powr input circuit. The mains comes in from teh hot side, immediately through the power switch, then through two 5W resistors in series. 6.8 ohms each. The relay will come on and short across those resistors after a moment. That is a soft start circuit. If one of those resistors is open, the system will not wake up. And it is also likely they opened for a reason.

Beyond the resistors is the main fuse F401 and on to bridge rectifier D401. The other AC terminal of the bridge rectifier is the mains return or neutral.

If the fuse, switch and resistors are intact, then you should see about 170VDC across each C409,C410. or about 340vDC across the pair.

If you are getting that, then your switcher is not running for one of many reasons. Getting the mains voltage to that bridge rectifier is still table lamp. So that is the first question - is mains 120vAC getting to that bridge?

If it is, and your switcher is not running, first thing to check is for a blown power amp - shorted outputs in one chanel. And since the switcher feds the channels, the main rectifiers are on each channel, so check them too.
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Old 1st February 2010, 02:29 PM   #5
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Quote:
Originally Posted by Enzo View Post
The transformer primary is probably not shorted. If it were shorted, your mains fuses would blow immediately. if you are measuring its resistance, that will naturally be a low value.


Secondaries should not measure open. it is not likely more than one secondary would measure open even if one was.


I made an erroneouos assumption. This is more complex than a table lamp - this unit has a switching power supply.

I am looking at page 86 of hte manual which covers the powr input circuit. The mains comes in from teh hot side, immediately through the power switch, then through two 5W resistors in series. 6.8 ohms each. The relay will come on and short across those resistors after a moment. That is a soft start circuit. If one of those resistors is open, the system will not wake up. And it is also likely they opened for a reason.

Beyond the resistors is the main fuse F401 and on to bridge rectifier D401. The other AC terminal of the bridge rectifier is the mains return or neutral.

If the fuse, switch and resistors are intact, then you should see about 170VDC across each C409,C410. or about 340vDC across the pair.

If you are getting that, then your switcher is not running for one of many reasons. Getting the mains voltage to that bridge rectifier is still table lamp. So that is the first question - is mains 120vAC getting to that bridge?

If it is, and your switcher is not running, first thing to check is for a blown power amp - shorted outputs in one chanel. And since the switcher feds the channels, the main rectifiers are on each channel, so check them too.
The two 6.8 ohm resistors check good in the circuit at 6.8 ohms ea. The 170 Vdc is present at C409 and C410. I am having trouble finding Bridge Rec D401 on the PCB. Double checking the print now. Thanks for the help.
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Old 1st February 2010, 03:58 PM   #6
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Still can not find D401 on the PCB. On the drawing it is in a block with IC401 402 and a Op amp switch symbol?? There is no 120 Vac at T401 primary. (120 Vac should be there correct?)
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Old 2nd February 2010, 07:07 AM   #7
Enzo is offline Enzo  United States
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YIKES!!! NO there is not 120VAC at the primary of T401. This is a switching power supply. A very high frequency non-sinusoiodal waveform will be there. That primary will have those high DC voltage s switched off and on.

If those two caps have the 170VDC, the only way it could get there is through the missing bridge. You may not see it, but it is there and working.


This power supply is extremely dangerous to work on. The mains power is directly rectified and filtered. Nothing between you and the mains. grab the wrong thing and you can find yourself dead.


Turn up your scope, and hold the probe right next to T401. If there is any activity in it, your probe will pick it up. it could possibly mention it in the amp specs, but I have no idea what it is, but I'd expect the waveform to be something in the 50kHz and up region. I bet there is nothing going on.

Note that the drive section, IC 402 and Q406,407 are not refernced to ground. They are referenced to the -170VDC rail. DO NOT connect your scope ground to any point in this circuit.

You see D429 there charging up that 1uf C435? look straight down from the fuse. From there that rectified supply feeds through four series 120k resistors. And to the right of them is R440 in the box #4. 220 ohm 2w. Open?

R440 feeds +170VDC down to Q414 and into that 15v regulator.

The string of 120k needs to turn on that transistor to kick the 15v regulator into action to turn on IC401. Once that starts then the 15v circuit will run off the supply formed by D414,415. And at that time, that supply will also turn on Q415, whose job is to turn Q414, the start transistor back off.

NONE OF THESE VOLTAGES ARE REFERENCED TO GROUND. Your meter would take measurements against that -170vDC rail. SO the output of that 15v regulator will be +15 over the -170.


If one of the 120ks open, then they can;t turn on Q414. And if Q414 is not getting +170 from R440, then it can;t pass it along.

I suppose one quick check would be to see if there is in fact 15v coming out of that rectifer. Look at IC401. "ground" is pin 12 and pin 15 if V+. SO is there 15v between those two pins? if so, then 401 is a suspect.

I suspect your main switcher transistors are OK or you'd be blowing fuses.
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Old 4th February 2010, 01:27 PM   #8
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R440 measures open in the circuit and shows slight signs of over heat. Two other resistors close by R414,R415 may have heated up too.
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Old 5th February 2010, 01:38 AM   #9
Enzo is offline Enzo  United States
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SO at least that is bad. remember, resistors don;t burn up on their own, something always causes the excess current to flow through them. So I'd be highly suspicious of the transistor that resistor feeds.
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Old 27th May 2013, 02:56 PM   #10
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Default EMX312SC

I hv a EMX312SC that comes on and den shuts off one minute or so....soft start relay shuts off...+ and _ 15volts shuts off,,,main + voltages for power amp section ok...cud sumone tell me why???if front control panel connected ok..bt if strip cable of of power amp connected it shuts down...
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