Yamaha emx512SC protection circuit

[rbilx]

I have three EMX512SC powered mixers that shut down after 2 seconds. All the PS outputs are good, all the output transistors check good and I replaced all of them on one unit. It powers up, relay clicks, fan starts, 15V -15V 12V 5V all come up, all the higher voltages in the output circuit seem ok and then the relay drops out and it's off - same on all three units. Has anyone had any experience with these units and the protection circuit? Yamaha seems to think it is a short in the output but I've looked at just about everything.
Thanks for any replies.

[oc disorder]

Ditto well almost I have the smaller model here in the "very hard basket" the 312sc.
Try disconnecting the high voltage 5 spade connectors maybe 4? and see if it still does it.

[rbilx]

Disconnect HV plugs

I just disconnected the 5 HV plugs and it still does it. If I disconnect the 14 wire flat cable it stays powered on (with the HV leads plugged in) - all three of them

[Jazz P Bass]

Stupid question.
Are you positive ALL of the grounds are in place.
I know Yamaha uses a very strange grounding scheme on some of there hard shell units.

[Enzo]

I'll have to look up the circuit later, but you need to isolate the problem. So at the moment I am assuming certain things, like no SMPS. Either there is a bad channel or other problem that is triggering the protection - so it is really just doing its job - or the protection circuit itself is faulty.

Just because you meter-test outputs doesn;t mean they MUST be good. And even if they are all good, that doesn't mean there is no DC offset creeping into an output bus.

Are we talking a main power relay? Or ar we talking speaker relays on the channels?

When a speaker relay drops out, one first step is to look at the output line on the amp side of the relay. In a conventional amp, I usually just grab one of the ballast resistor legs. Is it centerd on zero? Or is there some offset?

And look at what all controls the relay. SOme more sophisticated units will detect not only DC on the outputs, but also temperature problems of one or more circuit points, plus a timed power up sequence, and even excess output current. SO something like a broken leg on a temperature sensor could be triggering a shut down.

[oc disorder]

If I disconnect the 14 wire flat cable it stays powered on (with the HV leads plugged in)

I found exactly the same thing which made me initially think there was a problem with the low voltage supply (+ & - 15v).
There was an unusual amount of ac on one of the 15v supplies (for memory the positive) which made me think this was upsetting it
then after substituting capacitors and a new regulator came to the conclusion that there was interference with the meter due to
the way a smps works .I have been testing it without the mixer section plugged in.
I hope to jump back to it later today. The last thing I can remember doing is putting 5k resistors in series with the + & - 15 supply
to the power amp board and it stayed on. I checked that the first IC on the pwr amp board didn't have a short and the two filter caps
on the board were ok .Bear in mind I have a 230v supply and by the way it will function here via a 100w light bulb in series which
just glows dimly and the results were the same whether I had it in series or not.
So my conclusion so far is some strange fault in the 15v supply which upsets the protection which appears to be over enthusiastic
as the amp seems to work perfectly with the dropping resistors.?????!!!!!

Service Manual 5 part winrar archive
http://fileshare.eshop.bg/downloadsm...%20EMX512.html

[rbilx]

Reply to grounds

Stupid question.
Are you positive ALL of the grounds are in place.
I know Yamaha uses a very strange grounding scheme on some of there hard shell units.

This is not a stupid question at all. I had a Yamaha Motif 6 I repaired and whenever I powered it up it only played white noise no matter what key I played or sound I chose. After about a month I gave up and put it completely together to sell it as is and it worked fine. It must have been all the ground straps that weren't connected when I tested it. I did completely assemble one of these EMX512SC amps with all ground wires and shields in place and it still shuts down but I appreciate the advice.

[rbilx]

The relay comes in when the +15v and -15v supply turn on. After about 2 seconds, the relay drops out and it shuts down. I monitor the voltages going to the amp board and they look like what they should be. If I disconnect the + and - 15v lines going to this board the relay doesn't drop out. This is even with the higher 60 plus volts going to the output transistors. The circuit is pretty sophisticated so that is slowing me down. The funny thing is, I have three of these all doing the same thing. When I got them, there was quite a bit of damage to the SMPS. I had to replace two ICs and two transistors and three power resistors on all three. Even though the 15v supplies look good maybe I still have something else not quite right? Another person here with the same problem put a resistor in series with the + and - 15V lines going to the amp board and he says it stays powered on. I haven't tried that. If you get a chance, would you mind having a look at the schematic which has been posted here and help us make some sense out of the protection circuit? All thermal sensors are at the proper resistance and connected ok. Also, I replaced all of the power output transistors on one of the units with no luck.
Thanks.

[Jazz P Bass]

Cannot open the schematic from the link above.
It seems the file is in (2) parts. RAR type.
There is only one file on the download site, therefore the first part cannot be opened.
If you have the complete file, please upload it.
If it is too large, get CUTE Pdf Reader & knock it down by groups of pages that will meet the forums upload limits.

[rbilx]

I attached three pages from the schematic. They are - the PS itself, the PA board which has the output amps on it and seems to cause the problem, and the output board which the speakers plug into. I just included that so you have a point of reference for the outputs. I hope the attachments work.

[oc disorder]

Firstly JPBass the 5 parts are still there ( see attached e-service-info-page.jpg)
and winrar trial can be downloaded from RarLab

 http://www.rarlab.com/download.htm

.

Here is some info from the book of Enzo!

Enzo Yamaha EMX312SC
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.



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.

Can't remember where I found that !!

So far to me particuarly the 312sc (which doesn't have the comparator section) doesn't sense the power amp - (follow the DC Pin 11 on the poweramp plug CN415) but relies on both + and - 15volt supplies to be working to activate the 24vdc relay.

To clarify my experiment I removed the multiwire cable from between the smps and the power amp and the high voltage spade connectors for the power amp and only used 3 wires from the smps (CN415) to the power amp essentially only powering up an IC and a few transistors (ie +15v pin 1, 0 volts gnd pin 2 and - 15volts pin 3) , and this was enough to engage the protection relay even though I could see no evidence of loading of the 15v supply.

I've got the box out with all the bits and plan to spread them out and re-investigate if the phone will stop ringing. I have no isolation transformer as its on loan at the moment so hopefully can have a look at some wave forms ripple etc later. Hope thats a bit more fuel for the subject and that I haven't made any classic blunders in my ignorance!

[Jazz P Bass]

[QUOTE=oc disorder;231613]Firstly JPBass the 5 parts are still there ( see attached e-service-info-page.jpg)
and winrar trial can be downloaded from RarLab

 http://www.rarlab.com/download.htm

.QUOTE]
Thankyou.
I failed to see the multipart 0-4 header.

[Enzo]

I admit to not having studied this closely, but on that 14-pin ribbon, pin 11, called DC, connects to the output bus of each channel, and if DC is ther, it will trigger either Q411 or Q413 to shut it down through the opto. "Optocouper" as they typo.

SO if ther is DC on an output, that will trip it out, and pulling the ribbon will defeat that.

Also there is the temp sensor, which will mute the amp, but doesn;t seem to involve the SMPS.

The soft start relay is across the +/-15v rails, so if they come on it should fire. But that doesn't prevent something from shutting down the SMPS, which of course would remove the 15v supplies and drop the relay.

Let us assume we are all experienced techs with some idea of safety. You need an isolation transformer to work on the primary side of the SMPS. Specifically if you want to connect any real world stuff to it, like a scope. But you can scope the secondary side all you like, Other than the freq it runs at, it is rectifiers and filters like any other. We know the 15v is going away and causing the relay to drop, but I doubt it is a direct failure of the 15v rails. On the secondary side, things are referenced to ground. On the schematic, anything left of that dotted line is dangerous.

The SMPS sits ther running if all the rest of the system is disconnected, right? You get +/-15 and the control voltages, and the two sets of HV rails? If so, it probably works. Wanna be hard core? Stick a load on each supply in turn and see if it handles it. The two control voltages don't even go to the power amp board, so I'd ignore them, at least initially. The loads are specified, so I wouldn;t drag more than half an amp from either 15v. And The HV rails are probably OK if they come up to voltage, but maybe 1 amp tops should be sufficient for test.

My money is still on a power amp. With the ribbon in place, watch pin 11. In the few seconds you have before it drops, what does pin 11 do? For that matter look at the output bus for each channel. In those few seconds, does it sit near zero, or does any of them go offset? That is on the output bus itself, not the speaker connectors outside. Little 2-pin connectors CN102,CN103 look to be test points for bias current monitoring, but would be handy points to connect to the output bus.

[rbilx]

I disconnected the HV leads and just left the + - 15v lines on. There is a 1 second voltage pulse on pin 11 and it shuts down. I opened pin 11 and the SMPS stays on. I have two of these hooked up right now and they both act identical. With everything connected except the HV lines, I read 9.75v DC on D127 and D125 on the output circuit and then the relay drops out. With pin 11 disconnected, it will run forever but of course there is no amp working. Is it possible the shutdown circuit fails after the amps have another failure? Remember, all three of mine had blow SMPS power supplies and two had a blown power transistor in the output on one channel.

[Enzo]

SOunds more like the shutdown is working and doing its job.