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Yamaha EMX512SC not much output

Started by DrGonz78, November 23, 2014, 03:26:14 AM

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So I am attempting to troubleshoot this beast the Yamaha EMX512SC. Lots of surface mount parts and very tough to take it all apart. Had to go buy a huge screwdriver just to get the front panel off, very long hole shafts.

The behaviour is that all the channels except ch3 are not passing a strong signal. There are LED's (Label Name: FCL) that are associated with each channel. All the channels except ch3 gradually light up the FCL LED as you turn up the compressor for that channel. I can observe on a scope the signal is passing through the channels, but it is very low. This is whether or not I scope from the speaker out or the main out(preamp). Channel 3 is loud and working great, except that FCL light is fully illuminated. The strange behaviour leads me to look towards the compression part of the mixer as a potential culprit for the problem.

I am going to take some voltages as well but just going slow first to understand which ground points I should choose on the preamp part of the amp. It is a switch mode power supply and I am not very experienced dealing with these types of units. Any help would be greatly appreciated and I have attached a link to the HUGE service manual, since it is way too big to upload here. Thanks.

"A person who never made a mistake never tried anything new." -Albert Einstein


Yep, that's a really detailed manual (and thanks to electrotanya for hosting it).

Okay, we all get a bit jittery around SMPS (well, I know I do  :o ), so just do the conventional and check that all the supplies are present and correct, and you can ignore it after that.   :tu:

I'll go for a dig on the circuit, but an obvious question has to be the Mic/Line switch for each of the channels 1-4; are they all set the same?

Looking at page 129 it would seem that you have already localised the fault to the input "Jack" board or the early part of the "Mix" board, which is a major step forward.

For the input earth you can use pin  on the XLR's, or the sleeve on the TRS sockets.

Assuming the SMPS checks out, are the required voltages actually making it to the "Jacks" board via the interconnects?

I'd be inclined to try and defeat/minimise the effect of the compressor by turning the Threshold control to the least sensitive position (I presume 10, but it may be 1), and see if you can get matching or similar behaviour on each channel.

Take heart, a mixer is like a stereo except that you have more channels to compare, and there is a lot of the same stuff repeated, so they aren't quite as complicated as the circuit looks intimidating.

You certainly have a rather odd set of symptoms, but you also have it well backed into a corner already, and a large service manual to beat it with.   ;)
If you say theory and practice don't agree you haven't applied enough theory.


  Certainly a very strange set of symptoms.  As all channels but 3 have the same problem, normally you would suspect something common like low voltage power supply, but ch.3 working throws a wrench into the works.  I would suggest you leave the ch.3 issue till you have the other channels sorted.
Find out if the FCL's are lighting when they should, if so, the signal at that point should be correct and the problem is downstream somewhere.
Levels for FCL threshold are shown on page 129 up by where it says "mix".


Well this thing gets stranger by the moment. I went to go check it out again tonight and scope it before testing voltages. Well the ch3 is not behaving the same as it was and is now acting just like the other channels. All the FCL LED's check out as I bring up volume levels to check thresholds(Edit: When I say volume levels I am talking about signal gain at the generator.) I am not using any of XLR inputs to monitor signal on the scope and can only input the TRS 1/4 inch jacks. Now there is a mic/line button and all my signal checks have been with the line button selected. If I select mic button option my signal are very healthy and strong. I know for fact though that last night ch3 was getting strong volume even with the Line option selected. Strange...

I opened it up and started to prepare to read some voltages. When I click on the unit with the front face removed it starts for a second and turns off. I think that has something to do with some grounding shields that connect when the unit is all snug and put together. Put it all back together and it starts up great! So I guess I need to just jumper a connection point to get the unit to stay on while testing voltages. It was a long day so I just gave up for tonight  :trouble. At this point I need to check some more things tomorrow, but I am really not feeling good about this one. Just a PITA. 
"A person who never made a mistake never tried anything new." -Albert Einstein


Quote from: DrGonz78ch3 was getting strong volume even with the Line option selected

...and if the Mic/Line switch had dirty contacts... ?
If you say theory and practice don't agree you haven't applied enough theory.


Doc, there was a thread over at MEF that went into detail about the ground scheme for this mixer.  JPB made up a pdf of the ground info, but there was more said about it later so I'll just link the thread instead of posting the pdf.
The ground info starts here and carries on further down:


Yeah Roly I thought that too but it does seem strange that all of them are behaving in the same manner. Further more ch5-6 yield same problems at line level and there is no switch there for those channels. I will have to cover my bases no doubt.

G-one thank you for that post and the link. That is exactly the info I needed confirmed in regards to grounding schemes. I am taking this very slow as a precaution that the grounding scheme for this unit is pretty elaborate. Now I can see that the power amp and preamp grounds are different. That is great info!
"A person who never made a mistake never tried anything new." -Albert Einstein


Honestly I think I am on one of those wild goose chases. All voltages check out and nothing seems bad. Anyway the customer also said the monitor out was not putting out signal. I flipped the switch for monitor/LR and it scopes out a healthy signal. I was told it was too quiet by the friend who dropped it off and he talked to the owner. He then said all that was wrong was scratchy pots and monitor out not working. Appears that I just gave the amp a basic check up and a bill of good health. I am still not convinced the line level thing is not a problem, so I will check a few other things. I swear I had stronger signal with line level engaged on ch3, but perhaps that was my own error. There is always a chance I had ch3 at mic level.  :loco Wish I had a known working unit to compare it to so I could have more clarity. Still I think the Pad is just doing it's job and I can crank a stronger signal in than with mic line.

Moral of the story is that some things are not broken, but when told they are can cause all sorts of confusion. Still some learning lessons to be had, but not much to be fixed.
"A person who never made a mistake never tried anything new." -Albert Einstein

J M Fahey

Quotesome things are not broken, but when told they are can cause all sorts of confusion.

Always happens  :P