Yamaha EMX 640 schematic

[keithsplace]

Does anyone have or can post a schematic for a Yamaha Emx640 powered mixer?
I purchased one used and it works great except the effects are creating noise when engaged, horrible distorded noise. It can be engaged, with all gain levels down, no noise, when the Effect RTN is turned even slightly, it gets an almost 56k modem noise, thru mains or monitors.
Works fine otherwise. I did have to take it apart and re-solder two of the channels XLR connectors when I got it, they had become loose from the board. I can't remember if the effects worked after that or not, but I'm pretty sure they did. Anyone have clue where to start looking?
I'm not a tech, but I can trace circuits and know what components do and have built amps, and radio equipment.

[g1]

Here's a link to the service manual. DON"T click the big blue button that says "download" (that is for a pdf reader program). Look under the picture of the manual for where it says "get manual" and click on that.
YAMAHA EMX-640 Service Manual free download, schematics, eeprom, repair info for electronics

[Jazz P Bass]

The EMX has the FX card under a cover.
There is a 'slip in' ribbon cable for it.
#1, make sure it is inserted into the FX card.
#2, see that it did not get damaged at reinsertion.
The little connectors can peel up if you are not extremely careful.
Also, there is a footswitch jack to disable the FX.
Does that work?
Can you turn off the FX there or on the front panel?

[keithsplace1]

Reply

The EMX has the FX card under a cover.
There is a 'slip in' ribbon cable for it.
#1, make sure it is inserted into the FX card.
#2, see that it did not get damaged at reinsertion.
The little connectors can peel up if you are not extremely careful.
Also, there is a footswitch jack to disable the FX.
Does that work?
Can you turn off the FX there or on the front panel?

Thanks, got the schematics.

Yes effects turn on with the effects button on the front, but there is all noise when the Effect RTN gain is turned up, which kind of leads me to the effects board.
Is this board behind the front panel? I have had the back parts, PS, and amp section, off.
Didn't see anything burned, blown, or broken. Fuses or ok.

[Jazz P Bass]

The FX board is on the rear of the front panel.
It is under a small metal cover.
Just to clarify, when you disengage the FX, with the front panel switch or the footswitch, the FX return still makes noise?
If it does, then something else is going on, other than the FX card.
With the FX switch in the disable position the FX return pot is basically grounded.

[keithsplace1]

The FX board is on the rear of the front panel.
It is under a small metal cover.
Just to clarify, when you disengage the FX, with the front panel switch or the footswitch, the FX return still makes noise?
If it does, then something else is going on, other than the FX card.
With the FX switch in the disable position the FX return pot is basically grounded.

No no, with the effects switch desengaged there is no noise. Just when you should be getting effects is when you get the noise. Like it is in the effects circuit somewhere. It is also with all effects, L Hall, S Hall, and Vocal, makes no difference.
Effects on LED lights up as well, so I don't think it's a power issue to the effects sections.

[Jazz P Bass]

There are a number of opamps on the FX card.
You could try testing each output pin for unwanted Vdc. (no signal)
They are dual supply opamps so the outputs should sit at zero volts.
Do you have a scope?
If not you could use your meter.
Set it to read Vac & look at the opamp output pins.
The noise should show up as a reading on the meter.
Hopefully the noise is in the opamp circuits.
If it is showing on the main IC's output pin, the ic itself may be at fault.

[keithsplace1]

I assume your referring to IC 905. &. IC 906?

[Jazz P Bass]

Work your way backwrds from the FX pot.
IC 904 & 903.

[keithsplace1]

Thanks much, I will post results, What a great forum.
Thanks again.

[Jazz P Bass]

While you are in there, the first line item should have been to check the power supplies going to the FX card.
Pin 1 & 2.
Pin 3 & 4.
Pin 8 & 10.
Pin 9 & 10.
Check them with the meter set to read volts dc.
Them check them to read volts ac.
The volts ac should ideally read zero.