Behringer Xenyx1202FX mixer

[JP-Stereo]

Hi everybody !
Strange problem with this mixer.
I wonder if someone already see this ?



On this unit, the Line 1 to 4 are Mic inputs with XLR and TR mono connector.
When i plug a Mic, i have tried 3 different models yet with the same result.
I put the Level knob at 50%, the EQ is neutral with or without Low Cut.
When trying to adjust the Input Trim to reach the Clip level and lower a little, i have no output on the Mains and HeadPhone/Control until reaching 90% or the pot travel.
Then SUDDENTLY the signal overload and is just too strong.

If i use a Line out from a CD player via the TS connector, eveything is fine, i have the signal from very low to strong and it still overload suddently at the same point.
The FX line is also OK as i have my Mic signal with effects even at low TRIM level.
All other Line Inputs are fine and respond from low to high predicably, without jumping suddently.

I had to change the Power blue LED wich was open, leaving the Left meter line dead.
Now this is working.
The Regulators outputs are there but one seem stressed.
I don't have the schematic of this (FX) model, but it is very near the plane Jane Xenyx1202...
Except for the FX line and the Regulators.


The problem is visible on a scope as early as R28.
The FX version use an external Powerpack and on board +/- 12vdc regulators.
There is also a 7805 in serie with the 7812.

The VCC at IC4B is -11.8vdc and +12.0vdc, is this unballance can cause the problem with those differential amplifier at the low Mic signal input level ?

Thanks for inputs...

[x-pro]

R45, R46 in what condition?

[g1]

Since you say the FX buss is fine even using mic input, I don't think this non-FX version schematic is going to help.
Post a schematic request in that section of the forum.

[JP-Stereo]

Good morning everybody !
It's always cool to get answers : )

The circuit is not exactly the same, but parts layout is very similar with about the same parts numbers with some exeptions.
It's more like the original 1202 compare to the schematics i've seen for bigger "FX" model.
In this one it look like they used the AUX line for the FX bus.
I will post a request immediatly !

So on my board, the first in serie resistors are R46 = 4.765 ohm and R47 = 4.755 ohm.
The marking on the board is 4E7 beside the part number, so the value is not the same as the +/-15 v schematic.
They must have change some components values probably to operate at +/- 12 vdc.

Not absolutly certain if the FX buss is perfect, but at least i have signal from low to high TRIM setting on it.
Even this bus have the excessive Bump in the gain at 90% travel.
And it's not at the individual channel component level because ALL 4 Mic inputs are acting the same...

The -11.8vdc doesn't look suspect for you ?

[x-pro]

The -11.8vdc doesn't look suspect for you ?

with a +/-12V supply is not the cause.

[x-pro]

And it's not at the individual channel component level because ALL 4 Mic inputs are acting the same...

In use, these all 4 inputs could have broken not all at once, but gradually. For example, by pulling out the XLR connector when the phantom voltage is turned on.

[x-pro]

" i have tried 3 different models yet with the same result."

Either specify the brand or specify the output. Is there a balance transformer?

[JP-Stereo]

After experimenting the problem with a Berhringer XM8500, i've tried a Shure SV100.
I don't have the model of the third Mic because i've asked a musician i know to test it with it's own mic for the same result.
3 different cables too, both XLR and TS.
No Tx from what i understand, unless there is one in these mic...

You are right x-pro, "individual channel components" should not be excluded.
It is a possibility and in fact, once such problem is found the repairs may be all alike.
But it's also very possible that a commun cause affect all the four inputs, it's was my first feeling.

The little heat sink for the 3 regulators is getting pretty hot after a few minutes.

[x-pro]

This radiator is always warm, almost to hot.
It would not hurt to check the power supply. Especially capacitors 100uF 100V - 2 pcs. and 470uF 25V - 2 pcs. and 330uF 25V.
They often lose their nominal value.

[g1]

When trying to adjust the Input Trim to reach the Clip level and lower a little, i have no output on the Mains and HeadPhone/Control until reaching 90% or the pot travel.
Then SUDDENTLY the signal overload and is just too strong.
...

The FX line is also OK as i have my Mic signal with effects even at low TRIM level.

The only thing between the aux buss and the L/R busses is the pan pot. Maybe I'm not understanding the first post.
With low gain trimpot setting and mic input, you have clear signal at Aux/Fx buss but not at mains output?

[x-pro]

That could be the reason. You should check this point on all microphone channels.
If it will be on all of them, then check the power supply of microchips directly on their pins, 4 and 8.

I apologize. I misunderstood.
Of course, 0.2V difference in chip power supply is not the reason. I thought that on the output of this IC4B (-11.8V).

If IC6B pins 5 and 6 (1202FX schematic) have a voltage of (-6.7V) at the chip inputs and about (0V) at pin 7, the input is OK.

[x-pro]

I put the Level knob at 50%, the EQ is neutral with or without Low Cut.
When trying to adjust the Input Trim to reach the Clip level and lower a little, i have no output on the Mains and HeadPhone/Control until reaching 90% or the pot travel.
Then SUDDENTLY the signal overload and is just too strong.

GAIN adjustment is exactly the same there.
But in order for the signal to go to "MAIN OUT" and "PHONES", you must raise the "FX TO CTRL RM" button.

I currently have a model 1002FX on hand (under repair).

[JP-Stereo]

Hi everybody, i was away for a while,

I just took the following measurments;
U6B
Pin 5 to Pin 6 = +11.6 vdc differential.
Pin 5 = +11.6 vdc, Pin 6 = 0 v, regarding to ground.
Pin 7 = -7.06 vdc regarding to ground.

Now that i understand the level adjustment of the FX and the CTRL RM/Phone output, i can confirm that both FX buss and L/R have the same behavior.
I was getting a stronger signal on the FX bus because of the two gain adjustment, if i ramp up the Phones-CTRL RM pot both are now the same.

Very weak signal, barely audible even with all the other levels set to high.
And at around 90% of the TRIM setting the signal burst to saturation/overload.

[JP-Stereo]

Another strange thing...
I am reading 31.15 vac RMS on the powerpack connector on the board with my Agilent multimeter.



But my scope tell me 83.5-84 vac, with 42.7 vac from each leg to center tap.



Is it the RMS thing that bring this difference ?

[x-pro]

You don't need to measure the AC voltage there with an oscilloscope in the transformer power supply. A multimeter will suffice.