Behringer Xenyx1202FX mixer

[x-pro]

like this

[x-pro]

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.


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.

You have to look at the schematic and repair each channel.
Usually burn out the transistors and diodes before the IC6B.
In order not to tread on the same place, it is better to change transistors and diodes at once.

To remove them easily, you have to unsolder the TRS connectors.
In the meantime, check the resistors around these transistors for their rating.

[g1]

Is it the RMS thing that bring this difference ?

Yes.
Supply says 14.8-0-14.8VAC. Times 2.8 to go from RMS to p-p equals approx. 41.5V p-p per half. You are close enough.

[JP-Stereo]

Something was inconsistent in some of my previous measurment or else something changed.
Now i have a good weak signal at R76 and R77, no burst.
The gain burst appear at R36 and R37.
Very flat weak gain slope until the burst point where the signal is going 10x suddently.

C24 and R84 are on specs.
It's must be around T1, T11, T7 or T15...

I think i have to remove D12 and D13 for testing therm ?
I will go for the transistor too, removing and testing too ?

[x-pro]

Take action
Don't forget to check the resistors when you unsolder the diodes and transistors.
Voltages on the chip in the operating mode, indicated earlier.

[JP-Stereo]

The D12 and D13 BAV103 diodes and the T11 and T15 MMBTA6 are located.
For T1 and T7 i physicaly have 2SV888 from CoolAudio wich are no longer available.
But the schematic lable them as 2SA970, these are still available in NOS.
Proceeding !

[JP-Stereo]

Finally i replaced the preamp transistors and diodes on channel 1.
T1, T7, T11, T15 + D12, D13.
No improvement or very little.

Testing everything around on channel 1, R29 is testing 3.9k but R28 wich should be the same was testing 1.2k.
It look like a good imballance, so i am waiting now for these resistor around IC6B.

Ordering...

[The Dude]

Resistors don't lower in value. I'd be more apt to believe you are measuring across something else in the circuit.

[JP-Stereo]

I will remove it !
It's a "through hole" it's not a surface mount.

[x-pro]

Check the transition holes that supply power and the common "A" bus.

[JP-Stereo]

O boy, this is a good one : )
It's not obvious to locate such a point in the circuit.
On the schematic the IC's Vcc supplies are not traced.
How can i locate the "transition hole" for the supply ?
And what are you refering to with "common A bus" ?
Sorry to not following...

I will also measure R28 off circuit right away.

Beginning to think about swapping the opamp...
A Cool Audio V4580 look the same as a TI RC4580, am i missing something ?

Thanks to all !

[x-pro]

V4580 - RC4580, JRC4580, BA4580, NJM4580
you can also use 4560/ 4558/ 5532
The power supply should probe at 0 ohms with other similar chips at pins 4 and 8.
Refer to the schematic for common pin "A", e.g. right pin R28.

[JP-Stereo]

OK i figure i tap a probe on the A buss at one place and i check for continuity to all other ?
Same for a +15 and a -15 Vdc.

By the way, even out of circuit R28 act weird, from 1.2 to 1.9 kohm in 5 second, somewhat like a charging cap...
R29 have the same color and is testing exactly in the specs at 3.9k.
I've ordered a few, along with some RC4580.
To follow in a few days.

[x-pro]

OK i figure i tap a probe on the A buss at one place and i check for continuity to all other ?
Same for a +15 and a -15 Vdc.

By the way, even out of circuit R28 act weird, from 1.2 to 1.9 kohm in 5 second, somewhat like a charging cap...

On nutrition, the answer is yes.
For R28. Compare its readings with another of the same in other channels.
Measurement of part ratings should be carried out with the unit de-energized.
To check, you can unsolder the chip from another channel.

[JP-Stereo]

Hi again.

On the schematic R28 is the same as R29 and in reality also, same color stripes.
Measurment are taken off circuit, removed.

Must be carefull comparing because the other 3 channels are showing the same crazy behavior.
So R28 really look suspect, if it's the problem, all other should also, since they all have the same symptoms.