Behringer Xenyx1202FX mixer

[JP-Stereo]

Finally i change both R28 and R29 to no improvement.
The input stage discrete components has been completely checked and replaced.
I will try to get IC26 out by removing C230 to see.
Then i will replace both IC6 and IC26 Opamps.

[x-pro]

Why? What is your rationale for removing IC26?

[x-pro]

"If IC6B pins 5 and 6 (1202FX schematic) have a voltage of (-6.7V) at the chip inputs and about (0V) at pin 7"
This is an indication of the operating status of the device.

Tomorrow I will check these voltages again on another model of the 1204FX. They have the same input structure.

[JP-Stereo]

Since it's linked to the output of U6a, i was thinking that it can remotely be a source of the problem.
But for now U6b is the prime suspect i think.
The thing is that it work but not in a linear or normal log scale.

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.

Thanks for verifying.

[x-pro]

I measured resistors R28, R29 directly on the board and IC6(1202FX). Resistors are 1.38k and 3.92k respectively. I haven't figured it out yet. But if it's like this on my remote control, it's normal.

[x-pro]

The sequence of the pin voltages shown in the photo (model 1204FX):

1. IC6/7
2. IC6/6
3. IC6/5
4. IC6/2 = -8.48B
5. IC6/1 = -0.39V
6. IC6/3 = -8.48V
7. T18k = 0,61V
8. T14k = 0.61V
9. T18b = -7.87V
   10. T14b = -7.87V

[x-pro]

Why are the resistances different?
Between the lines (-15) and (A) are installed series resistors R10+R28.
And so in each channel from the corresponding position designations of resistors.
All these "twos" are connected in parallel. When measuring the resistor R28, it is necessary to take into account these parallel elements. From the calculations it turns out to be 1.49kOhm.

[x-pro]

Since there is such a dependency between channels, it is better to remove all ICs in the other 4 channels similar to IC6, as well as 4 transistors. And restore the inputs in series, installing new serviceable components.

[JP-Stereo]

Hi, my current reading are a lot different.

1. IC6/7...................11.879
2. IC6/6.................(-) 6.684
3. IC6/5.................(-) 6.684
4. IC6/2.....................0.0004
5. IC6/1.....................0.008
6. IC6/3.....................0.0
7. T18k...................No T18 on Mic 1
8. T14k...................No T14 on Mic 1
9. T18b = -7.87V
10. T14b = -7.87V

The circuit look different, the opamp is not wired the same way.
IC6b is the first opamp in the 1202fx but it look like it's IC 6a in 1204fx.

[x-pro]

I have indicated that my positional part designations are from the 1204FX model.
For 1202FX important values for IC6b and T11, T15

[JP-Stereo]

I finally change IC6 and loss the sound completely, no more "clipping".
I double check everything and locate this oddly mounted IC5:


I corrected the worst pins on the right and now i still have the clipping point around 4 o'clock but i have sound at lower level before the clipping point.
How can we compare the response of the preamplier ?

With the LEVEL and EQ at mid position, the FX at 0 and the PHONR/CTRL ROOM at mid position.
Moving the TRIM, i have no background noise until passed 4 o'clock where a white noise appear, corresponding to the clipping point with a mic.

I clip a probe on the right pin of the LEVEL pot.
Singing some "Hoolaa" (around 300 hz) at normal speaking level on a mic.
The LEVEL at mid position, the TRIM at minimum, i have a sine signal of around 50 mv speaking in the mic.
The TRIM at mid position give a signal of 400 mv and the TRIM just before clipping give just over 1 volt.
On more degree of rotation and the signal jump at 10 volts !!!
I will be very interested to know how this compare to your 1204fx gain ???

At least now i have a good signal before the clipping occur...

[x-pro]

This IC5 represents the effects circuitry. It worked somehow.
As long as there is no normal voltage on the pins of IC6b and other components, the console will not operate properly.
Please specify which parts have been replaced.
Did you install IC6 correctly?

[JP-Stereo]

I replace T1, T11, T7, T15, D12, D13, R28, R29, IC6.
About all other components around those were tested too.

For IC6 the IC has no standard DOT to id Pin 1, it have a Line like surface mount capacitors.


It's not exactly the same package but it fit.
I figures that Pin 1 is the bottom left one...

OK i will remove IC7, IC8 and IC9.
What about IC13, IC14, IC22 and IC23 ?

I notice that the units we are comparing are not working on the same supply potential.
So some component doesn't have the same value, how can this affect the measurment we are comparing ?
Do you think that at the IC pins the signals should be alike anyway ?

[x-pro]

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.

Hi, my current reading are a lot different.[LIST=1][*]IC6/7...................11.879[*]IC6/6.................(-) 6.684[*]IC6/5.................(-) 6.684

how so?

[x-pro]

What about IC13, IC14, IC22 and IC23 ?

Do you think that at the IC pins the signals should be alike anyway ?

IC13, IC14, IC22 and IC23 - you don't have to delete
The voltage on IC6b/7 should be close to 0.
Check the power on this chip, directly on pins 8 and 4 relative to the midpoint and on the top pins R12, R13(+15 in the schematic).
Also, is the supply from the transformer exactly at the midpoint?
It should be as in post 17, the voltages may be slightly different but relative to the middle pin should be symmetrical.