Behringer 1280s no sound on left side

[Enzo]

Heck I don't even use special equipment here in my commercial shop. I use Chip Quik. There are other brands. It is a very low melting temperatuer solder you melt onto the part to remove. It mixes with the old solder, and stays liquid for long enough to melt all around the IC, then lift it off on the tip of an Xacto blade or something.

If you can't get on R274 itself, get on what it connects to. One end is wired to the control and pin 3 of IC28. The other end is wired to R259 and C112. Any of those accessible?

Good luck coming up with a board. Possible, but don;t hold your breath. Won't be cheap. IN fact, if I were looking for a mixer board for that model, I'd look for one of those units with a blown up power amp or power supply, buy it cheap and take the mixer panel out of THAT.

The master control? The wiper is grounded, and one end is wired to pin 3 of IC28. SO set your ohm meter between pin 3 of the IC and ground. Turn the control through its range. At one4 end it should essentially ground off pin 3, and at the othe end the 50k pot resistance should be there. From pin 3 to C134 should measure the resistance of the pot itself.

[Muzicman76]

does power need to be on? if so, dont i need to be carefull not to have the probe touch pins 3 & 4 since 4 is power? they are VERY small

[Enzo]

Yes, to look for signal, things have to be powered. Sorry. Gotta be careful, gotta be STEADY. And y'know what? It helps to keep the meter probe tips REALLY SHARP. You poke them at the solder and they dig in a little rather than slip aside.

Yes, they are tiny. I have a few magnifiers on my bench for just that. Plus my ancient eyes are less and less good at this.


Resistance readings are done with power off though. You have that much going for you.

[Muzicman76]

OK I checked for resistance at pin 3, when i set the knob all the way down I only get 15k. When U say "to ground" U mean to one of the lugs actually on the master volume pot right? I did try it at pin 5, which i believe is the input for the Right channel..? and got same reading?

is there a way I could take a practice amp, plug in a guitar cable, cut the + end off the other end and stick it on pin 3 and check for signal? or check somewhere else for signal?
I cant even find, R259 and C112, the writing is too small on the board. I barely found 274

[Enzo]

I admit the tiny surface mount stuff can be difficult to navigate around.

To ground means to circuit common to me. I usuall;y stick my probe on the sleeve part of a jack for ground. If you can determine which pins of the pot are the wiper, those are grounded. What I do in a case like that is find the wiper, then measure for continuity between that and some jack on the panel, and if I have continuity, then move my black probe to the jack. THEN I take readings to other points. You need to check because some jacks ground themselves by their mounting nut and others are grounded through circuit board traces. Never assume.

Can you cut off the end of a cord and plug it into a practice amp? Yes, but you really need to addd a series cap. IN post #10 I suggested googling "signal tracer". Really, do it. It describes exactly what you suggest. SOme guys make a whole thing, amp and all, but really, just a properly made probe p[lugged into an amp works fine. It is not remotely complicated, just a cap and a resistor, but by reading one of the little articles, you might get some ideas on a more convenient way to put it all together.

[Muzicman76]

OK, i really apprecitate your help. I think this whole process may be above my head. I dont have any caps on hand. by the time I go around town to buy some, come home and make the signal tracer...its all just a huge hassle.
but to sum up. if it IS an op amp its got to be IC28 cause thats the 1st amp in a line of functions that dont work correct?

[Enzo]

Not because it is first, but because it is after the last working point and before the first non-working point.


What is your practice amp? It may have a cap at its input already.