Behringer 1280s no sound on left side

[Muzicman76]

Hi, (first time posting )
I have a Behringer 1280s powered mixer. Last night the left side output stopped working at the line level output and the powered output. If I switch to Mono-Monitor both powered outputs work.
just seeing if anyone has experience with these or has any suggestions.
I have the schematic but havnt been able to find a exploded view diagram to disassemble (if I need to).
any help is appreciated.

Thanx in advance

[Enzo]

There are schematics, and not much else.

So your amps work, the mixer is not feeding one output. Clean the amp assign slide switch, look for open on the master fader, explore the op amps that seem involved with the master section and see if any have DC on their output pins. Places to start.

[Muzicman76]

Thanx for the reply
The switch does work cause it switches between L/R & mono/monitor mode. when im in mono mode if i pan left the signal goes away.

any ideas on how to get to the circuit board? do I need to take all the knobs and nuts off the front panel? I saw there were 3 fuses on the rear circuit board, do you know if there are any other fuses? ( I scanned the schematic but I didnt see any)

[Enzo]

I don't know how many fuses are in it, but they wouldn;t affect individual channels on the mixer.

Welcome to the world of fixing mixers. yes, to get at the board, all those knobs and things have to come off. I always make a chart so I know which color goes where when I reasemble it.

[Muzicman76]

Gotcha, Ya i have consulted another amp guru and everything is pointing to a failed Op-amp. Hopefully the part is readily availiable.

[Enzo]

The op amps are just generics, duals are usuall 4580, and quads are still TL074 almost always.

Hmmm, there are no slide faders in that model are there? In the desk mixers, one common failure is the sliders themselves, but rotaries fail FAR less often. SO I;d agree, an op amp seems likely.

[Muzicman76]

(schematic attached)
OK I got the circuit board out (wow, it was in there, ) I can kinda follow the schematic but Im having trouble pinpointing where to start looking for trouble. I see where the main L/R comes out on page 7, X15 & Xbehringer 1280s scheumatic.pdf there is no vu meter reading for the left side so the signal dies before it gets there, Does the signal go through the VU and then to the outputs? or is it a parallel run in two directions? if anyone could tell me which Op amps are in the chain from the times it leaves the channel Left send. (again, there is no signal for any channel on the left side so its the after the master section. )
also, how to I test the Op-amp? they are 4580's.

thanx in advance

[Enzo]

Page 7 IC9, IC18 run the graphic EQ, then feed main L and R through R274, 275. From there, they go to the two halves of R86, the MAIN volume control. MAKE SURE that control is OK. Is there signal at the left end of R274,275? You can't measure signal at the input of the op amps. From the control, L and R are buffered by IC28. The two outputs of IC28 also go up to IC19 to make the mono signal. If you have a missing side in the mono mix, then we assume the trouble is at IC28 or earlier.


C148,149 come off the outputs of IC28. They serve the MAIN OUT jacks as well as making the MAIN_L and MAIN_R signals, which do go to the VUs, but also to the power amp switch on page 6, where it gets processed and sent to the mix out connectors for feed to the power amps

On the way into the graaphic, IC40 does the job., taking the main L/R bus and buffering it. It feeds the TAPE OUT jacks. Is there signal on both TAPE OUT jacks or is the left one silent there? If IC40 doesn;t feed anything to those jacks, it has nothing to feed teh graphic either.

[Muzicman76]

-So how do I test for signal ? Do I just use a Volt meter w/ a continuity tone?
-I never got a chance to test the "tape Out" jack. Nor did I check to see if there was signal going to the graphics (it has the FBQ feature so U can see how hot the frequencies are as they are coming in.) I do have a missing side when running in "mono/Monitor "mode using powered outs. as i panned left, the signal went down and eventually out. So lets say for arguement that I have no signal at the EQ, that means the trouble is at least from before that back to where the signals sum out of the channels and start to become L/R, correct? so where does that put me? do I just replace all the Op amps from the point of the channel sum to right before the EQ's? or am I looking for resistors that dont read resistance as the culprits?

[Enzo]

Can you not go now and check for signal at the tape jacks?

How do you test for signal? You put some signal to an input just as you would in general use, then you see if it is present at whatever point is of interest. Might be at a jack, might be at an op amp output pin, might be at a resistor, whatever.

A scope is the best way, in my book, but a signal tracer handles it easily too Google "signal tracer" and get a bunch of takes on the idea. And amp can be used for one, you have another amp of some sort? The idea is to make a probe to connect to the input of the other amp. Have a cap in series to block any DC. You then can listen to what is at any point in the circuit. And the crudest method is to set your meter to AC volts and follow the signal with it.

The input channels are mono until you get to the pan pot. That splits the signal onto the L and R main busses. From that point on there are two duplicate circuits through the mixer. I recdommend working backwards from the output end, and problems are a lot more likely there than at the L and R busses. But anything is possible. Any point in the circuit that is missing the L signal is after the problem. So I don't care where you pick, EQ, whatever, wherever it is, if signal is missing, look farther back. But start where I suggested, the master volume control, those ICs, etc. And the whole point of looking at the TAPE OUT jacks was they are right there on the outside, you can simply connect them to an amp and find out. And that tells you the problem is either before or after that point before you even open it up.


We are trying to do one thing... isolate the problem. Until we know where it is, there is no point planning what parts to swap. What if it isn;t a bad part at all? Maybe it is a broken solder connection. We find out if signal is at each point. If it is on both ends of a resistor in the R channel but only on one end of the corresponding resistor in the L chanbnel, well then we suspect an open resistor, OR something on the silent end of the resistor shorting the signal path to ground. And if it is an op amp, then I want to find out which one and change it, not wholesale change all the op amps in blind hope it turns out to be what was wrong.

If you have ten op amps in a row, one going to the nexst, with a signal into the first one. If no signal comes out the other end, I don;t replace all ten chips, I look at the output of the first one, then the second one, then the third one, and so forth, until I find the point at whwich the signal disappears, then I replace THAT op amp.

[Muzicman76]

Actually i did just go test it, (had to reassembe it a bit but was able to do it minimally) Plugged my Ipod into "tape in"

Heres what I found: there IS signal to the EQ; there IS signal at the Left "tape out", the monitor send is outputing both L/R signals (original problem)-> there is NO VU signal on Master Left, and NO signal on Left "main out" line out jack NO signal when panned to left in mono/monitor mode using power amp outs.

So going by your post about signal flow, U said "C148,149 come off the outputs of IC28. They serve the MAIN OUT jacks as well as making the MAIN_L and MAIN_R signals, which do go to the VUs, but also to the power amp switch on page 6, where it gets processed and sent to the mix out connectors for feed to the power amps

Are you saying that after it makes the L/R signal, it goes in 2 different directions? to the VU AND to the switch separatley ? or, passing through the VU moving on to the switch ? The power switch is working properly in terms of swithcing from stereo to Mono/Monitor. So I dont think thats it...

it wouldnt be the main volume control cause that feeds "tape out" and thats working.

Idea's?

[Enzo]

Any circuit has a signal flow. Signal goes in one end, moves stage to stage through the thing, then comes out the output. So your signal makes it to the L and R on the tape out. We know that because you can find it there. So the signal path is ok at least up to that point.

Look at your schematic. IC40 drives the TAPE OUT jacks. SO we know we are good up to there. Then on into IC9,18 for the GEQ. Then through R274,275 and on into the master volume and IC28. The outputs of IC28 go to IC19 to make a mono mix, AND they go on through caps 148,149 and to the MAIN OUT jacks. Note the little MAIN_L branch sticking up just right of the cap? That is the branch to the VU and to the power amp selector switch. ANy place on the drawing you see MAIN_L is connected to the same place. Nothing goes "through" anything. It is like a Y-cord. THE VU is not moving because nothing is getting there.

So you have it at the TAPE OUT, do you also have it at the MAIN OUT jacks? If so, it is getting through C148,149. If not then it is not making it through the master/IC28/C148


I didn;t suggest the power amp switch was involved, it is just an easily found point on the schematic that was a destination for the signals MAIN_L. I believe you when you put it on mono, and the pan drops out your signal. The power amp switch could not do that to it.

The main volume control does NOT feed the TAPE OUT jacks. Not on your schematic anyway. IC40 drives the TAPE OUTs, as well as sending the signal on to the GEQ through IC9,18. It is only AFTER IC9,18 that the signal reaches the master volume control at the input to IC28. The tape outs are before the master volume and even before the graphic.

SO until I know we have signal at the output pin of IC9,18 or at resistors R274,275, I don;t know really that both sides of the GEQ are working.

[Muzicman76]

OK.....
"So you have it at the TAPE OUT, do you also have it at the MAIN OUT jacks? If so, it is getting through C148,149. If not then it is not making it through the master/IC28/C148"
I do NOT have it at the main out L jack. So if there no sound at the L output, Nothing on the VU and nothing on the power amp, then IC28 would be the most likey culprit, agreed?

the problem is i have no way to test other than a volt meter.

[Enzo]

IC28, the master pot, and the cap are ALL suspects.

You have a meter, and you have a good channel. Your meter can certainly check the master volume control for opens. COmpare readings between channels. If they are more or less the same, then that is probably good. If they differ susbstantially, that points to the problem.

And don't forget to check R274,275 for open while you are at it.

[Muzicman76]

How exactly do I check for opens?
the problem w/ R 274, 275 is they are 1)TINY and 2) buried under the volume pot.
also, I dont have the tool needed to remove the op amp properly. and at 100$ its not worth purchasingjust for one use, (i dont fix boards enough to make it worth while)


The more I look at this it may be easier to see if I can get another circuit board. I have a local Behringer repair shop checking to see if they can get one. But they said they have 5 units under warranty that have been waiting for 4 months for parts. (thats the issue w/ behringer. )