hi guys, i'm trying to fix a Behringer Eurorack MX-802, a 4 mono + 2 stereo input mixer. I've been almost a month figuring out whats going on with no luck, i've repaired audio stuff before but never mixers, also i'm new to SMD components , the design does not help much at all, but after all i feel like i can get it working again.
The issue with this mixer is it does a very nasty crackling noise randomly on the #3 and #4 mono inputs, the noise volume varies from very subtle to suddenly very loud. Fortunatelly, some good samaritan on reddit gave me the schematic ,tracking out some minor problems and isolating which stage makes this noise. The circuit uses SMD RC4580 dual opamps for every amplifing stage. Initially one opamp and one diode was blown on line #4, i replaced them and it was working again but later line #3 and #4 started showing this noise and undefined behavior. The first input opamps are the source of this noise.
My main suspects is that there must be an invisible cold solder somewhere, input jack, power supply pins that feed the opamp, an invisible broken trace, a bad
smd capacitor or resistor, altrough i've removed them, measured and all seemed ok, the traces also were ok. I also checked the voltages on the V+ and V- pins, showed 15.6v and -15.6v respectly.
Also it could be an invisible short, but there are no carbon traces or smoke, and this possibility does not make sense because the rest of the mixer works OK, the leds could be flashing because of the shorts, the inputs #1 and #2 work perfectly despite the noise coming out from the other channels. Basically I discard checking the power supply module because the noise would come out from all channels if were because of this(?)
I don't know how sensitive are SMD opamps, maybe i shorted and damaged them without knowing, altrough i've working carefully with them, but i'm pretty sure this does not makes sense at all too because a short on the power lines would affect the entire board
Any help would be really apreciated, and honestly it's driving me crazy, i'll post the schematic and point out which stage the noise is coming out.
also a demo of the noise is on the zip file, maybe you can recognize it.
thanks in advance
The issue with this mixer is it does a very nasty crackling noise randomly on the #3 and #4 mono inputs, the noise volume varies from very subtle to suddenly very loud. Fortunatelly, some good samaritan on reddit gave me the schematic ,tracking out some minor problems and isolating which stage makes this noise. The circuit uses SMD RC4580 dual opamps for every amplifing stage. Initially one opamp and one diode was blown on line #4, i replaced them and it was working again but later line #3 and #4 started showing this noise and undefined behavior. The first input opamps are the source of this noise.
My main suspects is that there must be an invisible cold solder somewhere, input jack, power supply pins that feed the opamp, an invisible broken trace, a bad
smd capacitor or resistor, altrough i've removed them, measured and all seemed ok, the traces also were ok. I also checked the voltages on the V+ and V- pins, showed 15.6v and -15.6v respectly.
Also it could be an invisible short, but there are no carbon traces or smoke, and this possibility does not make sense because the rest of the mixer works OK, the leds could be flashing because of the shorts, the inputs #1 and #2 work perfectly despite the noise coming out from the other channels. Basically I discard checking the power supply module because the noise would come out from all channels if were because of this(?)
I don't know how sensitive are SMD opamps, maybe i shorted and damaged them without knowing, altrough i've working carefully with them, but i'm pretty sure this does not makes sense at all too because a short on the power lines would affect the entire board
Any help would be really apreciated, and honestly it's driving me crazy, i'll post the schematic and point out which stage the noise is coming out.
also a demo of the noise is on the zip file, maybe you can recognize it.
thanks in advance
CAps and resistors that are off value doesn't mean they are noisy..
You isolated the noise source to a particular IC? Great, then replace the IC.
Just for reference, I have been soldering electronic things for over 65 years, and so far I have not ruined an op amp from such activity. They are much more resilient than you might think.
You isolated the noise source to a particular IC? Great, then replace the IC.
Just for reference, I have been soldering electronic things for over 65 years, and so far I have not ruined an op amp from such activity. They are much more resilient than you might think.
Hi enzo thanks for the reply, the issue is i've replaced that particular ic many times
The noise does not come from the ic, i mean, it just amplifies it
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Noisy transistors can certainly happen. Not open or short does not necessarily imply quiet. How much could those 4 transistors possibly cost?
And what about the 48v phantom power? Could that be injecting noise? Normally, it would be common mode, but could be noisy enough that the CMRR of the input stage isn’t enough to take it out anymore. That input pair does not use a true current source in the tail.
There are only so many things it could possibly be, and none particularly hard to change out. shouldn’t be that hard to eliminate all the possibilities.
And what about the 48v phantom power? Could that be injecting noise? Normally, it would be common mode, but could be noisy enough that the CMRR of the input stage isn’t enough to take it out anymore. That input pair does not use a true current source in the tail.
There are only so many things it could possibly be, and none particularly hard to change out. shouldn’t be that hard to eliminate all the possibilities.
Aha, didn't realize it had the transistor front end. AS said above, surely one of those transistors can be noisy.
Is phantom power turned on when the problem occurs? I wouldn't call using 50 V rated input coupling caps with P48V particularly generous, you typically see 63 V parts there. Input protection would also appear to be inadequate; look up the aricles on the "Phantom Menace" (by one Neil Muncy, if memory serves).
When P48V is on, you may even run into issues with microphonic cables... I certainly have.
If the noise appears at the noninverting opamp input, it has to come from somewhere within the discrete input stage. Unfortunately the fault could be literally anything, from a bum solder joint, trace or capacitor to a bad transistor (note: 2SB737 have been discontinued for years and 100% equivalent transistors are not being made any more, although ZTX550 or 2N4403 should be able to stand in if you account for the different pinout).
When P48V is on, you may even run into issues with microphonic cables... I certainly have.
If the noise appears at the noninverting opamp input, it has to come from somewhere within the discrete input stage. Unfortunately the fault could be literally anything, from a bum solder joint, trace or capacitor to a bad transistor (note: 2SB737 have been discontinued for years and 100% equivalent transistors are not being made any more, although ZTX550 or 2N4403 should be able to stand in if you account for the different pinout).
the problem either happens if the phantom supply is on or off.
"phantom menace", thats a funny title hehe.
gotta replace the transistors on the input stages, seems to be general purpose ones so i'll replace them with good 2n3904's. If this doesn't fix the issue at least i discard the possibility of the transistors being the problem
Ok, found a dead transistor on the input quad, replaced the transistor and the thing started to oscillate, the transistors i used had similar gain values, 200-300 hFE, then i remove it to check again and noticed that without the front end transistors the noise was still there, i checked removing c176 capacitor (input of the opamp after the first stage) to check from the signal path to the input of the second stage.
That cap is after the noise source, i replaced it anyway because it was a bit burnt by the solder iron
it is part but not inside the loop, it just decouples the opamp output
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just now removed R104 and and R103, both 3,9K resistors and the noise stopped, gotta replace them now and see if the noise quits.
if it doesn't work, it could be a bad solder between these resistors and the IC or the IC just got fried, i hope are just the resistors.
ok, i replaced both resistors and removed diodes D20 and D21, the noise still around
Also checked pin 7 opamp output voltage and it was going crazy between 14v and 0v , maybe there is a short somewere, it even could be under the IC
Also checked pin 7 opamp output voltage and it was going crazy between 14v and 0v , maybe there is a short somewere, it even could be under the IC
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well it's starting to seem that the opamp at IC6 is bad....i'd still try C101 given that you said the noise stopped when you removed R104,103.
i dont have the full schematic in front of me but there must also be supply line bypasses close to the IC those could also be suspected.
i dont have the full schematic in front of me but there must also be supply line bypasses close to the IC those could also be suspected.
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i'm starting to think is the goddamn flux i use, i once got a bit of smoke between the pins i mentioned, power rail and pin 7 i've desoldering this IC so many times that flux must have turned out conductive, shorting out these pins and giving an undefined behaviour.