Hi.
A friend of mine has 2 dbx 286S. When turning on phantom power on this units, hum is present in one, hiss in the other unit. Any suggestions on how to fix it?
This apparently happened after they fixed a UPS in their studio...
A friend of mine has 2 dbx 286S. When turning on phantom power on this units, hum is present in one, hiss in the other unit. Any suggestions on how to fix it?
This apparently happened after they fixed a UPS in their studio...
I talked to my friend yesterday who finally gave me both damaged DBX.
The problem happened after they repaired a voltage regulator (the size of a refrigerator!), not a simple UPS. they use it for the whole studio consisting of 11 separate control / recording spaces. Each control room has one DBX286S preamps in it.
The info he gave me at first was wrong, as it happens many times. Talking about handing down defective equipment with partial info from one person to another to another and so on...
Supposedly one unit has a hum when activating phantom power and the other one should have a ground issue with phantom on. I did not notice the ground issue but there is a barely present hum on the 1st one.
What I did notice immediately is that as soon as the processors are activated, especially the compressors, noise level rises significantly (I don't know yet if this is normal) and there are erratic crackling noises coming out of both units.
Taking these DBX to a tech is not an option since these equipments were purchased in the US and we are located outside the US. Also, these DBXs were made in China...
I am the tech.
The problem happened after they repaired a voltage regulator (the size of a refrigerator!), not a simple UPS. they use it for the whole studio consisting of 11 separate control / recording spaces. Each control room has one DBX286S preamps in it.
The info he gave me at first was wrong, as it happens many times. Talking about handing down defective equipment with partial info from one person to another to another and so on...
Supposedly one unit has a hum when activating phantom power and the other one should have a ground issue with phantom on. I did not notice the ground issue but there is a barely present hum on the 1st one.
What I did notice immediately is that as soon as the processors are activated, especially the compressors, noise level rises significantly (I don't know yet if this is normal) and there are erratic crackling noises coming out of both units.
Taking these DBX to a tech is not an option since these equipments were purchased in the US and we are located outside the US. Also, these DBXs were made in China...
I am the tech.
The hum problem on the first unit leads me to believe that these use a traditional linear power supply, with either a dedicated winding or more likely a voltage doubler / tripler for phantom power generation. I would not be surprised if you find one or multiple bad electrolytics of questionable quality in there. Do you have any cap testing facilities, however primitive they may be? (For example, a setup that allows inserting the DUT between an amplifier and a speaker makes a half-decent ghetto ESR tester - with some exercise you can tell by ear whether a cap is bad by sound, at least from about 10 to maybe 1000 µF.)
It is also worth checking the two 6k8 P48V dropper resistors, these should measure basically identical.
The erratic noises could be just bad solder joints, or possibly damaged voltage regulators. If a loupe does not turn up anything suspicious, a scope may be helpful.
Next step definitely is cracking one open, poking around inside and getting a good idea of the power supply.
It is also worth checking the two 6k8 P48V dropper resistors, these should measure basically identical.
The erratic noises could be just bad solder joints, or possibly damaged voltage regulators. If a loupe does not turn up anything suspicious, a scope may be helpful.
Next step definitely is cracking one open, poking around inside and getting a good idea of the power supply.
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I Opened both units today. 1st impression: SMD architecture! I can deal with some components but the rest are a p-i-t-a. Especially the tiny (barely recognizable) ones.
Obviously both units are identical. The have a traditional transformer that converts into +/- 15VDC by 7815 and 7915 SMD regulators.
Up to now, I couldn't find a voltage tripler, but that doesn't mean its not there...
The following happens when nothing is connected to MIC or LINE input:
There are 3 electrolytic caps (2x 470uF 63V and a 1000uF 6.3V) that make a buzzing sound when I touch the top of it. They are firmly in place on the PCB. And they are the same caps in both units.
When activating the processors, there is an enhancer on the units with low freq and high freq controls. The LF control induces an awful hum that gets louder when turned clockwise reaching the 12 o'clock position and then disappears when getting to 5 o'clock.
Tomorrow I will continue checking...
BTW, wenn Du willst koennen wir uns auch auf Deutsch unterhalten...
Obviously both units are identical. The have a traditional transformer that converts into +/- 15VDC by 7815 and 7915 SMD regulators.
Up to now, I couldn't find a voltage tripler, but that doesn't mean its not there...
The following happens when nothing is connected to MIC or LINE input:
There are 3 electrolytic caps (2x 470uF 63V and a 1000uF 6.3V) that make a buzzing sound when I touch the top of it. They are firmly in place on the PCB. And they are the same caps in both units.
When activating the processors, there is an enhancer on the units with low freq and high freq controls. The LF control induces an awful hum that gets louder when turned clockwise reaching the 12 o'clock position and then disappears when getting to 5 o'clock.
Tomorrow I will continue checking...
BTW, wenn Du willst koennen wir uns auch auf Deutsch unterhalten...
Da wäre ich von allein nie drauf gekommen!
Since the DBX286S is just a preamp, setting it between a an amp and a speaker would certainly damage it...
I do have a "Parallax USB Oscilloscope (#28014)", but I don't know jet exactly how to use it. It has 20 Vpp max. input, and I don't know if this is enough.
(I have to get a proper ESR tester, soon...)
Since the DBX286S is just a preamp, setting it between a an amp and a speaker would certainly damage it...
I do have a "Parallax USB Oscilloscope (#28014)", but I don't know jet exactly how to use it. It has 20 Vpp max. input, and I don't know if this is enough.
(I have to get a proper ESR tester, soon...)
Hmm. But phantom power has to come from somewhere. Maybe a small DC/DC converter fed from + or -15 V then? (Look out for a chip with a surface mount inductor with ferrite core next to it.) But if so, why the hum - is said DC/DC dead and shorting its supply out, causing the voltage to go out of regulation? A quick check of supplies and actual P48V output (if any) ought to show that.
I'd guess the two 470µ/63V are the smoothing caps for the unregulated voltages that +/- 15 V are generated from. Not sure about the 1000µ, but if it's in the same vicinity, that might be for a digital +5V/+3.3 V supply or something.
The buzzing may be the result of the whole device floating, i.e. not being mains earth referenced. That kind of makes sense in a device partly using unbalanced connections. But doesn't the thing have an IEC power connector? Where's the PE connection going, if anywhere at all?
Sounds like there is some electrostatic coupling to the node the wiper pot is on. Source impedance of a pot tends to be highest at midpoint. That may be normal, especially with the case open.
Got something resembling a camera and some half-decent light at hand? The equivalent of a couple thousand words may provide some more insight, especially some sharp shots of both sides of the PCB.
Oh, da hätte ich jetzt nicht mit gerechnet. Das wird hier aber nicht so gern gesehen, weil dann u.U. kein anderer was versteht. Kann man ja auch nachvollziehen.
Alles klar. Also kein Deutsch...
Here are some pic I took from the upside of the mainboard as well as the underside. I cannot make out any kind of ferrite core inductor.
There is also a schematic for this DBX286S, and it seams like there are no 6.81 but 2.0 kohm matched resistors feeding +15V to pin 2 and pin 3 of the XLR mic input.
I had no time jet to take a really close look at the schematics, but will do so as soon as I return from my 2nd job. That is a mechanic for european vehicles...
Here are some pic I took from the upside of the mainboard as well as the underside. I cannot make out any kind of ferrite core inductor.
There is also a schematic for this DBX286S, and it seams like there are no 6.81 but 2.0 kohm matched resistors feeding +15V to pin 2 and pin 3 of the XLR mic input.
I had no time jet to take a really close look at the schematics, but will do so as soon as I return from my 2nd job. That is a mechanic for european vehicles...
Attachments
ftp://ftp.dbxpro.com/pub/pdfs/Manuals/English/dbx286sOwnersManual.pdf
Read the manual, hiss and hum issues according to processor settings are covered.
Dan.
Read the manual, hiss and hum issues according to processor settings are covered.
Dan.
Hi Dan.
The manual talks about hiss and hum, but not about crackling noises like pop-corning.
That is exactly the issue I have.
The manual talks about hiss and hum, but not about crackling noises like pop-corning.
That is exactly the issue I have.
There are three pcb to chassis earth points, make sure that they are 100%.
Also unpug/replug the ribbon cables, these can cause trouble and tell me what you get.
Problematic is that the 286A2 pcb does not correspond to the schematics available.
Dan.
Also unpug/replug the ribbon cables, these can cause trouble and tell me what you get.
Problematic is that the 286A2 pcb does not correspond to the schematics available.
Dan.
Hi Dan.
Thanks for the tip.
Some time ago I had a Mackie 32x8x2 8-bus console with similar issues for service. By the way, no one else would receive it and after opening it I understood why...
Tomorrow I'll check the ribbon cable on the DBX.
Thanks for the tip.
Some time ago I had a Mackie 32x8x2 8-bus console with similar issues for service. By the way, no one else would receive it and after opening it I understood why...
Tomorrow I'll check the ribbon cable on the DBX.
BTW, I was not aware that schematics and actual pcb does not match. Sorry for that little detail.
Yeah, the pcb component identifiers do not match.
Also I don't see the input series inductors, cap values are different too, I think.
Dan.
Also I don't see the input series inductors, cap values are different too, I think.
Dan.
On the first picture on the top left are the rectifiers, stabis and caps for the +/- voltage. On the right it looks a lot like a cascade to double or triple the voltage, probably followed by some kind of regulation. At least the high voltage rating of the caps (63V) support that theory, and nowhere else than in the phantom power supply such a voltage rating would make sense.
I would check the diodes and change all the caps there.
I think I fixed it.
As Dan pointed me to the ribbon connector, and I recalled doing the same job to the Mackie console, I removed the ribbon from both dbx, unplugged / replugged them a couple of times and the crackling disappeared.
Tomorrow I will check if the crackling really is gone for good.
BTW, thanks, Dan!
As Dan pointed me to the ribbon connector, and I recalled doing the same job to the Mackie console, I removed the ribbon from both dbx, unplugged / replugged them a couple of times and the crackling disappeared.
Tomorrow I will check if the crackling really is gone for good.
BTW, thanks, Dan!
Fingers crossed that the problem is sorted. 😎
It may be that the problems are nothing to do with the studio UPS failure, and got 'noticed' after.
This ribbon cable connector problem is pretty common, Behringer gear especially.
Dan.
I don't know about Behringer but this is dbx-Harman. ANyways, both brands manufacture in China. That doesn't necessarily mean that it is all poor quality stuff. Unfortunately, most of the times competitive prices come with cuts in quality...
I think the parts and both Chinese assembly factories are perfectly fine.
First up is that the connectors are fitted together ONE time only in production.
Usual practice is to protect connectors etc with tape before conformal lacquer coating of the pcb assembly.
It may be that a small residual of the tape adhesive is left behind and causes problems after the device is put in service and/or oxides/contaminants are not fully scraped through.
It's only a theory/hunch, but working these connectors usually effects permanent solution to contact issues like these.
Dan.
First up is that the connectors are fitted together ONE time only in production.
Usual practice is to protect connectors etc with tape before conformal lacquer coating of the pcb assembly.
It may be that a small residual of the tape adhesive is left behind and causes problems after the device is put in service and/or oxides/contaminants are not fully scraped through.
It's only a theory/hunch, but working these connectors usually effects permanent solution to contact issues like these.
Dan.
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