Main Forums >> DIY, Electronics, Studio Design & Acoustics
        Print Thread

Pages: 1
Kendo Phil



Joined: 27/11/11
Posts: 70
Loc: South West
Behringer DI 4000 hummm
      #1013574 - 14/10/12 01:33 PM
I have just been given a Behringer DI 4000 unit which hums on every channel, the display shows 3 or 4 LEDs on the DB meter with no signal, boosting and cutting signals makes no difference. The hum occurs regardless of if anything is connected to the input. I thought it has to be a power supply problem.

As I was given it for free I decided to open it up and look for any clues. Opening the thing up I found all the boards are in tip top order and all wires connected. The earth is connected and good.

The set up appears to be that the main transformer has 4 no. 24 volt taps coming off it. The 4 DI's then have their own bridge rectifier and smoothing circuits and a DC power track runs to each DI along the main board. I measured the voltage at the the DI circuits and they run about 11.7 volts and steady. Each board appears to be electronically isolated from the next, so I cannot figure out why each channel is humming.

The only thing I did note was that the capacitors for the smoothing part of the power supplied all run very hot, even after only being on for a minute or so.

Could it be a bad batch of capacitors?

Any ideas would be greatly welcomed!

Note: Now that I have it apart and running on the bench all the lights are out but the hum still remains.

--------------------
Dazed and confused


Post Extras: Print Post   Remind Me!   Notify Moderator     Back to top
shufflebeat



Joined: 09/12/07
Posts: 3248
Loc: Manchester, UK
Re: Behringer DI 4000 hummm new [Re: Kendo Phil]
      #1013589 - 14/10/12 03:36 PM
There's at least one Behringer multi DI that can run on phantom power instead of the mains supply. I can't see any mention of this in easily accessible literature for the DI 4000 but might be worth a look if only to find out it doesn't help.

--------------------
Dear Mr God,
We called but you were out - B Dylan Deliveries (Intntl)


Post Extras: Print Post   Remind Me!   Notify Moderator     Back to top
ef37a



Joined: 29/05/06
Posts: 6790
Loc: northampton uk
Re: Behringer DI 4000 hummm new [Re: Kendo Phil]
      #1013602 - 14/10/12 04:35 PM
Hi Phil,
How do you know the transformer taps are 24volts? If this is so then the rectified supply should be much more than 11.7V. If it is a split, neg and pos supply I would still expect 13-14V per side? Are there any 708/709 regulators in there?

I think it is most likley a bad batch of caps, there was quite a few found their way mosly into MOBOs I believe. What is the value and voltage rating of the caps? Maplin have a fair range if you want quick. Do not by the way go for a larger cacitance than fitted in the hope of even lower hum, won't work. In a low drain "pre amp" type circuit such as that 1000mfds is more than big enough.

Dave.


Post Extras: Print Post   Remind Me!   Notify Moderator     Back to top
Hugh RobjohnsAdministrator
SOS Technical Editor


Joined: 25/07/03
Posts: 21819
Loc: Worcestershire
Re: Behringer DI 4000 hummm new [Re: Kendo Phil]
      #1013608 - 14/10/12 05:10 PM
24V AC feeds should, as Dave says, produce a rectified DC of a lot more than 11.7V! Are you sure it doesn't provide symmetrical +/-12VDC supplies into the active circuitry? That would be a lot more realistic -- although I'd actually expect it to be more like +/-15V, really. What regulators are in use?

It could be that one (or more) of the op-amps has been fried, and it's dumping one power rail straight into the ground line, injecting power-rail hum into everything else sharing the same ground rail.

That's what I'd be looking form, anyway. It could be a failed electrolytic too, of course. It would have a similar effect.

Are the IC's socketed? Can you op them out easily.

I don't know what your electronics skills are like, but if possible I'd suggest you need to start by isolating the four transformer windings and power up each DI circuit individually to see if it's just one that's failed -- which is quite possible. NAd then work through the failed one checking the rectifiers, smoothing capacitors, regulators and op-amps to find what's failed.

hugh

--------------------
Technical Editor, Sound On Sound


Post Extras: Print Post   Remind Me!   Notify Moderator     Back to top
Kendo Phil



Joined: 27/11/11
Posts: 70
Loc: South West
Re: Behringer DI 4000 hummm new [Re: ef37a]
      #1013609 - 14/10/12 05:15 PM
There are 4 pairs of wires from the transformer, yellow, green, orange and grey. They have an open circuit voltage of 24 VAC between pairs measured on my old faithful multimeter.

The coloured pairs each feed into a circuit containing a 220uf capacity rated at 50v then a 47uf capacity rated at 35 volts. Both of these are post bridge rectifier circuits. There is a transistor, MC78L24ACP, which I believe is a 24 volt regulator.

I have just spotted that on the edge of each DI circuit it says 24 volts. I rechecked the voltage reading and confirmed that it is indeed supplying 24 volts to each circuit. This time I checked the voltage where it comes into the circuit - must have picked an odd point when I measured 12 volts last time.

I think that I will order a bunch of replacement capacitors from Maplins and see if cures the problem.

--------------------
Dazed and confused


Post Extras: Print Post   Remind Me!   Notify Moderator     Back to top
Hugh RobjohnsAdministrator
SOS Technical Editor


Joined: 25/07/03
Posts: 21819
Loc: Worcestershire
Re: Behringer DI 4000 hummm new [Re: Kendo Phil]
      #1013611 - 14/10/12 05:23 PM
Quote Kendo Phil:

There are 4 pairs of wires from the transformer, yellow, green, orange and grey. They have an open circuit voltage of 24 VAC between pairs measured on my old faithful multimeter.




Okay. That's a good place to start...

Quote:

The coloured pairs each feed into a circuit containing a 220uf capacity rated at 50v then a 47uf capacity rated at 35 volts.




Okay. We're talking technical here, so we need to get the terms right so everyone knows what's what. They are 'capacitors'. The voltage ratings are appropriate given the AC supply.

Quote:

Both of these are post bridge rectifier circuits.




Yep, makes sense. I imagine there's an inductor, or more likely a high wattage resistor between the two of them to help with smoothing.

Quote:

There is a transistor, MC78L24ACP, which I believe is a 24 volt regulator.




Yes, it's a three-terminal, low current 24V DC regulator. I know it looks like a transistor, but it is a lot more than that (it actually contains 12 transistors!)

Quote:

I think that I will order a bunch of replacement capacitors from Maplins and see if cures the problem.




I wouldn't recommend that. It's pointless randomly changing components, and you stand more risk of damaging the circuit board or other components that way. Far better to work methodically and find the real fault...

hugh

--------------------
Technical Editor, Sound On Sound


Post Extras: Print Post   Remind Me!   Notify Moderator     Back to top
Kendo Phil



Joined: 27/11/11
Posts: 70
Loc: South West
Re: Behringer DI 4000 hummm new [Re: Kendo Phil]
      #1013621 - 14/10/12 06:30 PM
Thank you for the top replies.

I studied electronics at college back in 1995, so its slowly coming back to me. I have to admit all the pages of my notes have gone brown and curled up at the edges. We built power supplies as part of the course, so rattling around at the back of my brain somewhere is a memory of how they are put together.

I am confident that the transformer is supplying the correct voltage and each set of windings is intact. I am reasonably confident that the bridge rectifiers are all functioning correctly - though I currently have no oscilloscope to really get into the workings.

From the rectifiers the circuit feeds into the large capacitor, the power regulator then the smaller capacitor. There does not appear to be a resistor in sight till the circuits for the DI units, where it feeds the power into the DI.

I had intended in replacing the two capacitors at the power end. When you get to the DI circuitry everything is soldered to the board and I would rather not start de-soldering chips and bits here.

For the cost of 8 capacitors (2 capacitors * 4 DI power circuits) I think its worth the chance to see if it cures the problem, else its the bin or a really odd paperweight for the unit.

--------------------
Dazed and confused


Post Extras: Print Post   Remind Me!   Notify Moderator     Back to top
Hugh RobjohnsAdministrator
SOS Technical Editor


Joined: 25/07/03
Posts: 21819
Loc: Worcestershire
Re: Behringer DI 4000 hummm new [Re: Kendo Phil]
      #1013644 - 14/10/12 09:02 PM
Quote Kendo Phil:

From the rectifiers the circuit feeds into the large capacitor, the power regulator then the smaller capacitor.




Ah, yes, that makes more sense!

Quote:

For the cost of 8 capacitors (2 capacitors * 4 DI power circuits) I think its worth the chance to see if it cures the problem, else its the bin or a really odd paperweight for the unit.




Fair enough. Please let us know how it goes.

H

--------------------
Technical Editor, Sound On Sound


Post Extras: Print Post   Remind Me!   Notify Moderator     Back to top
Folderol



Joined: 15/11/08
Posts: 3651
Loc: Rochester, UK
Re: Behringer DI 4000 hummm new [Re: Kendo Phil]
      #1013648 - 14/10/12 09:50 PM
A couple of points:
You said you measured the open circuit AC voltages. Does this mean you unplugged the leads? - I don't know the kit so don't know whether or not they are on plugs.

Which capacitors are getting hot? All of them? Just the bigger ones? Just one?

A group of capacitors getting hot often means they are handling excessive ripple current, and the commonest cause of that is an O/C or S/C rectifier diode. If one of the diodes was S/C - and the transformer was relatively low powered - this would dramatically reduce one half/cycle to all the lines resulting in them all stressing the caps.

If the AC side is on plugs, try unplugging them all, then plugging them back in one at a time.

--------------------
It wasn't me!
(Well, actually, it probably was)


Post Extras: Print Post   Remind Me!   Notify Moderator     Back to top
Kendo Phil



Joined: 27/11/11
Posts: 70
Loc: South West
Re: Behringer DI 4000 hummm new [Re: Folderol]
      #1013672 - 15/10/12 05:40 AM
Quote Folderol:

A couple of points:
You said you measured the open circuit AC voltages. Does this mean you unplugged the leads? - I don't know the kit so don't know whether or not they are on plugs.

Which capacitors are getting hot? All of them? Just the bigger ones? Just one?

If the AC side is on plugs, try unplugging them all, then plugging them back in one at a time.




The wires are on a common strip connector so they can be unplugged but all together.

The smaller capacitors are the the ones getting hot, after the voltage regulator. They all seem hot, some more than others.

I have priced up all the replacement capacitors for the power supply - the princely sum of £3.83.

--------------------
Dazed and confused


Post Extras: Print Post   Remind Me!   Notify Moderator     Back to top
Kendo Phil



Joined: 27/11/11
Posts: 70
Loc: South West
Re: Behringer DI 4000 hummm new [Re: Kendo Phil]
      #1014914 - 22/10/12 05:31 PM
Capacitors ordered, should be here in the next few days.

Now where is my soldering iron and solder?

--------------------
Dazed and confused


Post Extras: Print Post   Remind Me!   Notify Moderator     Back to top
Kendo Phil



Joined: 27/11/11
Posts: 70
Loc: South West
Re: Behringer DI 4000 hummm new [Re: Kendo Phil]
      #1015846 - 27/10/12 06:00 PM
OK the story so far ........



I opened up the unit to find it consists of 4 completely separate circuits an a common board. The transformer provides 4 coloured feeds (Yellow, orange, grey and green). First order was to put an RCD on the power supply, then I unhooked the AC lines from the circuit board to confirm the voltages.



I measured the voltages between coloured pairs and then every other combination, varying voltages occurs between colour combinations with 28 VAC between all coloured pairs. I then set about tracing all the circuits through to try and find any faults.

I thought I would check the DC supply to the each circuit, this is when something strange happened. I noticed that if you even brush the body of the power circuit capacitors with the tip of an insulted probe the LEDs go nuts. In fact if you touch any of the capacitors in the power circuit the whole thing lights up like a Christmas tree. I think I might have found the guilty party.

Also the voltage seems very unstable on the boards, reading 24 VDC, then dropping to 11.7 VDC. The power capacitors are very hot even after a few seconds. I have a spare capacitor for every one in the power circuit so I am going to spend this evening with the soldering iron.

--------------------
Dazed and confused


Post Extras: Print Post   Remind Me!   Notify Moderator     Back to top
Kendo Phil



Joined: 27/11/11
Posts: 70
Loc: South West
Re: Behringer DI 4000 hummm new [Re: Kendo Phil]
      #1015857 - 27/10/12 07:59 PM
OK having replaced all the capacitors in the power circuitry the hum is much less pronounced and the LEDs no longer go nuts if you touch any of the capacitors.

There is still one channel that is very noisy, I will look for any loose components in this channel.

Is there anyway to test the capacitors that I have removed?

--------------------
Dazed and confused


Post Extras: Print Post   Remind Me!   Notify Moderator     Back to top
Folderol



Joined: 15/11/08
Posts: 3651
Loc: Rochester, UK
Re: Behringer DI 4000 hummm new [Re: Kendo Phil]
      #1015866 - 27/10/12 09:56 PM
Hum is reduced, or effectively removed? I'm still finding the whole situation rather weird - especially the business of making the LEDs go nuts. I'm wondering if the caps are almost open circuit and one or more of the voltage regulators are oscillating at some insane frequency. I've had that happen on older ones and it's the only thing I can think of that would give all those symptoms.

If the caps were getting hot there's not a lot of point keeping them, but the usual quickie is to use a test meter, switched to continuity/diode check. The length of time the beep lasts will depend on how quickly the caps charge. Open Circuit ones will give no beep. Short circuit or extremely leaky ones will give a continuous tone.

--------------------
It wasn't me!
(Well, actually, it probably was)


Post Extras: Print Post   Remind Me!   Notify Moderator     Back to top
Pages: 1

Rate this thread

Jump to

Extra Information
1 registered and 8 anonymous users are browsing this forum.

Moderator:  David Etheridge, James Perrett, zenguitar, Martin Walker, Forum Admin, Hugh Robjohns, Zukan, Frank Eleveld, SOS News Editor,  
Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled
Rating:
Thread views: 5018

October 2014
On sale now at main newsagents and bookstores (or buy direct from the
SOS Web Shop)
SOS current Print Magazine: click here for FULL Contents list
Click image for October 2014
DAW Tips from SOS

 

Home | Search | News | Current Issue | Tablet Mag | Articles | Forum | Subscribe | Shop | Readers Ads

Advertise | Information | Privacy Policy | Support | Login Help

 

Email: Contact SOS

Telephone: +44 (0)1954 789888

Fax: +44 (0)1954 789895

Registered Office: Media House, Trafalgar Way, Bar Hill, Cambridge, CB23 8SQ, United Kingdom.

Sound On Sound Ltd is registered in England and Wales.

Company number: 3015516 VAT number: GB 638 5307 26

         

All contents copyright © SOS Publications Group and/or its licensors, 1985-2014. All rights reserved.
The contents of this article are subject to worldwide copyright protection and reproduction in whole or part, whether mechanical or electronic, is expressly forbidden without the prior written consent of the Publishers. Great care has been taken to ensure accuracy in the preparation of this article but neither Sound On Sound Limited nor the publishers can be held responsible for its contents. The views expressed are those of the contributors and not necessarily those of the publishers.

Web site designed & maintained by PB Associates | SOS | Relative Media