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NEED HELP with broken channel on dbx 386.

I am not experienced in this area, but there are no places within a 2.5 hour drive to take this thing to. If you think you can help, here's what I've already ruled out.

Channel 2 is working great. Analog or digital, the inputs are lighting up in the expected way. I have tested all other possibilities in the signal chain and it is definitely the channel. Nothing lights up at all when I plug a mic in, and obviously I'm getting no input either. I've looked inside and don't see any obvious problems like severed connections. I switched the tubes and this made no difference, so I know it's not the actual tube. Any thoughts on the next angle?


RemyRAD Tue, 06/05/2012 - 19:37

In that particular unit is a combination of both transistors and tubes. Both require completely different kinds of power supply feeds. Does the tube light up? If it lights up, it is getting the heater voltage supply. No lights would indicate a problem with the bipolar power supply fee to the dysfunctional channel. Since the LEDs are generally fed from the positive supply rail, a resistor feeding the dysfunctional channel from the power supply for that particular channel would be my first thought. Carbon film resistors can sometimes crack without looking burned. You'll need the schematic to trace the power supply output which will split off to 2 independent resistors one going to each channel. The negative side of the power supply may still be fine but without the positive side, you will not get any LEDs and you will certainly not get any audio. There is likely no individual voltage regulators powering each side. Without a schematic I can't possibly know that for certain. Some folks do send out a higher unregulated voltage from the power supply and do put individual voltage regulators to supply each channel but generally that is not the case. Especially on more budget oriented equipment.

If and when you locate that positive supply resistor, you shouldn't have to pull it out from the circuit board in order to test it. You certainly do not want to test it with the power on as this would damage the ohm meter. And you do want to let the voltage dissipate after disconnecting the power. Make sure you disconnect the power and not just turn the unit off.

Another issue could be that an IC chip has failed and shorted? Though this would generally then sink more current through the power supply resistor causing it to overheat and turn brown and is not black. A shorted IC even if the resistor was of a high enough value to not overheat and burn out, would still sink enough current to prevent the LED indicators from lighting at all. So if any IC chips are on sockets, you may want to carefully utilize an IC chip puller or carefully put a miniature screwdriver underneath the chip to help dislodge it from the socket. If you pull it out by hand, you risk bending or breaking the leads. Make sure you observe the polarity of the chip within the socket (provided it has a socket?) And without any extra spare IC chips, you could carefully swap them from the working channel to see if that corrects the problem.

With transistors, in order to test them, you would actually need to remove the leads from the circuit board when utilizing your ohm meter. Testing transistors with an ohm meter is fairly straightforward. You should see a high resistance and by reversing your test leads you should see a lower resistance. But unless the transistor is actually shorted, you should still have LEDs that work.

Now when removing a transistor from a circuit board, only a low wattage soldering pencil should be used. The lower the wattage, the better which translates to lesser heat. You can cook a transistor to death relatively quickly. Utilizing a solder wick (it's a braided piece of wire designed to suck up the solder and available at most electronics stores) or, you can utilize a solder sucker. I like the big blue ones with the yellow tip plunger known as a " SOLDAPULLIT " Which generally run around 15-$20. Though they are not completely necessary the solder will fill in the hole where the transistor is mounted if you don't use one. Then it becomes a little more of a challenge to get that transistor lead back into the hole of the PCB board. And this is known as component level troubleshooting an art by itself. Remember to heat the PC board solder pad and not the transistor directly. And try to do this as quickly as possible in order not to overheat the transistor. This can take some time to perfect but when done correctly, the benefits can be very gratifying.

Replacing an IC chip that is soldered directly to the circuit board without a socket maintains the same issue of heat damage. And they are a heck of a lot harder to un-solder. This is where that's solder wick or a solder sucker becomes a virtual necessity. Once most of the solder has been cleared from the solder pads on the PCB board, you'll still need a slender small screwdriver to pry it loose from the PC board. Do not force it. Let the IC chip cool and try to determine which lead ain't lettin' go. Then without the IC chip installed, power up the device again and see if you get LED indicators.

These are my best suggestions without having to go run all over town. Though a local trip to your nearest Radio Shaft may be necessary? They have everything you need to effect this repair. Except, of course, precisely identical replacement parts. But even many general-purpose parts can be substituted to get the unit functional again, albeit, it may have a different sound, may have more noise, may have less gain? They should also have some kinds of substitution books to indicate if they in fact have any compatible replacement parts. Some IC chips that are of the 8 pin dip style can have either one operational amplifier within them or two. This is a big factor. Just because it might look the same does not make it the same. Nor will it have the same pinouts, power supply connections, input and output connections. Unfortunately, I'm too lazy to go online and start searching for the schematic for your device. Your manual (hopefully) should have been included with your device. I will tell you one thing, Avid/Digi design does not include any schematics with any of their hardware and does not even make them available online or by mail. They gave me a bunch of BS about proprietary blah blah, which is just a load of BS and completely unprofessional as far as I'm concerned. That's why I hate their crap. You're left to strictly trace everything out yourself. And that requires a certain amount of understanding. I have that understanding. I've done that in the past. But virtually every piece of professional audio equipment through time has always included schematics for the technicians who need to fix them. So when no schematics are included or online, I don't consider the product professional in any stretch of the imagination. There ain't nothing proprietary in Avid's crap. Many people who have taken the time to trace out their circuitry modify them. And there's good reasons to do so with their crap. DBX usually provides schematics. Even though they have utilized certain devices that are proprietary, they at least give you the schematic and show the proprietary device but not its internal circuitry.

I'm burnt out on equipment maintenance
Mx. Remy Ann David

Boswell Wed, 06/06/2012 - 04:14

First of all, check [=""]this[/]="http://recording.or…"]this[/] thread.

You have to break the problem down as far as you can, and that probably involves some creative use of the insert jack. The block diagram on P19 of the [[url=http://="…"]DBX386 manual[/]="…"]DBX386 manual[/] shows that the signal chain has sections that you can test individually. I'm assuming you have the "mic/line" switch set correctly.

Start by using a line-level signal into the line or instrument input of the faulty channel with the switch set to "line". Do you get any signal on the analog output or any indication on the meters? If not, plug a TS jack lead (e.g. a guitar cable) into the insert jack and take the other end into something like a guitar amp. Does your signal appear there? Try a microphone into the 386 microphone input (switch set to "mic") with this combination.

If everything works as expected when taking the signal out of the insert send, you either have a faulty insert jack (the tip and ring not shorting correctly with no plug inserted), or the fault is in the output stage.

Let us know how these tests go and we can guide you through the next stage.

Boswell Thu, 06/07/2012 - 04:01

Post #23 in the thread I linked to has links to Bushy's tracings of the DBX386 circuit. The schematics are not 100% complete and there are a few errors, but it's an otherwise excellent effort. The Imageshack site where the schematics are hosted posts annoying ads over sections of the schematics, but if you click the magnifying icon and then dismiss the resulting ad page you get the full pictures.

Both the DBX block diagram in the manual and Bushy's schematics show that the LEDs and other indicators are a long way down the signal chain, so doing the tests I suggested using the insert jack as an output would yield useful diagnostic results for the earlier stages. To go into more detailed diagnosis may require an oscilloscope.

bentothesmith Thu, 06/07/2012 - 14:13

Wow. You guys are already above my head. I'm taking some engineering classes next semester, so maybe this will make sense then. I can assure you it's not the line/mic switch set in the wrong position, haha. But years ago I probably made mistakes like that. I just don't have much electrical knowledge outside of switching out tubes and checking for disconnected wires.

Boswell Fri, 06/08/2012 - 02:22

OK, so we won't ask you to get inside the box with a soldering iron. However, it's worth doing the test I mentioned with a TS jack lead connected to the insert jack. You could even plug the other end of the TS cable into the line input of the other (working) DBX386 channel and use the LEDs on that channel as indicators, so you don't need to go out and buy a 500W guitar amp.

RemyRAD Wed, 06/13/2012 - 11:46

He's not responding so he must have electrocuted himself on one of those 1/4 inch connectors? I think it's funny you didn't tell him that could happen when he had his guitar in his hand, the ground pin removed and plugged in upside down in the Edison outlet in his basement. We should have told him to wear some surgical gloves when attempting this feat.

My repairs are surgically sound
Mx. Remy Ann David

DrGonz Thu, 06/14/2012 - 08:51

Well if this thread ends in a similar way or whatever I am not fixing this unit... What value is the coupling resistor? >>> For those that only use surgical gloves while eating? Also, can you plz refer me a to a business that has the smallest type of soldering pencil equipment for this type type of work. I have a soldering station and the likes of ... But yet to find those really small units for tight soldering of small boards etc... :tongue:facepalm And don't think for a moment I am joking here... I read what I wrote and it turned out (accidently) to be kinda funny huh? But seriously I meant it in real way. lol. I have worked on old Carvin tube amps from the 70's where they had soldered some semi-large can capacitor to a PCB board. Not sure what type of tool they use to get in that tight of a space, but I could not do it w/ my small pencil soldering station. Then now I can't even find a can that is good enough and my B+ voltage is all whacked out w/ these diodes I added. Well, it's probably the caps nowadays are crap and the old ones are great. That is a fight in of itself but whatever...