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dbx 386 schematics/technical knowledge

Discussion in 'Tech Talk - Electronic Repair Modifications DIY' started by bushy, Apr 19, 2010.

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    The New AT5047 Premier Studio Microphone Purity Transformed

  1. Boswell

    Boswell Moderator Well-Known Member

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    Yes, do the cap replacement first and then give the unit a good checkout and listening to. You could even record a few semi-reproducable sound sources for comparison with the unit after the amplifier mods you plan to do.

    Before soldering the new SOICs in place, clean the pads with the solder wick, then position the new chip on the pads. Double-check the orientation, and tack one of the power pins (4 or 8) in place using a small amount of solder. With that pin temporarily holding the chip, you can use your free hand to adjust the chip position with tweezers or a knife while heating the tacked pin only. When happy with the positioning, tack the other power pin. That done, test your dexterity by pressing firmly on the top of the chip with the tweezers while soldering both power pins properly in turn. You can now go round soldering the signal pins. Finally, use the solder wick to remove most of the solder from the top side of the pins, leaving only a little solder under the chip legs. If you don't press on the chip while soldering the corner pins, the chip floats too high on its pads and the solder does not wick correctly under the legs. The reason for connecting the power pins first is that they are the least sensitive to static electricity.
     
  2. bushy

    bushy Guest

    great. thanks heaps, this will be a great help. I'll order caps today :)
     
  3. bushy

    bushy Guest

    ok. so with dbx being too secretive about their schematics, I've drawn my own. Of the signal path at least. Low(er) res jpegs are linked at the bottom of this post, high res pdf's available on request. (you'd better be grateful though, I've spent bloody hours with a torch and a pencil and paper trying to follow traces underneath capacitors and surface mount op amps).

    As the schematics show, yes boswell, pins 1 and 6 of the 12AU7 are joined through a resistor each side. The signal splits two ways through resistors on the way into the tube, goes through both sides, and then rejoins on the other side. Not sure why :)

    I was surprised to find that most of the caps associated with the signal path are surface mount ones, which more or less rules out upgrading them. But can anyone have a glance at the schematics and tell me which caps are the more critical ones to replace for a better sound (assuming they're not surface mount)?

    Also. Coupling caps - I had assumed that the two caps next to each other on the board were simply two coupling caps in paralell (I've found this in other devices) but have found this not to be the case. C62 and C63 are the two I thought were the caps, but it would appear that only C62 is going to be the one causing the problem, am I right? Or should I replace/upgrade them both anyway?


    Thanks everyone :) right, I think I've earned myself a drink... (y)

    View attachment 5621
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  4. Boswell

    Boswell Moderator Well-Known Member

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    Well, Bushy, you've excelled yourself there!

    It's only C62 that could cause d.c. to get through to the output stages, so that and its mate in the other channel are the ones to target. The two halves of the 12AU7 are indeed used in parallel. I think you may find that R114-6 make up the anode resistor, and that the top end of C63 is actually the HT+ rail. It's not unusual to use several SM resistors in series or parallel where power dissipation needs to be shared, or sometimes to get a resistance that is not a preferred value.

    It amazes me to see all those 4580s walking across the board, and especially in the mic pre-amp section. With all this excitement about the schematics, I've lost the detail of whether you are using the unit with mic, instrument or line inputs, but I would change C62 and its mate, and then look at replacing the 4580s starting with U19. Don't change the NE5532A that makes up the electronic transformer in the output stage.
     
  5. bushy

    bushy Guest

    yeah I was quite surprised to see how much they're used! mind you, there's 6 4580's on each channel for a total of 12 op amps, so not sure what they're using the rest for yet...

    I'm using a keyboard into the front instrument input, although having established that it's different to the back line input, I'm wondering if I should be using a different input? One of my main reasons for picking this model was that it had front inputs and rear outputs, so I can plug straight into the front and loop out of the back to my rack mount DI, and never have to go poking around in the back of a rack case...

    so beyond replacing any electrolytics in the signal path, and upgrading the coupling caps, is there any benefit to be had upgrading the other 40+ electrolytic caps involved in the power supply? once again, they're all 2cent beauty's...
     
  6. bushy

    bushy Guest

    I've just created my list of parts to upgrade...before I buy it can somebody tell me whether it's worth upgrading the electrolytics in the power supply section? There's about 40 of them, and all cheap and nasty ones...i have to order most of the parts from the US so it's better if i can get it all at once :)
     
  7. Boswell

    Boswell Moderator Well-Known Member

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    If you have a feeling in your water that you ought to replace the electrolytics in the power supply, then go ahead, but you have not presented any evidence that there is a need to do so. Low-cost parts are not necessarily unreliable.
     
  8. bushy

    bushy Guest

    great. one more question, sorry :) as I was going to order my parts I found this, speaking of Dynamicaps.

    "The ELECTRONICS version is optimized for a high Z, low I and potentially high V environment…whereas the LOUDSPEAKER version is optimized for a very low Z, high I and low V application. They differ in the damping material used, (the Electronics (E version) uses smooth plastic RED tape, whereas the LoudSpeaker (L version) uses a special thick RED damping material, which is painted onto each cap by hand), utilize different dielectric materials, and are made with different plate/conductor materials."

    I'm going to use one of these to replace the anode coupling cap as it's in such an important spot (and dead anyway). I assumed that I'd be using the electronics version, but can someone confirm that for me? When it says that the loudspeaker is for low V applications that seemed to indicate that I'd need an electronics version, but the loudspeaker versions are rated at least 300V anyway, so I'm not sure why they say they're for low V applications. The electronics version is rated at 425V as opposed to the original cap's 250V, but that won't give me any grief surely? I thought the anode had 300V on it anyway, so I was surprised when the coupling cap was only a 250V one, maybe that has something to do with it failing? The rating is 0.33uF, for the record.

    Thanks!
     
  9. Boswell

    Boswell Moderator Well-Known Member

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    Umm, I don't buy the "lush, smooth, and warm" bit in the Dynamicap page, and I wouldn't pay that sort of money for an anode coupling capacitor. Unlike power supply reservoirs or speaker crossover components, low ESR (equivalent series resistance) is not an issue in that position. But again, if you feel you want to go for this type, then do so. You should get the electronics version.
     
  10. MrEase

    MrEase Active Member

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    Oh, I'm not so sure. I thought that RED damping (?) material gave you a really hot sound though whereas the PINK damping gives you the warmth. Of course avoid BLUE unless you want a really precise sound and GREEN is just downright clinical. I hear ORANGE is good for jazz but the one to avoid at all costs is GREY!

    :<)
     
  11. MrEase

    MrEase Active Member

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    More seriously I am completely befuddled by the description of a damping material. If there truly is any damping of the capacitor, this should just result in an increased tan(delta). Surely this could be equally and more accurately be achieved with a resistor...

    EDIT: Also surely the Z is determined by the capacitance and frequency, not the intended application! Maybe I'm just behind the latest technology which has apparently redefined a capacitor. Maybe "magic smoke" has been replaced with either paint or tape - just so long as it's RED.
     
  12. bushy

    bushy Guest

    what if i get one of each and make a rainbow capacitor? :rolleyes:

    whoops, back to reality we go :) i think, although don't quote me on it, the damping as actually referring to physical damping against vibration...I guess they figure that being mounted in a speaker there WILL be vibration, and maybe this vibration has audible effects? Or maybe it's for the longevity of the connecting leads etc? Don't know how much difference it would ACTUALLY make, but hey, you got to find something to talk about when you're trying to sell a capacitor :wink:

    anyway. i did a bit more research and thinking and i think you're right boswell. I'm just going to get $2 silver mica caps :) 0.33uF, 450V...the fact that the voltage rating is almost twice the original shouldn't hurt this application, right?
     
  13. MrEase

    MrEase Active Member

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    Ah, so you think this is to do with microphony! The only time I've seen anything worth noting with microphony is with surface mount ceramic cap's in an RF circuit on a vibrating board. That was caused by actual flexing of the capacitor. I very much doubt that painting a capacitor would do much to reduce any such or similar microphony. For use as a coupling capacitor in this circuit I can't believe you will notice any effect whatsoever. I don't think I'd bother with Silvered Mica either. These are normally reserved for very accurate capacitance values in RF circuits. There is nothing to be gained by spending more money than you need on an inappropriate capacitor.

    I really don't see that you'd have any problem with a simple plastic film capacitor in this position or even a ceramic! I think you are deliberating unnecessarily. If you really think it will make a difference to the sound, the best way to find out would be to buy several different types and try them out. To do this though I'd set up each channel differently (using one as a constant "reference") so you could do A-B comparisons without the time taken to swap out the cap - this time would "kill" your sonic memory which makes "slow" comparisons virtually useless.
     
  14. bushy

    bushy Guest

    hmmm, you might be right. I think I'll replace the plastic film in one side, put a silver mica in the other and do an AB test. I can't get the original coupling cap from the manufacturer without fulfilling (or paying for anyway) their minimum order, so I'll just try a basic green cap, and a silver mica, and see what sounds best :)

    thanks for the advice!
     
  15. Peter Cornell

    Peter Cornell Active Member

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    Hi all

    Does anyone have the 386 schems handy? Original links are down.

    Many thanks
    Peter
     
  16. Boswell

    Boswell Moderator Well-Known Member

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    It's a shame that the original links are no longer operational. I would suggest you send a message to Bushy, as he was the OP in this thread and the one who traced out the salient parts of the circuit. However, it looks as though he was never a full member of these forums, so we have no public contact information. Perhaps Chris (@audiokid) could advise.
     
  17. Peter Cornell

    Peter Cornell Active Member

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    Ok thanks for the tip, will try Bushy and @audiokid

    Peter
     
  18. audiokid

    audiokid Chris Staff

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    Hi Peter,

    There are no records, contact info of a Guest.
    If a link in a post no longer points to its original destination, our sever gets a security message to close it. Closing a link usually means the url has become dead or has been redirected to a url no longer accurate.
     
  19. Boswell

    Boswell Moderator Well-Known Member

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    I've repaired and serviced a fair number of DBX products here in Europe, often without the benefit of schematics. Was there a specific point about the 386 that you were chasing, or were you just interested in having a copy of the schematics?
     
  20. Peter Cornell

    Peter Cornell Active Member

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    Thanks so much for the offer of help!

    I have tracked the the fault to one of the output driver ICs, cant remember the the number right now, but one of the 8 pin chips close to the back panel.

    The output from the Insert jack tip is good, but the output from the XLR is low and very fuzzy. I managed to do a DC check between channel 1 and channel 2 pins 1, 2, 3, 5, 6, and 7 are all reading -8v or so whereas on the good channel they are all close to 0v.

    I measured the components around the opamp and their values are the same between the good and bad channels, so my guess is the output balancing chip is bad.

    I was also interested in the schems but it is not a big issue.

    Regards Peter
     
  • AT5047

    The New AT5047 Premier Studio Microphone Purity Transformed

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