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dbx 386 schematics/technical knowledge

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

  1. bushy

    bushy Guest

    Hi all,
    my dbx 386 has started giving me some horrendous noises on one channel. I've tried switching the tubes between channel 1 and 2, and the same channel still has the problem, so obviously my tubes are fine. Does anyone know where I might get a hold of some schematics so I can track down the problem?

    Alternatively, can anyone suggest where to start looking?

    The output meter shows signal (not constant, but varying) with nothing plugged into it. It varies with the input/output controls, but is not affected by any of the other controls (phase, phantom, pad, dig/analog, line/mic). When you plug the output into a PA you get a wonderful static type noise.:confused:
  2. Boswell

    Boswell Moderator Distinguished Member

    Could be any number of things: dry solder joint, leaky decoupling cap, noisy resistor.

    How experienced are you at faultfinding?
  3. bushy

    bushy Guest

    im an industrial electronics technician so ive got a good mind for fault finding, got a little bit of experience with audio gear and that sort of electronics, but not a heap. It happened all of a sudden with no prior warning, it went from nothing to really bad, so I'm inclined to lean away from dry joints etc toward something like a blown cap or something that's failed, rather than just a manufacture defect. Can't see any obviously faulty caps/resistors or signs of overheating though
  4. Boswell

    Boswell Moderator Distinguished Member

    ** Don't do this if you are unhappy about working with high voltages. **

    You need to get access to the component side of the PCB while the unit is powered up. Go round with a plastic or wooden rod (NOT a pencil!) giving each component a tap and a firm push while listening to the output. If it's a bad joint or noisy resistor, physical movement usually changes the character of the noise as you hit the sensitive part. This technique will not identify leaky caps, so further work would be needed there.
  5. bushy

    bushy Guest

    thanks, ill start with that. and don't worry, in my trade it's not considered high voltage until its over 1000V :)

    any luck with schematics??
  6. bushy

    bushy Guest

    :rolleyes:unfortunately nothing conclusive from that approach. Some more info:

    Varying the input pot does nothing, so the problem's probably in the output path somewhere. Increasing the analog output pot increases the static noise, up until about 3 oclock, when the static suddenly disappears. from there up there's no static. On the digital side, the same effect is had by varying the digital output pot, except it doesn't completely cut the static at high levels, just reduces the volume of static. So the problem must be before the "branch" to the digital section.
    If I plug a keyboard into it, it still appears to function as normal - varying the input/output varies the keyboard, so the signal is still getting right through from the input to the output, just somewhere we're getting a lot of added static.
    I don't have a scope at the moment to properly look at the static, but a DC voltmeter measuring the analog output measures a DC voltage of 0V to about 10V as you vary the output pot from 0-100%. It doesn't pick up an AC voltage, and my meter's supposedly good up to 20kHz (it's a $1000 fluke too, not a $100 radio shack job), but again it's not a scope. The good channel obviously has 0.000V-0.002V across the analog output as you vary controls. Again, varying the input pot, while it still varies any input signal you're actually giving the amp, doesn't affect the static, nor the measured DC voltage, no matter what you do with it.

    oh, for some schematics :rolleyes:
  7. Boswell

    Boswell Moderator Distinguished Member

    Great work! I think you may have identified a leaky coupling cap from the anode (plate) of the 12AU7. DC applied to pots can cause static of the sort you describe.

    The next check is to plug an insert cable cable into the insert jack on that channel and check two things: (1) that the d.c. is present on the send (tip) line and (b) that injecting a separate signal into the return (ring) line results in that signal appearing at both the analog and digital outputs without added noise over the range of their respective output level controls.

    If those tests show the d.c is present on the send and that the return working normally, I think you are into identifying the anode coupling capacitor on that channel and replacing it. It is likely to be something like a 0.1uF 300V component.

    It is possible that having d.c. on the level controls has made them permanently noisy when being rotated, but they should not generate noise at any one position.
  8. bushy

    bushy Guest

    Aha, thanks heaps! I'll try this and see what results I get. The pots are noisy even when stationary though, the pots on this are not a smooth pot but rather ones with little indentations all the way round so there are about 30-40 positions they can be in. Don't know if that makes any difference. I measured the output pot from centre "leg" to outside and it ranged from 0 ohms to around 20k, although interestingly after about 3 o'clock it didn't go any higher, and in fact dropped back a little to around 18k. The one on the good channel did the same though so it must be meant to do that :)
  9. bushy

    bushy Guest

    Yep, that's exactly what I found. I've removed the main board and found what I believe to be the coupling cap, it's a grey box shaped cap with .1J 400 written on the top, which I take to be .1uF, 400V, +/-5%. There's another red capacitor next to it, (and I think more or less in paralell with it) similar size but more rounded box shape, marked .33K 250, which I take to mean .33uF, 250V, +/-10%. If someone could confirm this for me that'd be sweet.
    While in the board both ch 1 and ch 2 measures about the same, but that's not a reliable test, so I'll take them out and see what I find. Besides, it takes a good 10-15 seconds from power up for the static to appear, so I probably won't be able to find a fault on the bench at any rate.

    It's going to be a bastard to get out too.
  10. Boswell

    Boswell Moderator Distinguished Member

    Try tracing the capacitors that connect to pins 1 and 6 of the 12AU7 base. They will be the coupling caps for the two channels, but you have no way of knowing which belongs to which channel. The other capacitors you describe are probably for power supply decoupling, and as yet you have no evidence that they are giving a problem.

    You may be able to see the other end of the coupling caps going to the channel 1 and 2 insert jacks, which would identify them. However, having got this far, there is a case for replacing the caps on both channels, as one has been seen to go leaky.
  11. bushy

    bushy Guest

    Both channels have their own 12AU7 on the 386. Pins 1 and 6 are joined together and go to a surface mount resistor bank, then through these two capacitors. I swapped the two with the identical ones from channel 2 and hey presto, static on channel 2! It's a bit hard to trace things out further than this, there's a lot of surface mount and very fine pcb work going on.
    I've emailed the australian distributor and they've told me they're MTPT film RF TA capacitors (can anyone enlighten me as to what that means?). I've got a quote for 4 new caps, as you suggested boswell, I'm going to do both channels.

    However...I'm thinking about doing a few mods, apparently the design of the 386 is great but let down in the execution by using cheap caps all round, basic op amps etc...so I'm thinking about buying a heap of elna audio caps and some burr brown op amps and giving the unit an overhaul. With that in mind, there might be some better options for coupling caps as well.

    Does anyone know what sort of caps (i.e. film, electrolytic, tantalum, ceramic...) are best suited to use as a coupling cap with a 12AU7? Can anyone suggest some that might be ideal? I need a .1uF and a .33uF, rated aroundabout the 300V mark...
  12. Boswell

    Boswell Moderator Distinguished Member

    Looks like you have nailed the problem - well done! I was surprised to hear that pins 1 and 6 of the 12AU7 were joined together - are the two halves of the double triode used in parallel in this design?

    MTPT is not a standard abbreviation, but in this case it probably is Metallized Thin Polyester Film, i.e. pretty much the standard construction for medium-voltage capacitors. If you want to go ahead and replace them with Elna audio caps, fine, but I would steer clear of the aluminium types for the high-voltage area. You must NOT use electrolytic or tantalum types for the anode coupling position. Tantalum is only a low-voltage dielectric, and is fine for op-amp decoupling, but don't use them in the signal path or in the high-voltage section.

    As for replacing the op-amps, you have to be a bit careful. Make a list of the make and type numbers of the op-amps that you see fitted in the original design, and we can comment on the possibility of replacement with other types.
  13. bushy

    bushy Guest

    thanks a heap for your help on this one boswell :) I'm not 100% sure on the pins 1 and 6 being joined, it's pretty fine tracework. I'll have a good look next time I'm at it and report further :)

    as for op amps, I have 1 5532A on each channel - this appears to be what the inputs run to first - and then a heap of JRC 4580's scattered all over the board. There's also one LF353 per channel, not sure what role it plays. From some preliminary research I'm looking at Burr Brown OPA2604's to replace the 5532A, and OPA2132's or OPA2134's for the 4580's. Although I'd still love a schematic to see which op amps are actually in the signal path. They'll all surface mount, so they'll be hard to replace...never worked with surface mount components before :)

    when you say "steer clear of the aluminium types for the high-voltage area", do you mean don't use them for the coupling caps? or don't use them in the power supply section?

    Oh, and what sort of caps would you recommend for coupling? Buying the original caps is looking like being a bit pricey as I have to fulfil their minimum order, or pay their minimum order anyway...and I can't find the exact caps on farnell or RS...if you can hit me with a link that'd be magic

    and one more thought...is it worth replacing diodes? there are a lot of them on the board, once again, not sure what role they play, but would i be likely to get significant benefit from upgrading diodes?

  14. bushy

    bushy Guest

    e.g....how good would these be in the anode coupling position? pricey, but from what I can find on the existing caps, they seem to be 15 cent jobs...and im not sure I want a 15 cent cap as a critical part of my signal chain :)
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  15. SammyD

    SammyD Guest

    cool thread guys. That sounds like a fun little mod to do. I actually have this pre and I really love its lay out and its tone is pretty good but it could use some cleaning up. I appreciate the leg work you've done already, it'll make my little mod a lot less of a head ache. Did anyone ever come across any Schematics for this unit?
  16. bushy

    bushy Guest

    unfortunately no, although believe me, im trying really hard. again, if we had a schematic, we'd be able to do one channel at a time and an AB test to see how much benefit we were actually getting. Tough to see where everything goes without one. If I do manage to con somebody into giving me a schematic I'll PM you :)

    if anyone out there is sitting on one, please feel free to PM me any time! id be happy with just the schematic on the analog side of the unit, and maybe power supply...i suspect the reason dbx hold the schematics for this unit so tightly is the top secret type IV digital conversion, and I'm happy to be without that...
  17. Boswell

    Boswell Moderator Distinguished Member

    If you fancy tackling the op-amp replacement job, I would start with the 4580s. My normal recommendation as a replacement is the BB (TI) OPA2134, but I would usually want to look at the schematics first. In this case, we don't have them, but it's not too much of a risk to do the replacement. What type of package are the 4580s in?

    I would not have thought that it would be worth replacing the NE5532s or the LF353 without further information.

    For anode coupling capacitors, I use the standard 400V poly film types. There are plenty to choose from, but you will want to get as similar a package type to the original as possible.

    Don't start replacing diodes unless you have direct evidence that you have a faulty one.
  18. bushy

    bushy Guest

    The only information I've been able to find on these op amps is:
    LF353M: Dual wide bandwidth JFET input opamp, SO8 (i presume S08 is the package type?)
    5532MD: Dual bipolar opamp, SO8-200
    NJM4580: Dual opamp, SO8

    For the coupling caps, when you say to get as similar a package type as possible, is this just so they mount and fit in the board properly, or are there other considerations? I'm struggling to find decent quality caps in the same package type. These are the closest I've found to the suspect 2 caps, and they're still only 50c caps...
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  19. Boswell

    Boswell Moderator Distinguished Member

    SO8 is a surface-mount Small Outline 8-pin (SOIC). The full part number for the BB part is therefore OPA2134UA. The SOIC-8 package is relatively straightforward to remove using a fine iron and solder wick. The technique is to wick all the solder off all the pads, and then under very short heat applied to the pin, "click" each pin off its pad with a scalpel blade using the adjacent pin as a pry point. Take care, as releasing the last pin causes the chip to come free.

    Yes, the reason for choosing a similar package for the coupling caps is purely physical ease of making the replacement. Either or the type you linked to would be OK, but don't stray from the 0.1uF value of the original. The Vishay part may be a little easier to coerce into fitting the pad separation of the original.
  20. bushy

    bushy Guest

    great, thanks, I'll make a start on this tomorrow and report any findings as I go :) is there any useful techniques for installing the new op amps once I've got the old ones out? other than patience and a steady hand of course.

    I'll probably start by ordering some new coupling caps and just making the thing go again with no static, to give myself a platform to work from. I'll also try to trace out some of the signal path to give us all an idea of what I'm actually playing with. If it turns out that I get a really good result by the time I'm finished I'll upload some photos or something so that hopefully somebody else can learn from my experience/mistakes :)

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