EDIT: DDA DMR12 maintenance/recap thread.

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thecr4ne

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EDIT: I'm renaming the thread to reflect my hijacking of it to keep documenting my progress on the DMR12. Originally began trying to diagnose phantom power issues.

Original post:

I just finished my first lap around my newly acquired DDA DMR12 console, all channels pass signal but 19 of the 48 inputs are having a common issue with Phantom power (the rest seem to be functional). I tested first with a dynamic then with an SDC and the symptoms go like this: Engage Phantom push-button, peak and signlal LEDS lights light up, then go out (pop if gain and fader up), no signal passes, disengage phantom push-button, signal rises and fades, like it's really only working momentarily as the caps drain or something.

Got out the multi-meter to measure voltages. PSU puts out a steady +50VDC (and +/- 18VDC). Good channels measure +48~+50VDC on XLR Pins 2 & 3. Bad channels measure about +30~+32VDC on one or both of XLR pins 2 & 3. Dug up the schematic (down below) and measured a bit more

With the SDC mic connected and the Phantom power engaged, good channels measure a voltage drop of about 7.4VDC across R1 and R2, Bad channels measure a voltage drop of about 18VDC across R1 and R2.

Is C81 leaking/failing maybe? or C1 & C2?

FlyCp3o.jpg
 
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thecr4ne

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Maybe the issue isn't the phantom power, but just a really dirty board.

R138 measures close enough, 455~470 depending on the channel. Voltage is +50 on the switch side and +47~+48 on the other side...pretty consistently. And re checking my previous measurements, things are different...and the two "bad" channels I was testing passed some signal when repeatedly rotating the gain knob. I'm starting to think the pots and switches are just dirty...still...after blasting everything with alcohol 4 times over already and exercising everything a bunch.

I'm taking a break from this for now. and will come back at it again. Frustrating.
 

jensenmann

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I haven´t seen a DMR from inside, but I´ve refurbished a D-Series, which is supposed to be DDA´s livesound counterpart, years ago. They´ve been using molex connectors for the rail voltages. It turned out that on each (!) channel´s Molex some solderspots were broken. So if your DMR uses these, too, then resolder all the Molexes. It could be possible that these broken connections create a contact resistance which could corrupt your 48V rail in the faulty channels.

Another spot which calls for trouble with age is the selfcutting connection in Molex plugs. Try to wiggle the wires while measuring phantom power to see if the contact is flawed. Anyway, for a troublefree life it makes sense to renew all the selfcutting connections.
 

thecr4ne

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And the winner is:
My money is on them.

Yeah. I should've been able to figure that out the first time around, but apparently it was an off day. Measuring for DC on either side of C1 and C2, good channels show phantom on the + side, nothing on - side. Bad channels are the same on both sides. So it looks like that will be the first wave of recapping.

Stock values are 33uf/63V any reason to change that up? I think I'd maybe go with a higher voltage tolerance since phantom is measuring 50ishV, but I don't know of a reason to change the capacitance value if there is one.
 

thecr4ne

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Happy to report that replacing C1 and C2 on each channel (with 33uf 100v Panasonic FC's) did the trick and I've now got happy phantom power on all but 6 channels. The last 6 will be getting the Neutrik NTM1 input transformers instead of new caps, once they arrive.

Now the real question is this: Does the failure of C1 and C2 suggest an entire recap is needed or should it be expected that the phantom blocking caps would fail sooner than those in the rest of the circuit? And is there a way to check for this besides pulling caps and measuring them?

I do get sound through every channel, but I haven't done any serious listening yet so I can't say if anything sounds noticeably off.
 

abbey road d enfer

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Now the real question is this: Does the failure of C1 and C2 suggest an entire recap is needed or should it be expected that the phantom blocking caps would fail sooner than those in the rest of the circuit?
Good practice is to replace them all, because they are next in the list.
And is there a way to check for this besides pulling caps and measuring them?
Some people advocate the use of an ESR tester. If you had a brand new capacitor (not an NOS), you could compare with those on the boards, and that would give a comparative evaluation. But it is not likely, since no mfgr makes capacitors like they did 30 years ago, you're left to conjecture.
An ESR tester will almost certainly indicate a (half) faulty capacitor because capacitors made 30 years ago had larger ESR, because almost nobody cared.
There are basically two answers:
One is to measure the channels and leave them alone if the measure OK. If not, trace the culprit and replace all capacitors of same make/value on all channels.
The other answer is recap everything. Compare immobilisation of the mixer for one week once, or one day several times (bound to happen at the worst moment).
 

thecr4ne

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Do you have/use the MIDI Muteing system on your DMR12 ?
I do have the midi muting system. I only recently recently acquired the desk, so haven't yet figured out how I might use it in practice. A few channels weren't working but I quickly discovered one of the ribbon cables for the muting system was disconnected.
 

JohnRoberts

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Happy to report that replacing C1 and C2 on each channel (with 33uf 100v Panasonic FC's) did the trick and I've now got happy phantom power on all but 6 channels. The last 6 will be getting the Neutrik NTM1 input transformers instead of new caps, once they arrive.

Now the real question is this: Does the failure of C1 and C2 suggest an entire recap is needed or should it be expected that the phantom blocking caps would fail sooner than those in the rest of the circuit? And is there a way to check for this besides pulling caps and measuring them?

I do get sound through every channel, but I haven't done any serious listening yet so I can't say if anything sounds noticeably off.
+1 it is good practice to replace all of the same capacitance/voltage capacitors when faulty ones are identified. They were likely from the same production run, and experience the same life stress.

63V seems adequate for that application.

I repeat my same old advice for troubleshooting consoles. Measure the frequency response of all the individual channels while looking for outliers with compromised LF response (a symptom of dried out capacitors).

If/when bad caps are identified replace all of those same value components.

JR
 

thecr4ne

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+1 it is good practice to replace all of the same capacitance/voltage capacitors when faulty ones are identified. They were likely from the same production run, and experience the same life stress.

Thanks. It is worth noting the Phantom blocking caps, as well as a few others (all axial) are NOT the same as the rest (all radial, different manufacturer). They are all original, but being different makes, I'll wait till I can do some further testing before placing an order for new caps. I still need to work through cleaning switches (again) on the output strips before I'll get a chance to test further.
 

Newmarket

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I do have the midi muting system. I only recently recently acquired the desk, so haven't yet figured out how I might use it in practice. A few channels weren't working but I quickly discovered one of the ribbon cables for the muting system was disconnected.
Ah - I knew that my spelling "Muteing" looked wrong ?
Anyway I hope the desk goes well for you.
And if the MIDI Mute is problematic then it's definitely nothing to do with me okay :)
Well I wrote the firmware so that shouldn't break !
IIRC correctly I think it used a dedicated switch scanning ic (but don't quote me - long time past). Likely not available now so don't break it !
 

thecr4ne

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Well I wrote the firmware so that shouldn't break !

Awesome. So far it seems to work as described in the manual, though I haven't gone through everything yet. Would you happen to have info on updating the firmware, and the latest/most robust version by chance? The documentation I have is frustratingly reliant on "contact DDA for more info" which isn't exactly helpful 30 years and multiple corporate buyouts later. Luckily you stumbled onto my thread here and I found Nigel Turner in a DDA facebook group.
 

thecr4ne

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Have you found this site for schematics etc:

DDA | Audio Mixing Consoles. Better by Design

I don't have any info on the firmware - it was a long time ago now - and I can't recall the processor - maybe 6803.
But I don't recall any outstanding issues with the code so if it's in there it should be okay if the hardware remains functional.
Yes, I've got all the docs there. Certain features in the manual however are just followed by "contact DDA for more info".

Glad to hear there weren't any noteworthy issues with the firmware.
 

thecr4ne

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Just placed the "full recap" order. from Mouser. Panasonic FC's as far as the eye can see...

After going back and forth on this I decided to go ahead. One of the 6 channels I installed the Neutrik NTM1 input transformer in (instead of the phantom blocking caps) was still noisy with phantom power engaged. My guess is C81 on that channel is being weird.

Anyway after lots of indecision and some realizations about supply chain issues, I managed to order everything I'd need from one source and most of it will ship immediately. Not really looking forward to finding out how much time each channel strip will require, but hopefully it will all come together and prove worth the effort (and cost)
 
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