DDX3216 - Distorted Sound

[notundercommand]

Hi, love this forum and the helpful information - hoping for some pointers for my DDX3216 audio issue.

I bought the unit very cheap about 12 years and then moved away so it has sat in it's box all this time. Now my son is starting to make music with his friends I thought it might be worth seeing if I get it working.

Overall it seems in very good condition, although it had the usual issues powering on. I replaced the electrolytics in the PSU + Q3 for good measure and it now powers on OK and seems stable with all functionality.

However there remains a problem with audio distortion which I am struggling with.

The problem is really similar to this post https://music-electronics-forum.com/...istorted-sound however when I disconnected X14 from the anaout board the distortion still persists so I don't think it's the same.

Distortion is the present with mic, line, spdif and test tone signals. Similarly it is present on all the outputs, including spdif. The distortion is a function of the audio signal - if there is no audio (or the level is very low) there is no audible distortion but as the signal gets louder the distortion also increases. It sounds a bit like an effect on top of the audio signal.

Any ideas how to diagnose where this is coming from?

Kind Regards
Chris

[The Dude]

Do you have a schematic. The first thing I'd do would be to check all power supply voltages and make sure everything is up to snuff- especially since you know that you've already had supply problems.

[notundercommand]

Thanks for the reply The Dude - I do have the schematics (available here: https://www.electronica-pt.com/esque...hematic-27415/)

After changing the electrolytics I checked the voltages from the PSU, first offline and then with the mixer powered on - I did adjust the +/-17V and 5V/3.3V slightly with the power on.

One clue perhaps is that the distortion is present on the spdif input. And also when listening to the spdif output, i.e. with spdif input routed to Ch13/14 and listening to the spdif output, there is still distortion even though the signal is purely digital.

It did make me wonder about the 5V and 3.3V rails. My DMM shows no AC component on the 5V but about 10mV AC on the 3.3V. Could this be an issue? I have changed the ceramic C45 to a polyester film capacitor just because I had one on the bench but it hasn't made any difference. Incidentally the schematics call for a 104 capacitor here and in a number of other places, but the ones installed are 103M.

Any thoughts on where next?

[The Dude]

Have you checked C43 &C44? 10mV doesn't seem like a ton of noise, but some digital circuits are pretty picky. On those supplies, I usually ESR test all electrolytic caps. It's not always just bulged caps that are bad. Pay particular attention to any electrolytics that are right next to heat sources (regulators, heat sinks, etc.). Heat is the number one enemy of electrolytic caps. There are a couple of reasons I still suspect power supply. 1) I've seen it multiple times on these units. 2) It seems like you have the distortion on most all outputs and, of course, power supply is common to all.

[g1]

Also worth checking for any AC voltage on those supply rails.

[notundercommand]

ok, I should clarify that I did change all the electrolytics in the PSU already, so eg. C43 & C44 have been replaced.

I have done some more reasonably accurate testing on the DC voltages with the unit fully powered up and idle.

I used a basic Fluke DMM for this:



Also just for fun I changed the transistor Q6 as mentioned in the original post as a potential cause but it didn't make any difference.

I wonder if I should make the same test as above, ie test all voltages but with a distorted audio signal present and see if any of the voltages look off? I'll try that next if no better ideas.

thanks!

[notundercommand]

looking at the voltage results, it's just the 48V and the +/-17V with any AC component on them. When I just test these with the PSU on the bench there is no ripple, but once it is connected in the desk and powered up the ripple is there. Looking at this part of the circuit I wondered if it might be C63 or C68 - again, these are marked as 104 on the schematics but the ceramics installed are 103M:



I changed both C63 and C68 (I have a bag of 10nF polyester film caps) the ripple is still 11mV on these rails. One questions - are the polyester caps OK to leave in or should I replace them with ceramics (assuming I get this thing fixed)?

Anyway, next I think I'll change C59 and C64 and see if that eliminates the ripple, after that C54 and C58 is all that's left on the secondary side.

Oh also I checked the voltages with a mic connected creating distorted sound, the 3.3V DC line showed a slight blip when there was a transition from silence to a noise (and vice-versa) but it stabilised quickly (to 3.25V in both cases). There was no effect I could see on the AC component or any of the other DC voltages.

[notundercommand]

mods - please delete this posted in error

[notundercommand]

ok so I have now changed all the caps on the +48V and +/- 17V DC secondary sides, still there is 11-12mV AC ripple on these 3 lines, but interestingly not on "5A" which is also fed from TR3. If there was a problem on the primary side, like C47, C50 etc., I would have thought that it would also affect this 5V supply?



I do wonder if the distortion can be due to the above supply rails though, as when using SPDIF for both in and out neither of the analogue sides AD or DA should be in use?

[chilidawg]

+48V is for phantom power.
+/-17V is for the analog circuit.
+5V is for the digital circuit.
+3.3V is also for the digital circuit.

Ripple noise is measured in DC voltage, and you need an oscilloscope. That 10mVAC you measured with your Fluke meter is not a ripple noise.

And since the power supply is able to provide stable output DC voltages according to the factory specs, it's no longer an issue and you should move on.

You said you disconnected the analog audio outputs board, and the distortion is still there.

Then how would you be able to hear the distortion???

Also, this mixer has digital and analog audio inputs. The flow is:

digital inputs -> DSP
analog inputs -> A to D converter -> DSP

so if the distortion is always there, regardless of which input is selected, then the prime suspect is definitely with the digital signal processing circuit.

Perhaps a minor component fault like dried up SMD electrolytic caps, or a minor water damage from wet moisture. Must fully inspect the mainboard, which means you have to disassemble the mixer.

[Helmholtz]

Ripple noise is measured in DC voltage, and you need an oscilloscope. That 10mVAC you measured with your Fluke meter is not a ripple noise.

Ripple noise is ACV riding on DCV. An ACV meter reads the AC RMS value of the noise because it is (should be) AC coupled. That noise will contain power supply ripple and maybe some noise from other sources.
But if the ACV reading is low, we know that ripple is low. That is within the frequency range of the meter.

[notundercommand]

the Fluke meter is not a true RMS model so yeah, the ACV measured will be RMS value. I would love the excuse to buy an oscilloscope and see exactly what that ACV component is made of but in this case it's probably not the cause of the distortion so I guess I can't justify it right now.

You said you disconnected the analog audio outputs board, and the distortion is still there.
Then how would you be able to hear the distortion???

The connector I disconnected was the 4 pin shown at the bottom right of the image in post #9 which carries a Mute signal to the ANAOUT board which I think mutes the outputs when when the PSU is starting or shutting down. In the referenced other thread, dutch_anykey was able to eliminate a similar distortion issue by removing this connector, however in my case, removing the 4 pin connector made no difference.

I wonder if it would be possible to bypass the DSP board completely? i.e. pulling the ribbon cable carrying the I2SIN and I2SOUT from the DSP board and linking for example I2SIN1 on pin 33 to I2SOUT1 on pin 32?

In the meantime I'll pull the board out and check it for any obvious issues. That said, there are 4 DSP chips on that board, so the issue would have to be common to all of them.

[notundercommand]

Some progress - I managed to bypass the DSP board by removing the 34 pin cable that carries the data to and from the DSP and linking pins 33 (ch1/2) and 26 (monitor/phones). With the link in place the audio signal from is perfectly clean in the headset so the DSP board does seem to be the problem.

I have pulled it out and it looks very clean - no moisture marks, heat marks, leaky caps etc. The only part that looks less than pristine is where there are some resistors soldered on the top of the board which have then had hot glue applied to keep them in place. First I thought this was a previous repair but from pics online it seems this is as built.

I did change the two 470uF electrolytics on the 3.3V and 5V right by the power connector with no effect.

I could replace the board but they're not cheap and my worry is it could still be a noisy supply if the frequency is higher than my meter can see (500Hz or 1KHz depending on the range). I'm at the point now where all I have left is finding duplicated SMD components and comparing the resistance values which so far hasn't turned anything up. Maybe I should invest in something like this: https://www.amazon.com/Smart-Tweezer.../dp/B0068QJ5Z6 ? Or perhaps a cheap USB scope to check the 3.3V and 5V.

Any tips on how to check it further?