Mixer Fault, possibly VCA related

[indyld]

One for the audio electronics enthusiasts.  A fault analogue mixer has appeared in my vicinity (Ecler nuo 2.0), basically a DJ mixer with a few channels, EQ and a crossfader. I'm interested in the failure mode of VCA ICs if anyone has an information.

I believe the PSU was replaced after the last one went bang. This may be relevant or not.

The key voltages (+/- 18v, 5v, 2.9v) are all reasonable and in general the audio elements tested seem to work. The bit that doesn't is the crossfader which, on one channel and with Normal output fader orientation, gets to 10%, makes a loud pop and then sound drops out until about 90% when it pops again and then sound returns. This doesn't happen with the other channel NOR when the fader is Reverse(d).  I have the schematic and this looks like it could be either the quad VCA or associated op-amp that seems to be the core of the this part. Am pretty sure it's not a physical problem, it sounds like transistor kicking out and in again.

I 'solved' the problem by reversing the pot connections of the channel and flipping it into reverse. But of course, this also reverses the other channel meaning the faders top out at opposite ends. Swapping the other channel around puts the faders back into the same orientation but just moves the popping onto the other channel. I'm pretty sure there is a bit of the schematic that deals with reversing that I can't see which would help me understand this. 

E2A: Given the symptoms and 'solution', my gut feeling is this is a CTRL-A / CTRL-B issue which means it is before the VCA perhaps? I've done a few more non-destructive wiring swaps and realise that the pop is basically an issue when a particular output fader is up rather than down. This is why the reverse/channel swaps seem significant but is just actually a function of which end of the pot is 'up' and the fader state.

Am now looking more closely at the crossfader stuff before it gets to the VCA.

Anyway, the question was really about what a faulty VCA might present as. If I just need to order one before disassembly then I will.

Thanks,

ecler_nuo2_controller copy.pdf

Edited March 31, 2022 by indyld

[sunray]

I assume X fade; WI107 = J129, vol CH1 WI109 = J133 vol CH2 WI110 = J133 &  board interlink WI111 = J136

What do the voltages on TP202 and TP203 do at the point of 'failure' ? and the same again for TP189 & TP190

Edited March 31, 2022 by sunray
more info

[sunray]

deleted

Edited March 31, 2022 by sunray
double post

[indyld]

1 hour ago, sunray said:

I assume X fade; WI107 = J129, vol CH1 WI109 = J133 vol CH2 WI110 = J133 &  board interlink WI111 = J136

What do the voltages on TP202 and TP203 do at the point of 'failure' ? and the same again for TP189 & TP190

Yes, J136 is the interlink to the main board from the front face control board which also terminates the looms from the Ch1 and Ch2 level pots as well as that from the xfader on the connections mentioned. However, J129 (or at least the header from the xfader board) isn't 3 way but 5 way and there is a quad op amp and some associated stuff on the xfader board which I don't think I've seen on the schematic. I have asked the owner as they mentioned a new crossfader and I'm not sure if it was due to pot wear or if they were trying to address the current issue. 

The TP voltages mentioned (broke out the analogue meter for that) tracks with what can hear e.g jumps happen towards each end of the track. So, from this I assume that the VCA is just doing what is asked and the issue is further forward like IC129 op amp or something. IC128 is a 3 legged zener regulator.

I tried the meter on what came off the xfader board but I need to take a closer look at the tracks as which pin does what isn't totally obvious and only 1 voltage changes during xfader operation. Update: Xfader board control output goes smoothly from -10v to +10 end to end. Interesting that the static voltage at the board is +/- 15v when the rails seem be mostly +/-18v.

One thing I didn't mention in the OP was that any fiddling with the envelope controls etc. didn't seem to change anything.

 

e2A: Owner confirms the issue existed before the new crossfader.

Edited March 31, 2022 by indyld

[sunray]

To double check:

You say the problem only exists on one channel.

I see two quad op-amps  and one dual op-amp on the X fader board

I 'solved' the problem by reversing the pot connections of the channel and flipping it into reverse. But of course, this also reverses the other channel meaning the faders top out at opposite ends. Swapping the other channel around puts the faders back into the same orientation but just moves the popping onto the other channel. I'm pretty sure there is a bit of the schematic that deals with reversing that I can't see which would help me understand this. 

This confuses me, what are you flipping? The X fader pot only has a single control output, flipping that (turning the plug round?) should make no difference.

TP175 should be stable at about half of TP176, (I predict something like 3V & 6V but I could be wildly out!), TP177 should change smoothly between those 2 values.

The voltages on TP189 & TP190 should alter inverted to each other and the same again for TP202 & TP203, that invertion is created by IC134:1. I believe TP190 & TP203 should follow the direction of TP177.

[indyld]

The reversing turned out to be a red herring during functional testing. There is a fader reverse switch that I can't see obvious on the schematic. It turns the channel faders upside down. I used a combination of that and reversing the pot connections and swapping which channel went where.

In the end, the actual condition is that the problem happens when both channels are up e.g the way you actually use the cross fader. Also might be worth mentioning that all this happens even with no source material. (pop, VU hits the roof etc).

Edited March 31, 2022 by indyld

[indyld]

I've taken some relevant pics for context as this bit of the schematic isn't clear compared to what am seeing.  One is the crossfader board itself, the other is the other end of the 5 way header and the dual op amp. The 5 way headers shown are both ends of the loom (not Wl111, this is elsewhere). Tying up TPs on the schematic with actual things isn't all that easy in places. (Plus, I've avoided taking the main board out so far as I am hoping this is solved without all that malarkey as the key culprits may be more easily accessed).

The crossfader dual op am is the one on the front control panel board,  between the shunt regulators and another quad op amp. (the fat dual is the headphone jack one)

The blue sharpie mark at the crossfader is to remind me which is ground. 🙂 The other connections are + and - 15v, -10 to +10V xfader output (to GND), and a solid around 3v. The xfader output is smooth at that point, so it's being odd later on and before it gets to the VCA control.

 

Edited March 31, 2022 by indyld

[sunray]

So that's a completely different construction to the service manual you have offered, can you check model details on the product label match the manual?

[sunray]

This comes from the manual for the NU 03, the component numbers match the X Fade board.

 

ecler_nuo3_sm.pdf

Edited March 31, 2022 by sunray

[indyld]

Great stuff. Will have a better look through that SM tomorrow. It's a totally different model but at first look the xfader circuit has more similarities including the TL074 up front, 5 way header with pinout that fits with measurements. I looked to see if I could find this model which is a 2.0, just not the revision of the SM I found. 

Have ordered TL072 just in case, plus plan to see if anything odd happens to the shunt regulators during the fault.

edit: actually I already know this as the +/-15v is solid.

 

I can't quite picture the condition that would cause what seems like a slump / overload / shutdown or whatever of a stage during this xfader operation. Also whatever it is happens on both L and R so seems unlikely to be, say, part of an op amp. I guess the VCA can't be ruled out yet just because the input voltage is being jumpy as I assume this can still be a result of the VCA itself.

Edited March 31, 2022 by indyld

[sunray]

46 minutes ago, indyld said:

I can't quite picture the condition that would cause what seems like a slump / overload / shutdown or whatever of a stage during this xfader operation. Also whatever it is happens on both L and R so seems unlikely to be, say, part of an op amp. I guess the VCA can't be ruled out yet just because the input voltage is being jumpy as I assume this can still be a result of the VCA itself.

If you are seeing the voltage glitches on the control lines I doubt there is any problem with the VCA's.

If you have measured the control voltage at the VCA's there is nothing to stop you carefully simultaing the voltage directly with a pot to test the VCA.

[indyld]

Short update on this. After the last post, the job was at that place which really needs a speculative parts order so I got some op amps and the quad VCA in (or, at least a drop in replacement). The 'bang' crossfader issues WAS the VCA IC itself. The only thing outstanding is that having replaced it, there is a new problem in that one channel of one input is not outputting to PGM (I'm sure this is also VCA related, so either an op amp or perhaps a fault with the replacement IC).

Had some great info here and also via a tip-off from another member on here with audiophile connections. Some info on the IC here  The symptoms aren't exactly described, but power issues can bust the chip.

Now I just need to work out what is causing the new problem....