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Old 02-04-2019, 09:20 AM   #1
Dannyx
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Question Channel fader not working on mixer (Omnitronix LS-622A).

Good day folks. A pretty difficult task was lain on me today and I think I'm getting close to solving it, but not quite and I'd like some help from those more skilled with pro audio gear. It's an Omnitronic LS-622A powered output mixer which works perfectly except for one important part: the channel faders are bypassed all the time. The only way to adjust the channel volume is with the gain control. Moving the slider up and down has no effect at all, at least on the "stereo" channels - I haven't tried the two mono channels yet to see if they behave the same. Even if the fader is all the way down, the music is still going through assuming the gain control is moved up, which is not how it should work - at least not on this model, since I'm aware there are some mixers which may have a "bypass" or "defeat" switch for the faders.

I attached both the manual and the schematic for your convenience, maybe there's something I'm missing, but from what my colleague told me, it started doing this after someone else messed around inside and, supposedly, reversed some connectors around. While it is true that there are many connectors with the same number of pins which can easily be swapped around, you'd have to be a complete idiot to do so because not only they're marked on either end on the boards, but they also had the idea to mark some of them, so there was nothing wrong there. Also, I can't imagine reversing the connectors would still allow the thing to work, even with bypassed faders !

So what could go wrong for the fader to be bypassed entirely ? I even removed one of them and checked its terminals and the traces for shorts, but couldn't find anything - no shorts, yet the signal is still going through when the slider's down....

The schematic looks homemade rather than official, but it's the best we'll get for such a niche product. The area in question seems to be on page 2 under the "LEVEL" label - R15 most likely represents our slider, though the pinout doesn't quite match. On my sliders, only one pair of contacts actually works when I move the slider, so assuming the wiper is connected to GND like in the schematic, one end of the pot goes from high to low impedance like you'd expect from a pot, but the other one is always the same value or "ramps up" to some inconsistent value as a cap charges or something (can't remember what it did out of circuit, but was still inconclusive). Either way, for the signal to pass through full-blast through that op-amp, resistor R50 would have to be pulled to GND via the pot to form what appears to be a negative feedback amp. As that GND wiper moves "DOWN" (towards the bottom of the page), the volume goes louder...is this correct ?

I though maybe the 4558 op-amp screws up, but both of them ? It would've been more plausible if only one channel did this, but both of them ? That's a rare situation ! What do you guys think ? What could be wrong here ? I read that manual front and back hoping I'm missing something about disabling the channel fader, but couldn't find anything....any ideas ? Cheers and thanks.
Attached Images
File Type: jpg 20190204_164700.jpg (1.02 MB, 13 views)
File Type: jpg 20190204_164707.jpg (1,005.8 KB, 12 views)
File Type: jpg 20190204_143405.jpg (857.5 KB, 11 views)
Attached Files
File Type: pdf omnitronic_ls-622a_ls-822a_ls-1222a_ver.1.0_mixer_sch.pdf (173.8 KB, 14 views)
File Type: pdf ls622a.pdf (1.20 MB, 11 views)
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Old 02-04-2019, 11:37 AM   #2
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Default Re: Channel fader not working on mixer (Omnitronix LS-622A).

What about the filters? Do they work?
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Old 02-04-2019, 11:59 AM   #3
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Default Re: Channel fader not working on mixer (Omnitronix LS-622A).

Yes, I believe so...the speaker I'm using is pretty crappy so I can't exactly pick up the subtle differences, but the LO MID HIGH pots do something at least and so do the 7 band EQs, which come after the up fader anyway.

EDIT: also, I misspelled the brand - it's OmnitroniC - no X there....

Last edited by Dannyx; 02-04-2019 at 12:13 PM..
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Old 02-04-2019, 01:34 PM   #4
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Default Re: Channel fader not working on mixer (Omnitronix LS-622A).

gains and pans working ?
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Old 02-04-2019, 01:38 PM   #5
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Default Re: Channel fader not working on mixer (Omnitronix LS-622A).

Yes, gain is working - it's the only way you can control the channel volume. According to the block diagram at the end of the user manual, the gain knob comes first, then the EQ unit, then the fader, then the balance control.
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Old 02-04-2019, 02:01 PM   #6
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Default Re: Channel fader not working on mixer (Omnitronix LS-622A).

probably lost a ground .
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Old 02-04-2019, 02:34 PM   #7
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Default Re: Channel fader not working on mixer (Omnitronix LS-622A).

This is a good idea - I'm still looking for "physical" stuff rather than complicated electronic faults, especially since someone had been in there before.

The only place I see where missing GND could cause issues would be right at R15 there if the wiper were somehow floating. I need to check the board interconnect cables for continuity I suppose, just to make sure GND is present across the whole device. Thing is it sounds GOOD, so wouldn't a missing GND cause distortion or muddy sound ?
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Old 02-04-2019, 02:43 PM   #8
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Default Re: Channel fader not working on mixer (Omnitronix LS-622A).

It seems to me that the level slider you are refering to is on page 5 (10k)
Page 2 is for the stereo input control
Does changing the bus select switch have any effect?
Where are you feeding your signal in?

Last edited by R_J; 02-04-2019 at 03:08 PM..
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Old 02-04-2019, 03:10 PM   #9
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Default Re: Channel fader not working on mixer (Omnitronix LS-622A).

Hmmm. That seems correct indeed, though page 5 seems to be for the "mono" and MIC channels (1 and 2). The problematic channels are the two stereo channels (3 and 4 from left to right on the mixer). I shall test channels 1 and 2 tomorrow just out of curiosity, even if it's in mono. Strangely, the silk screen under the slider for channel 3 I removed in the picture says 10K, which corresponds to the value on page 5...but the circuit on page 5 seems to be for the first two mono channels. The value doesn't match the actual slider anyway because it's neither 50k nor 10k - it was replaced at some point and it's now like a 22k or something...doesn't really matter since it SHOULD work at least partially, but it's got no effect at all on the audio, so I doubt the value is THAT critical, plus I think I remember it working at some point, before they got their hands on it !

I am using an RCA to 2 T-S jacks cable, so there's one jack for each channel (L and R), because pro audio equipment doesn't utilize stereo TRS jacks to carry BOTH the L an R channels on the same jack - the sleeve (S) is used for the shield connection, but since an RCA jack has no shield, the GND of the RCA also acts as the shield connection....I make it sound a lot more confusing that it really is, but this cable works because I tried it before on many other devices I got round here and it works as it should.

Last edited by Dannyx; 02-04-2019 at 03:17 PM..
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Old 02-04-2019, 03:24 PM   #10
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Red face Re: Channel fader not working on mixer (Omnitronix LS-622A).

Just saw you edited your post, so:
Quote:
Originally Posted by R_J View Post
Does changing the bus select switch have any effect?
If you're referring to the L-R buttons in the manual, depressing those simply mutes the channel entirely, as the manual describes. If you're referring to the G1-G2 buttons, then it's a bit trickier to explain: from what I understand, those buttons route the audio from a certain channel to a separate slider on the far right, which can THEN be assigned to the main output as a "group". Either way, no, pressing that doesn't "engage" the slider in any fashion as far as I can tell...
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Old 02-04-2019, 03:59 PM   #11
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Default Re: Channel fader not working on mixer (Omnitronix LS-622A).

It does seem a bit confusing, I guess the stereo input IS on page 2, R15/20 could be the gain and R17/21 the slider?
it is a dual pot so it would be for the st_l/r and G1 bus output
I suspect if the slider was replaced it was with one with the wrong configuration.
with the L/R switch in the down position you should have audio, with it up it should be muted.

Too bad its not a complete schematic.

Last edited by R_J; 02-04-2019 at 05:18 PM..
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Old 02-04-2019, 05:14 PM   #12
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Default Re: Channel fader not working on mixer (Omnitronix LS-622A).

check for proper grounding at the sliders .
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Old 02-04-2019, 05:38 PM   #13
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Default Re: Channel fader not working on mixer (Omnitronix LS-622A).

This is a behringer pot, If it is the same as your's, the wiper would be going to ground according to your picture, making it likely R15/r20 in the schematic

After a closer look, R46/R47 must be the gain controls
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Last edited by R_J; 02-04-2019 at 06:53 PM..
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Old 02-05-2019, 06:33 AM   #14
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Thumbs up Re: Channel fader not working on mixer (Omnitronix LS-622A).

Ok, here's the situation: I have it all figured out now and solved the issue. I took some more measurements today and I was dealing with two bad sliders indeed, though I have no idea how they "got there" since I remember the thing worked....anyway.

Page 2 most likely depicts just ONE stereo channel, so R15 is one gang of the slider and R20 the other gang of the slider - the notation is a bit confusing, since I would've marked them as R20/A and R/20B to signify they're the same slider just different gangs. Anyhow, thanks to the diagram of the slider R_J provided which was VERY handy (hats off to you sir !), I took some measurements and got pretty wild results: the sliders for channels 1/2 (mono) read perfectly fine and exactly as expected from a potentiometer: 20k across the ends of the "resistors" (since they're 20k sliders in my particular case - says 20KA on the sides) and 1-4ohms when the wiper was at either end. It will never be a dead short between the wiper and the end AFAIK, so it's acceptable. On channels 1-2, even though the sliders are stereo, only one gang is wired - the other is floating and soldered only for support.

Now for channels 3 and 4, both gangs are wired but they both gave incorrect readings: I get the correct value on just one gang and this is because someone, before me, replaced these with mono sliders. To confirm this, I pulled out one of them and cracked it open to have a look at the tracks, along with another one my colleague gave me to try out. The brown one is the slider that was in there and the white one next to it is a stereo one which I actually put in its place. Notice on the brown one the rightmost track is not wired to the pins and I confirmed this with my meter. The problem is now solved and the mixer cranks once again

The L/R signal wires going to the amp were also swapped around so L was R, so it's a good thing I went in there to clean up this mess...cheers guys. Thanks again
Attached Images
File Type: jpg 20190205_095328.jpg (556.7 KB, 11 views)
File Type: jpg 20190205_095350.jpg (474.2 KB, 9 views)
File Type: jpg 20190205_100512.jpg (522.4 KB, 11 views)
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Old 02-05-2019, 06:46 PM   #15
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Default Re: Channel fader not working on mixer (Omnitronix LS-622A).

never easy if someone has been playing around before you get there .
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Old 06-26-2020, 04:22 AM   #16
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Red face Re: Channel fader not working on mixer (Omnitronix LS-622A).

Hey guys ! Who still remembers this Omnitronic mixer thingy ? When I moved in to this new wing of our joint where it's just me another chap, this thing along with its speakers also found its way in here somehow, so now I'm using it to blast my tunes all day long

What I did today was insert a resistor in series with its fan, since it was way too loud. Understandably, it's meant to be used in noisy environments where it wouldn't be noticeable over the music itself, crowd noises or outside noise, but I can't really play music THAT loud in here, since it's a big empty room and the acoustics are terrible, plus I don't really like it that loud to begin with...I keep the master volume at 0dB and adjust from the input slider which I barely have to touch and it gets pretty loud.

Since the fan noise now went away, the "idle" hum and buzz noise from the speakers became pretty noticeable too. I doubt I screwed up anything inside which would've caused it, so I can only blame it on the fact that the fan is now quieter and I can actually hear it, since the unit is close to the speakers, so the fan was drowning out that noise unless I was placing my ear next to them. I also have ground loop going on (I think) between my laptop and the mixer, since the buzzing becomes worse when the laptop's plugged into the wall, plus it also adds what to me sounds like 2.4Ghz noise...

When I first put it back together today, I noticed the noise and thought I must've done something wrong, so I opened it back up and noticed the buzz goes away when the top panel where the faders and pots are is away from the body. The reason is simple: the board they're mounted on gets VERY close to the toroidal transformer when the unit is assembled...there's no other explanation that I can think of - the fat yellow wire going between the top part and the bottom cabinet is connected and so are the other connections, so it's not a connection issue.

As I was testing this, I did do something rather stupid: with the master volume sliders up and the power on, I unplugged one of the signal wires feeding the amp board: the speakers made a loud buzz, so loud in fact that it scared the [email protected] out of me and also caused some folks next door to come in and ask what happened a few moments later ...and these are concrete walls, to give you an idea of the level of noise ! It only lasted a few seconds, since I panicked and killed the power, but god damn was it ever a shock to the ticker ! The room was dead-silent too, so you can imagine how that felt...must've been at least 100+dB in here

I have no idea what caused this (floating GND on that channel probably) - my idea was to check if the noise is still there with the amp board disconnected from the top controls. I then unplugged BOTH signal wires (L and R) and it indeed no longer boomed like the first time and the buzz no longer got louder when I put the top cover back on, so the noise is being picked up by the controls and op-amps at the top and gets fed into the amp board...I find this a pretty stupid design choice: placing a toroid directly underneath a board full of op-amps and stuff with no shielding whatsoever....any way to fix such an issue, or is it something you have to learn to live with ? Caps look good inside - I was mostly interested in the two big ones feeding the amp board, but they appear to be fine, despite not having been replaced before. There's many more caps around, but replacing that many at random, hoping to solve the issue, would cost too much to make it worth it....

I was still in shock from that sonic boom, so I didn't take any pictures of the inside, so THIS will have to do if you want to get an idea of what's in the box Cheers guys
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