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.
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 Files
...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 
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