metroiss

I know it has been several year but i was finally able to find a schematic and apply an oscilloscope to the circuit and figure out exactly where the problem is. mpaquette was correct, the issue is in the coupling capacitors, specifically C140, C148, C149, and the last one i can't remember the number but it is next to C149. These are the 4 coupling capacitor associated with instrument input 1. There is a similar patter of capacitors near these on the circuit board that do the same job for instrument 2. what the design actually does is split the incoming signal into 2 differential waves that are then sent to a Cirrus Logic CS4272 ADC. in my case 1 of the coupling capacitors on each channel was failing causing the differential wave to shift and distort which caused a bad input to the ADC. I replaced all caps with 10uF 50V audio caps that i found on amazon: Nichicon Muse FG 10uf 50V Caps. If you have basic soldering skills you can fix the board in about 30 min. everything worked perfect after that.

After replacing the capacitors that i thought could be suspect there is no change to the instrument inputs. I also found that all the voltage values in the power supply area are correct, turns out my old DMM from college is no longer reliable. At this point I'm thinking the main DSP chip could be the issue, I believe the high/low gain channel switching logic is controlled by the main DSP. Without a schematic its really hard to trace any of the signals back to the DSP to test everything along the path. Has anyone been able to get a schematic from Line 6? I would think since this unit is out of production they shouldn't have any issues selling the schematic to users. Brad

I've has some time to play with this some more and mpaquette comment makes perfect sense as a fix. I'll try to determine which caps are the problem and try the fix over the weekend, hopefully with some pictures. If someone could give me direction on which caps to focus on that would be very helpful. Looking at the unit i'm seeing 6 470uF electrolytic caps around what looks like 5 voltage regulators. I was expecting to see the nice mushroomed tops but no luck. Are these the caps that need replacing? some of the numbers are C136, C219, C145, C133, C132, and C120. I actually took the time today to pull out the main board and find out what voltage measurements i should be getting. Luckily Line 6 put a nice set of test pads on the bottom of the main board and indicated what voltage you should see at those locations. What i found is that my +20 and -20 volt signals are showing somewhere around +-13V and what should be an 8V source is showing 3.13V, these are feeding +15V, -15V, and 3.3V regulators. I'm actually surprised i'm not finding more issues with the unit other than the instrument inputs. Surprisingly the regulators are all outputting what they should and my theory is that they are able to do this at the sacrifice of current and the instrument inputs require more current then the other inputs to operate correctly. This then starves the high/low gain switching unit of power causing the instant drop just after initial attack. My next approach will be to replace the coupling caps for the corresponding voltage sources. For those interested it is the set of 3 caps that are glued to a set of 3 inductors right next to the transformer. If this works i'll post my results.

Tried through the Morley Wah with no change. Also decided to try plugging in my gibson and run it through dry and found it does the same thing so its not an active vs. passive pickup problem. I did switch the input 1 to line in and the problem goes away so its only an issue when channel 1 is set to instrument.

I can't say that this is just happening now, normally i only run my guitar through the UX8 but my situation changed where i'm starting to run my bass through it as well. Strange that an active pickup would cause an over voltage scenario, input of the UX8 (as far as i can tell) is about 18V p-p which i don't believe an active pickup running 9V battery can achieve. All of the batteries are fresh and this also happens with my second bass, also with active pickups. I'll try running through a stompbox today (I have a Morley wah I can try) and see if there is any change.

I've had my UX8 for a long time now and have never seemed to have any major problems. Lately though while running my bass through both the Instrument inputs on my UX8 i noticed that there is a constant clip on the attack every time I hit the lower strings. Odd part is if I hit a low string and let it sustain and turn the volume up and down on my bass the clipping also occurs on every volume swell. The clip light does not light up on the input and the pad does not seem to make any difference. I can then move my bass onto input 3 for instance as a line in and have no problem what so ever. My bass is a Ibanez SR405 running active Seymour Duncan soapbars. Is there a protection circuit that may be prematurely going off? Any way to find out if an input transistor may be going bad? I've attached a picture of the clip i recorded and normalized. I appreciate any help or suggestions.