I am trying to track down the cause of some serious volume and sound changes coming out of my powered speakers.

It seems to be a power or voltage surge thing, which is not surprising considering the part of the world I live in. Which is why I have everything running off an APC Battery-Back Up and surge protector unit. 

But I can’t be sure WHERE the surging is entering the system. Is it my MOXF output or the speaker? 

My first thought is to try the headphones and see if the surging is happening there. It seems to NOT be in the phones. But does that mean anything? Could it still be the MOXF Line output or does the headphone test point to the speaker only getting the surge? 

All the plugs on the APC Back UPS have surge protection (supposedly) and half are designated for battery backup + surge protection. That is, if it’s working correctly. I have tried using one type or the other for both pieces of equipment and it has no audible effect. That is, the sudden volume (voltage surge?) happens regardless of being in surge protection plugs only or the dual surge/back-up plugs.

At this point I don’t want to plug straight into the wall, since that is the reason WHY I have the surge/UPS unit. (I’ve had some power hits that caused my MOXF to go crazy, or shut down (then fire back up) a couple times. Still, it might be that unit is the culprit, no? In which case I’m not really protecting my MOXF which is the most important thing to me. 

If I add a mixer into the flow, would that help or confuse the trouble shooting?

Any thoughts, much appreciated.

There’s very little circuitry from the point where the headphone output is taken off to the line outputs. Output muting transistors are the only active devices - they’ve been known to fail sometimes, but I wouldn’t jump to that conclusion.

Since apparently the sound is OK at the PHONES jack, you could use that output temporarily to test things. Disconnect the cables going to the MOXF L/MONO and R jacks, and plug one of them into the PHONES jack. If the volume level still changes, that should eliminate the MOXF as the source of the problem. You can try the same thing with the other cable. The signal coming from the MOXF will be just the left channel, and of course only one speaker can be checked at a time.

Let us know what you determine.

I unplugged my Yamaha volume pedal and things changed right away. So maybe a sticky switch on that foot controller?  The problem seems to have gone away since I removed the pedal, but I’m not entirely convinced because of the intermittent nature of the problem anyway.  I have an extra pedal and will put that into the system once I’ve played a while without it and the surging seems gone. 

I’m assuming that it’s just a physical problem with the pedal itself. But maybe there’s another possibility? (Hope not...)

Under normal conditions, MOXF headphone and line output levels are related. Perhaps the pedal wasn’t acting up when you listened with headphones. Certainly playing without the questionable pedal should help determine if it was the cause of the problem.

muscarella - 06 December 2016 01:42 PM

I unplugged my Yamaha volume pedal and things changed right away. So maybe a sticky switch on that foot controller?  The problem seems to have gone away since I removed the pedal, but I’m not entirely convinced because of the intermittent nature of the problem anyway.  I have an extra pedal and will put that into the system once I’ve played a while without it and the surging seems gone. 

I’m assuming that it’s just a physical problem with the pedal itself. But maybe there’s another possibility? (Hope not...)

If you are using a Yamaha FC7, I can tell you that I have had exactly the type of symptoms/issues you described in your initial post, with FC7 pots that have become dirty or otherwise broken and/or worn out.

Let us know.  I have replaced pots in my FC7 pedals before.  Not a particularly hard fix.