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Broken UltraSpound AG-50-D? Here'a fix.


sonfire

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"By God, Jim, I'm beginning to think I can cure a rainy day!" My UltraSound AG-50D acoustic guitar amp failed at a gig last Saturday, Having found out it's $300 to fix locally & getting no response from the company, I took it apart and fixed it myself. There's "negative feedback" used to cancel sound, which opens up when you plug into the guitar jack. I guessed that metal fatigue had affected the grippers in the jack. It was plastic, & it had. So I de-soldered the jack, ran wires from board to new metal jack, bypassing the cancel. Presto! It's fixed. The two posts on the left, as seen from the top of the circuit board, are ground and hot of uadio input the two at right control cancellation, so omit those.I always found this feature to be a pain in the tail anyway. I mean, what if you just wanted a small PA to use at a gig, with no guitar? You still had to plug it in. When this failed on me, I had to plug in to the PA system to finish the gig with guitar.

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Jack failures are common on all amps that have plastic jacks. Most are like this either 4 or 6 connectors. http://www.mojotone.com/Mojo-3-Conductor-PC-Mount-1-4-Phone-Jack-3-Pin-Plastic-Nut-image.jpg

 

You got lucky installing a metal jack however. The reason they put plastic jack in is, it does save time and money manufacturing the boards, but, they may also be insulating jacks. Not all amps can have their input jacks safely grounded to the chassis. Many use plastic jacks so they have a floating ground independent of the chassis. The Chassis may or may not be DC ground. Some are AC ground only and you set up a ground loop by connecting the preamp ground to the chassis.

 

You just wound up being lucky in this case and (so far) you didn't create a bigger problem by using a metal jack. Some amps would have blown your preamp, or introduced AC hum so be advised to use the right components next time. You can buy new plastic PC mount jacks for $1, plus you have a better resale on it when the work is done properly.

 

By the way, I have no idea what you mean by Negative feedback. Input jacks are normally switched and connect the hot lead to ground when you unplug them. This kills the input like you would turning the guitar volume down. Less chance of issues blowing a preamp when a guitar isn't plugged in too. In other amps, they may also have resistors for when you're using the first or second jack so the input impedance is balanced or you may have a hot and normal channel input.

 

You should always try and Google up the schematic and look it over so you're sure what you are attempting is doable. If you cant find a schematic let an experienced tech advise you. There are a few besides myself who visit here regularly who can at least point you in the right direction so you don't wind up with a mess. Like I said you got lucky, but its still a rig job and not something I'd be advising others as a "Fix" Thy might think its something they can do with other amps that have plastic jacks.

 

You can use metal if you also buy the insulating washers for mounting those kinds of jacks. http://www.mouser.com/ProductDetail/Eagle-Plastic-Devices/561-SW375/?qs=YidGUlH2%252b5K4A7QQEqWdhg%3D%3D and http://www.mouser.com/ProductDetail/Eagle-Plastic-Devices/561-D37562/?qs=MX0Fjf4sB%2FjRqA9ylKTyZw%3D%3D The T shaped goes on the jack, and the flat goes under the nut.

 

 

They should be switching jacks like these though. http://www.onlinecomponents.com/switchcraft-14b.html?p=12318566&ref=Nextag.com

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Jack failures are common on all amps that have plastic jacks. Most are like this either 4 or 6 connectors. http://www.mojotone.com/Mojo-3-Conductor-PC-Mount-1-4-Phone-Jack-3-Pin-Plastic-Nut-image.jpg

 

You got lucky installing a metal jack however. The reason they put plastic jack in is, it does save time and money manufacturing the boards, but, they may also be insulating jacks. Not all amps can have their input jacks safely grounded to the chassis. Many use plastic jacks so they have a floating ground independent of the chassis. The Chassis may or may not be DC ground. Some are AC ground only and you set up a ground loop by connecting the preamp ground to the chassis.

 

You just wound up being lucky in this case and (so far) you didn't create a bigger problem by using a metal jack. Some amps would have blown your preamp, or introduced AC hum so be advised to use the right components next time. You can buy new plastic PC mount jacks for $1, plus you have a better resale on it when the work is done properly.

 

By the way, I have no idea what you mean by Negative feedback. Input jacks are normally switched and connect the hot lead to ground when you unplug them. This kills the input like you would turning the guitar volume down. Less chance of issues blowing a preamp when a guitar isn't plugged in too. In other amps, they may also have resistors for when you're using the first or second jack so the input impedance is balanced or you may have a hot and normal channel input.

 

You should always try and Google up the schematic and look it over so you're sure what you are attempting is doable. If you cant find a schematic let an experienced tech advise you. There are a few besides myself who visit here regularly who can at least point you in the right direction so you don't wind up with a mess. Like I said you got lucky, but its still a rig job and not something I'd be advising others as a "Fix" Thy might think its something they can do with other amps that have plastic jacks.

 

You can use metal if you also buy the insulating washers for mounting those kinds of jacks. http://www.mouser.com/ProductDetail/Eagle-Plastic-Devices/561-SW375/?qs=YidGUlH2%252b5K4A7QQEqWdhg%3D%3D and http://www.mouser.com/ProductDetail/Eagle-Plastic-Devices/561-D37562/?qs=MX0Fjf4sB%2FjRqA9ylKTyZw%3D%3D The T shaped goes on the jack, and the flat goes under the nut.

 

 

They should be switching jacks like these though. http://www.onlinecomponents.com/switchcraft-14b.html?p=12318566&ref=Nextag.com

 

Where, if I may ask? I know a guy who could use a couple.

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