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#1
06-05-2015, 04:18 PM
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One mans trash........
Hi,
The "Audio In" jack on my First Act MA214 bass amp works fine. However, Plugging into the "Instrument In" jack yields a hum and no guitar sound. So, I'm thinking that the instrument in jack is at fault. If it is the jack can I just replace it with a regular guitar input jack? Any help is appreciated. Thanks.
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Paully  Yamaha FG700S Dread Epiphone Joe Pass Hollowbody Electric Epiphone Les Paul Special 1 p90's Squier Stratocaster SE Yamaha Thr 5 v.2 Amp Behringer Ultracoustic AT-108 Amp Bugera V5 Infinium Amp Bugera 112 TS Cab Peavey PVi 100 Microphone Tascam DR05 Digital Recorder Cubase AI 6 
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#2
06-05-2015, 04:45 PM
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If that is in fact the problem you might need a sealed/shielded 1/4" jack (some amps as well as guitars require them - my Epi Ultra 339 uses one for the magnetic pickup input) - other than that it's nothing special...
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#3
06-05-2015, 04:52 PM
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Mistakes with mains electricity can be fatal so don't open it up unless you are sure you know enough not to make mistakes. You really should have a schematic and/or layout that you understand.
If this is a tube amp it could contain lethal voltages even after the amp is unplugged (several hundred volts worth in the big power caps if they haven't been drained). Even something as simple as replacing a jack can be more dangerous than you think.
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#4
06-05-2015, 05:01 PM
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Most likely scenario is that the jack has broken loose from the circuit board (I've seen dozens of amps over the years do this). Second most likely scenario is that the jack components may have broken.
If it's the former, you can resolder the joints. Unfortunately, if the board faces downward, you'll need to remove the board to solder (may be difficult, may not). If you're lucky, the board is over tha jack and resoldering is easy. If the jack is broken, you may be able to use a shorting jack (don't use a standard guitar jack- the shorting jack will silence the input of the amp when the guitar cable is withdrawn). The shorting jack has a small tab which is connected to ground. The tab touches the input of the jack when nothing is plugged in. Whether you can use the jack depends on the distance available between the hole in the chassis and the board. Hope this helps! Bill.
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#5
06-17-2015, 08:32 PM
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Opened it up, one of the jack legs was unsoldered from the pcb. Reflowed the joint and it's working fine.
Thanks
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Paully Yamaha FG700S Dread Epiphone Joe Pass Hollowbody Electric Epiphone Les Paul Special 1 p90's Squier Stratocaster SE Yamaha Thr 5 v.2 Amp Behringer Ultracoustic AT-108 Amp Bugera V5 Infinium Amp Bugera 112 TS Cab Peavey PVi 100 Microphone Tascam DR05 Digital Recorder Cubase AI 6
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