Tweet
I recently purchased a Roland JC-120 from a music studio that had been sitting in a garage for a few decades. Upon plugging the amp in to begin tests, I noticed the amp was susceptible to noise such as lightly tapping the top of the cabinet or knobs. These taps would be amplified through the speakers. Any slight turn of the preamp knobs would result in loud scratchy noises. I will note that two of the input jacks (one for channel one and one for channel two) had lost their nuts and washers and were recessed inside of the amp. I removed the amp chassis from the cabinet and set it on top to take a closer look at the circuit boards. Upon checking things over I didn't see any blown fuses, degrading capacitors or blown resistors, just one of the washers for one of the fallen jacks.
I decided to plug my guitar into the amp and see if audio signal would pass through and to my surprise in channel two everything worked as it should. I even tested the chorus and vibrato which both worked however I was being very delicate with turning knobs and minimizing contact with the cabinet or top of the amp. After about 10 minutes of enjoying the beautiful tone of the amp, I decided to test channel one and when I grabbed the jack to plug in, all of a sudden both power amp 5A fuses blew. I was very careful about making sure no wires shorted when I grabbed the input jack but perhaps it was when the input jack came into contact with the chassis/faceplate.
I ordered new fuses with exact specs and replaced the two blown fuses however upon plugging the amp in I had a loud buzz and the fuses blew again. After this I began probing around with my multimeter looking for any shorts or bad grounds. What I have found is the input of the spring reverb tank leads are connected (could be a bad coil?).
I guess what is confusing me is the microphonic characteristics of the amp before the fuses blew. Any experience with a solid state amp that has had this issue before? Thanks!
I decided to plug my guitar into the amp and see if audio signal would pass through and to my surprise in channel two everything worked as it should. I even tested the chorus and vibrato which both worked however I was being very delicate with turning knobs and minimizing contact with the cabinet or top of the amp. After about 10 minutes of enjoying the beautiful tone of the amp, I decided to test channel one and when I grabbed the jack to plug in, all of a sudden both power amp 5A fuses blew. I was very careful about making sure no wires shorted when I grabbed the input jack but perhaps it was when the input jack came into contact with the chassis/faceplate.
I ordered new fuses with exact specs and replaced the two blown fuses however upon plugging the amp in I had a loud buzz and the fuses blew again. After this I began probing around with my multimeter looking for any shorts or bad grounds. What I have found is the input of the spring reverb tank leads are connected (could be a bad coil?).
I guess what is confusing me is the microphonic characteristics of the amp before the fuses blew. Any experience with a solid state amp that has had this issue before? Thanks!
Comment