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I've got a puzzle with a guitar amp that I'm determined to figure out. I'm pretty good at troubleshooting technology, but because I have so little experience with electronics, schematics, and amplification...I'm really having a problem tracking down the cause of this issue. I've been working on it for several months and it's become my personal Moby Dick...but I'm caving and asking for help from more experienced and intelligent people.
Device: Crate FXT-120 guitar amp (schematics attached)
Problem: When I power on the amp, there is a steady and loud hum coming from the speakers. This hum is not affected by turning any of the knobs or buttons, and is present whether there is anything plugged into the input. The hum is obviously not there when the speakers are disconnected. When I plug in a guitar, you can hear the tones coming from the guitar through the hum, but they are distorted.
Troubleshooting:The first thought I had was that there's a DC offset on the speakers. Measuring the output (TP13) with my DMM (red probe on the + speaker terminal, black probe on either a ground or the - speaker terminal), I get 0.04v DC. That doesn't seem like enough DC to cause the loud hum, but I'm not sure what else would cause it.
I've replaced a handful of capacitors (including the main filter caps), but the hum has not changed. However, it is worth noting that my soldering skills are abysmal at best and it is entirely possible my joints aren't any good (I'm having someone else double check me in a few days). The caps I've changed are highlighted in yellow on the 2nd page of the schematic.
The results of the DC test points are listed on page 2 as well (in orange). There are several values that are off, but I'm not experienced enough to know what kind of variance would be acceptable.
Any interesting thing that I stumbled across, is that if I short the collector and emitter leads on Q5 (red circle 1 on the 2nd page) while the amp it turned on, I get a spark from the transistor and the hum goes away. After several minutes of the amp being on after that point, the hum comes back. I have tried to trace what might be causing the spark (e.g. a capacitor discharging?) and why the hum goes away, but again my lack of experience is holding me back.
I have found that if I remove Q5 (red 1) from the circuit completely, the hum goes away (although there is a new buzzing present in the sound from my guitar that is affected by volume and tone controls). But when I also remove the sister transistor, Q23 (red 2), the hum stays gone. But this time, the sound coming from the guitar is heavily distorted just like it was when the hum was present. Also, removing both transistors does nothing to change the 0.04v DC on the speaker (TP13).
Thanks to this thread (http://music-electronics-forum.com/t34148/), I noticed that the 0.04v DC is also on the base of Q14 (red 3) and also on the drain of Q16 (red 4, page 1). Then on either side of C68 is 0.04v (red 5) and 0.01v (red 6). The thread talked about a leaky Q16, which I am going to replace next...but something tells me that's not my problem.
I've heard many a person tell me not to bother with fixing the amp, but like I said it's become a personal quest for me and I'm confident I can fix it...I just need some clues. I've got to believe that the behaviors that I'm seeing can help trace this to the root cause...I just haven't been able to see it.
Any guidance or thoughts on where I should be looking would be oh so appreciated!
Device: Crate FXT-120 guitar amp (schematics attached)
Problem: When I power on the amp, there is a steady and loud hum coming from the speakers. This hum is not affected by turning any of the knobs or buttons, and is present whether there is anything plugged into the input. The hum is obviously not there when the speakers are disconnected. When I plug in a guitar, you can hear the tones coming from the guitar through the hum, but they are distorted.
Troubleshooting:The first thought I had was that there's a DC offset on the speakers. Measuring the output (TP13) with my DMM (red probe on the + speaker terminal, black probe on either a ground or the - speaker terminal), I get 0.04v DC. That doesn't seem like enough DC to cause the loud hum, but I'm not sure what else would cause it.
I've replaced a handful of capacitors (including the main filter caps), but the hum has not changed. However, it is worth noting that my soldering skills are abysmal at best and it is entirely possible my joints aren't any good (I'm having someone else double check me in a few days). The caps I've changed are highlighted in yellow on the 2nd page of the schematic.
The results of the DC test points are listed on page 2 as well (in orange). There are several values that are off, but I'm not experienced enough to know what kind of variance would be acceptable.
Any interesting thing that I stumbled across, is that if I short the collector and emitter leads on Q5 (red circle 1 on the 2nd page) while the amp it turned on, I get a spark from the transistor and the hum goes away. After several minutes of the amp being on after that point, the hum comes back. I have tried to trace what might be causing the spark (e.g. a capacitor discharging?) and why the hum goes away, but again my lack of experience is holding me back.
I have found that if I remove Q5 (red 1) from the circuit completely, the hum goes away (although there is a new buzzing present in the sound from my guitar that is affected by volume and tone controls). But when I also remove the sister transistor, Q23 (red 2), the hum stays gone. But this time, the sound coming from the guitar is heavily distorted just like it was when the hum was present. Also, removing both transistors does nothing to change the 0.04v DC on the speaker (TP13).
Thanks to this thread (http://music-electronics-forum.com/t34148/), I noticed that the 0.04v DC is also on the base of Q14 (red 3) and also on the drain of Q16 (red 4, page 1). Then on either side of C68 is 0.04v (red 5) and 0.01v (red 6). The thread talked about a leaky Q16, which I am going to replace next...but something tells me that's not my problem.
I've heard many a person tell me not to bother with fixing the amp, but like I said it's become a personal quest for me and I'm confident I can fix it...I just need some clues. I've got to believe that the behaviors that I'm seeing can help trace this to the root cause...I just haven't been able to see it.
Any guidance or thoughts on where I should be looking would be oh so appreciated!
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