Hello! I am in the process of servicing a 2-Channel Dual Rec and everything has been great except for an annoying crackle. The crackle is definitely originating from the preamp section. It is a crackle that is more noticeable on the decay of louder notes and it sounds a lot like a bad speaker. To rule out the possibility of a bad speaker, I connected another speaker as well as a portable recorder at the EFX send and the noise was still present. (The signal was not clipping going into the recorder) The noise is also always present regardless of which switching and channel configuration I use. If I play through the EFX return everything sounds clear, but of course I would like to use the preamp section!
So far I have..
Eliminated possibility of power amp problems by listening to output from preamp section in multiple ways
Changed guitars and cables
Cleaned Preamp Tube Sockets
Cleaned input jack
Replaced preamp tubes (new JJ's and assortment of known working tubes)
Measured voltages at the plates and cathodes of the preamp tubes; all were within spec
I will probably go after the plate resistors next as well as resolder the preamp tube sockets. The PCB on this amp does have a little bit of flex to it as well, so maybe I'll turn the lights down and look for some arcing. If I can visualize the noise on the scope I may also see if I can narrow down which section is causing the problem. One thing that I have noticed is that when I assign the Red Channel to vintage, the Orange channel LED is dimly lit when the red channel is selected. Since the amp is barely ever used in this configuration, I am not sure if this is normal operation.
Natrually I've gone through a lot of the basics, but I am not sure if there is anything specific to this amp (in the switching section especially) that can cause the issues that I am having. So my question is are there are any parts or common problems specific to this amp that can cause this crackling noise? I really hope I don't have to deal with all that switching circuitry, but it never hurts to learn new things.
So far I have..
Eliminated possibility of power amp problems by listening to output from preamp section in multiple ways
Changed guitars and cables
Cleaned Preamp Tube Sockets
Cleaned input jack
Replaced preamp tubes (new JJ's and assortment of known working tubes)
Measured voltages at the plates and cathodes of the preamp tubes; all were within spec
I will probably go after the plate resistors next as well as resolder the preamp tube sockets. The PCB on this amp does have a little bit of flex to it as well, so maybe I'll turn the lights down and look for some arcing. If I can visualize the noise on the scope I may also see if I can narrow down which section is causing the problem. One thing that I have noticed is that when I assign the Red Channel to vintage, the Orange channel LED is dimly lit when the red channel is selected. Since the amp is barely ever used in this configuration, I am not sure if this is normal operation.
Natrually I've gone through a lot of the basics, but I am not sure if there is anything specific to this amp (in the switching section especially) that can cause the issues that I am having. So my question is are there are any parts or common problems specific to this amp that can cause this crackling noise? I really hope I don't have to deal with all that switching circuitry, but it never hurts to learn new things.