5e3 - crackling and whining on 2nd channel

[d. spree]

My first ever tweed deluxe build (and the only one I've built from a tubedepot kits with PCBoard) has developed crackling and whining sounds on the 2nd channel. It was fine for the first year or so of playing. These annoying noises start when the amp has been on a while (15 - 40 minutes) and whether or not the guitar is plugged in. They respond to the volume control on te 2nd channel- no volume no problem. When I plug into the first channel there is no problem.

I checked and switched out all the tubes and eliminated that as the problem.

So my question is where do I start? Could this be a problem in the volume pot itself, as in Bad Pot? I checked all the wiring and it all seems well soldered and connected. The grounding wiring also seems solid. Its a little tricky to check compared to my two other builds which were from Mission and which have the traditional and - in my view - far easier to check grommet boards.

Any advice will be greatly appreciated.

[soundguruman]

My first ever tweed deluxe build (and the only one I've built from a tubedepot kits with PCBoard) has developed crackling and whining sounds on the 2nd channel. It was fine for the first year or so of playing. These annoying noises start when the amp has been on a while (15 - 40 minutes) and whether or not the guitar is plugged in. They respond to the volume control on te 2nd channel- no volume no problem. When I plug into the first channel there is no problem.

I checked and switched out all the tubes and eliminated that as the problem.

So my question is where do I start? Could this be a problem in the volume pot itself, as in Bad Pot? I checked all the wiring and it all seems well soldered and connected. The grounding wiring also seems solid. Its a little tricky to check compared to my two other builds which were from Mission and which have the traditional and - in my view - far easier to check grommet boards.

Any advice will be greatly appreciated.

I would suspect bad contacts on the input jacks, which should be cleaned with deoxit 100...
I would suspect a bad 100K plate resistor especially if you used carbon composition resistors.
Anyhow, with carbon resistors, the leads are GLUED in, and tend to crack.
You can sometimes locate a thermal defect by spraying the components one at a time with freeze spray...then waiting a few moments for noise to start. The sudden contraction of the carbon will trigger the noise, and you can sometimes find the defective component...
Anyhow if you do replace the plate resistors, use 1 watt, and I recommend metal film.
Although the circuit does not come close to a 1 watt draw, the resistors tend to break down from the cycle of heating and cooling over time. That's why 1 watt resistors will increase the reliability.

[d. spree]

I would suspect bad contacts on the input jacks, which should be cleaned with deoxit 100...
I would suspect a bad 100K plate resistor especially if you used carbon composition resistors.
Anyhow, with carbon resistors, the leads are GLUED in, and tend to crack.
You can sometimes locate a thermal defect by spraying the components one at a time with freeze spray...then waiting a few moments for noise to start. The sudden contraction of the carbon will trigger the noise, and you can sometimes find the defective component...
Anyhow if you do replace the plate resistors, use 1 watt, and I recommend metal film.
Although the circuit does not come close to a 1 watt draw, the resistors tend to break down from the cycle of heating and cooling over time. That's why 1 watt resistors will increase the reliability.

Thanks for the immediate response - unfortunately the problem occurs when nothing is plugged in to the input jack on the second channel which is why I am trying to track back where in the circuit the problem is.

[Bruce / Mission Amps]

Get yourself a 12" to 20" long, alligator clip lead with a .1uF to.22uF 400v-630v plastic cap on one end.
Ground the clip end of this gadget and start poking around with the capacitor end... from the preamp stage to "ground out" the audio on grids and on plates.
You'll find where it does and "doesn't" do anything.
Eventually you'll then learn where this noise has to be generated, by the simple process of elimination.
By the way.... a .1uF to .22uF cap will hold a high voltage charge and startle you if you get between it and the ground... careful now.

[d. spree]

Get yourself a 12" to 20" long, alligator clip lead with a .1uF to.22uF 400v-630v plastic cap on one end.
Ground the clip end of this gadget and start poking around with the capacitor end... from the preamp stage to "ground out" the audio on grids and on plates.
You'll find where it does and "doesn't" do anything.
Eventually you'll then learn where this noise has to be generated, by the simple process of elimination.
By the way.... a .1uF to .22uF cap will hold a high voltage charge and startle you if you get between it and the ground... careful now.

Thanks Bruce - once again! On my chopstick test (which i decided to do one more although it revealed nothing the first time) I found that I got a huge crackling from the second coupling cap - the one wired from the 6 pin of the first preamp tube to the 2nd volume pot. I swapped out this capacitor and the problem is solved.

How does a bad cap affect the sound like that? This was an orange drop.

I also wondered if the problem was something I couldn't see behind the PCB - such as one of the leads from the capacitor making some contact with the chassis - but didn't want to try to take the board out. So I was careful not to push too much of the lead from the new capacitor through the tiny hole in the PCB.

Either way it felt good to repair it. The reason I started building these amps last year was after paying $300 to have someone repair a blues junior - no knock on any techs I just couldn't afford that and figured I wanted to build something I understood. Of course the problem with the tubedepot kit was, that while the instruction book was incredibly clear, the PCB didn't help me understand the circuits the way the real deaal does with the grommet board. So after building that kit I built two Mission kits. Much more satisfying, and I learned more about how the tweed actually works.

Anyhow all who responded, thanks again.