Pointers for diagnosing crackling 2-Channel Dual Rec?

[CJWalker]

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.

 

[vick1000]

Interesting. You say you have isolated it to the preamp? What tubes have you tested in it? Cleaned all the pots and jacks with Deoxit 5? Changed speaker cable (using a speaker cable right, not an instrument cable)?

Sounds like resistance where there should be none (dirty pots/jacks), allowing low signal to ground, or no resistance where there should enough to prevent low signal grounding (thermal resistor/open resistor). Does it do it cold/hot? Low/high volumes? High/low gain? FX Loop/no loop? Tube/silicon rectification? Etc...

Could be a diode I suppose, maybe power filtering. Tapping the signal chain at various points with a scope would be the only sure way to find it. Contact Mesa and they may have heard of similar issues.

 

[CJWalker]

Interesting. You say you have isolated it to the preamp? What tubes have you tested in it? Cleaned all the pots and jacks with Deoxit 5? Changed speaker cable (using a speaker cable right, not an instrument cable)?

Sounds like resistance where there should be none (dirty pots/jacks), allowing low signal to ground, or no resistance where there should enough to prevent low signal grounding (thermal resistor/open resistor). Does it do it cold/hot? Low/high volumes? High/low gain? FX Loop/no loop? Tube/silicon rectification? Etc...

Could be a diode I suppose, maybe power filtering. Tapping the signal chain at various points with a scope would be the only sure way to find it. Contact Mesa and they may have heard of similar issues.

Thanks for the tips! The sound gets louder/quieter with respect to the master volume(s). The crackle is audible no matter which channel/tone stack is used and in any configuration of the effects loop or recto's. Speaker cables were changed out for longer ones when I took the head off of the cabinet in hopes that there was a vibration or microphonic tube causing an issue. I also swapped out one of each of the cables since my cabinet is split and I wanted to eliminate the possibility of a bad speaker. Regardless, when I listened through the effects loop send, I could still hear the noise. Preamp tubes were swapped with a new set of JJ's, as well as some RCA's, Ruby's, Groove Tubes and some stock Mesa/EHX tubes. I used everything that I had here as well as a new set of tubes. The noise is completely eliminated when I plug right into the return of the effects loop no matter how hot of a signal I put into it. I feel that I can safely say that anything past the preamp section is very strong and sounds great to me.

In normal operation, the noise is much more evident when the gain of either channel is turned up. The gain controls almost act as thresholds where higher gain settings cause a greater frequency of the unwanted noise that I am hearing and was very easy to hear on the clean channel. At lower gain settings on both channels, the noise occurs at a substantially lower rate, but is still just as loud respectively. Loud transients cause a sound similar to digital clipping followed by a crackle that sounds similar to a failing speaker. I am not hearing any ghost note notes, just a tapering crackle.

I also cleaned the pots and reflowed the solder on the PCB mounted sockets this evening. After that, I went back over the preamp tube sockets and checked for proper tension and gave them another cleaning. So far I have not had luck getting rid of this ugly noise. I pumped a bunch of different signals in with a function generator and the waves looked fine. I could not get the amp to act up by gradually raising or lowering the levels. It seems to act up with loud notes and transients and I am not seeing any voltage drop when this happens if it matters. I did find some noise going on around the V2A grid resistor when I cooled things down. It was a 'bad op amp' kind of noise. I swapped the part out and nothing changed so I'm curious to see what's going on beneath. I am going to continue with the filter caps later this week and see what happens from there and post an update. Thanks!

 

[vick1000]

Filter caps are a good place to start, they are only good for around 15 years, and that's if the amp has not sat around unused for any length of time.

But I think I have heard what you are describing before. Almost like a fuzz effect or sizzle comes in at the end of a low palm mute, swells in and out.

Did you try cleaning all the jacks and pots, front and back? Tried combos of bold/spongy, tube/silicon rectification? Does FX send/return levels alter it?

I would suspect a grid resistor too, when they start to go bad, they can introduce a "hissing/fuzzY" artifact.

 

[CJWalker]

Filter caps are a good place to start, they are only good for around 15 years, and that's if the amp has not sat around unused for any length of time.

But I think I have heard what you are describing before. Almost like a fuzz effect or sizzle comes in at the end of a low palm mute, swells in and out.

Did you try cleaning all the jacks and pots, front and back? Tried combos of bold/spongy, tube/silicon rectification? Does FX send/return levels alter it?

I would suspect a grid resistor too, when they start to go bad, they can introduce a "hissing/fuzzY" artifact.

Yup I gave everything a good cleaning, especially those preamp tube sockets. Changing from tube to silicon didn't make a difference, dropping the voltage might have made the noise occur a bit sooner, but no major change there either. All is quiet when I plug into the effects return and drive the output section independently, but I always hear the noise when I no matter how I have the effects loop configured. Adjusting the send level only changes the level of the noise with the rest of the signal. I think something is unhappy around V2 which could have been from me agitating it. I replaced the plate resistors, the grid resistor and a bypass cap for the hell of it and nothing changed. :x Chopsticking around that grid resistor causes noise, so when I get the board out (shudder), I'll take a look underneath. I have a few soundclips of that noise I can post up too. Thanks again.

 

[vick1000]

Yeah, it's a major pain pulling the main board. but at least it will let you give the chassis a complete cleaning while it's out, ne telling what's under there. Could just be a reflow job needed, or a raoch egg

 

[CJWalker]

Yeah, it's a major pain pulling the main board. but at least it will let you give the chassis a complete cleaning while it's out, ne telling what's under there. Could just be a reflow job needed, or a raoch egg

Haha, yes indeed.

Here's what it sounds like coming right out of the preamp through the effects send.

https://drive.google.com/file/d/0BxTo6L1xF0KEUTFPYWh2UGhVREk/view?usp=sharing

 

[vick1000]

Wow, that's worse than I thougt it would be.

That sounds like signal degradation to me, like a bad solder joint, or connection. It could be a pot as well, or just a bad ground somewhere.

 

[CJWalker]

Wow, that's worse than I thougt it would be.

That sounds like signal degradation to me, like a bad solder joint, or connection. It could be a pot as well, or just a bad ground somewhere.

Ended up being 2 leaky caps in the red channel tone stack.

 

[vick1000]

Leaky? As in the fluid leaking out? How did you find them?

 

[CJWalker]

Leaky? As in the fluid leaking out? How did you find them?

No fluid leaking out; it was 2 of the film Orange Drop coupling caps. They were allowing DC to pass which in turn also fed back to one of the LDR's causing even more nastiness. I looked at a bunch of stuff and followed a signal with a scope and checked out the coupling caps between the stages with no luck so a few days later brought it to the doctor. He said those particular caps are more prone to failure and sure enough when he scoped it out the wave was all messed up. Learned a bunch of stuff and got some new screen resistors to replace the old crusty ones too.

 

[vick1000]

Prone because of the circuit design, or the capacitors themselves? In either case, do you know where they are specifically?

 

[CJWalker]

Prone because of the circuit design, or the capacitors themselves? In either case, do you know where they are specifically?

Pretty sure it's just which parts are going to fail first in a 20 year old amp. They're the film caps for mid and bass in the red channel tone stack. I may eventually replace the rest of the film caps in the signal path as well.