AC30CC2 crackling noise

[Ewizard]

I think it is possible that the C17 cap could be a part of the problem. To weed out the potential that your noise is coming from the Tremolo, you could remove C17 and see if the noise goes away. The symptoms you seem to be describing are not A typical of a bad cap, but a bad cap will do all kinds of strange things to a circuit including but not limited to crackling, popping, and other strange behaviors. It upsets the bias ( in most cases ) of the preceding circuit and this can cause the preceding circuit to go into cutoff or depending on how the cap failed, it can be any number of things that happen. In the case of a tremolo, it is an oscillating circuit that shifts bias in order to create the effect. That circuit in this case is connected to one of the PI's two primary inputs. Since the Tremolo is already acting weird, I would suspect that is the problem circuit. A way to test that theory is to remove C17 ( in this case ) to see if the problem goes away. You should not hear ANY remnants of the tremolo circuit at all with C17 removed. In which case, if your noise problems go away too, you know that the problem is in the Tremolo circuit for sure. I would assume based on the symptoms that the problem is because of a bad capacitor, and or in the switching circuit for the Tremolo. The tremolo not working when it should be, but only just audible at very high volume settings can mean the switching circuit is bad, or simply the bias shifting of the circuit has gone bad. The bias shifting of the circuit is created by the capacitors. Typically the only reason to change those capacitors is to alter the rate of the tremolo. The depth is controlled by resistance/pot, which can also be a failure point. The C17 cap you changed at the end of the Tremolo circuit likely has nothing to do with the actual functionality of the circuit and is only there for DC-decoupling and to potentially set the amount of bass the circuit will pass onto the next stage. A 100nf is larger than a .047uf cap and would allow all frequencies of interest to pass through, BUT, the Tremolo circuit doesn't pass audio per se, it shifts bias. So that change made at C17 to me would be a fruitless one. I would put it back to the original value.

Not saying this is what I would do, but If you don't care for Tremolo at all, removing C17 will rid the circuit of use and if that solves your problem, you could simply leave it that way? The schematics addition of having voltages is really helpful. You can test against that and probably nail down where in the Tremolo circuit the problem is located. Again, I suspect a failed capacitor, but that is just a hunch. I would weed out all other possibilities first before I shotgun a full circuit capacitor change.

 

[dudiluty]

I think it is possible that the C17 cap could be a part of the problem. To weed out the potential that your noise is coming from the Tremolo, you could remove C17 and see if the noise goes away. The symptoms you seem to be describing are not A typical of a bad cap, but a bad cap will do all kinds of strange things to a circuit including but not limited to crackling, popping, and other strange behaviors. It upsets the bias ( in most cases ) of the preceding circuit and this can cause the preceding circuit to go into cutoff or depending on how the cap failed, it can be any number of things that happen. In the case of a tremolo, it is an oscillating circuit that shifts bias in order to create the effect. That circuit in this case is connected to one of the PI's two primary inputs. Since the Tremolo is already acting weird, I would suspect that is the problem circuit. A way to test that theory is to remove C17 ( in this case ) to see if the problem goes away. You should not hear ANY remnants of the tremolo circuit at all with C17 removed. In which case, if your noise problems go away too, you know that the problem is in the Tremolo circuit for sure. I would assume based on the symptoms that the problem is because of a bad capacitor, and or in the switching circuit for the Tremolo. The tremolo not working when it should be, but only just audible at very high volume settings can mean the switching circuit is bad, or simply the bias shifting of the circuit has gone bad. The bias shifting of the circuit is created by the capacitors. Typically the only reason to change those capacitors is to alter the rate of the tremolo. The depth is controlled by resistance/pot, which can also be a failure point. The C17 cap you changed at the end of the Tremolo circuit likely has nothing to do with the actual functionality of the circuit and is only there for DC-decoupling and to potentially set the amount of bass the circuit will pass onto the next stage. A 100nf is larger than a .047uf cap and would allow all frequencies of interest to pass through, BUT, the Tremolo circuit doesn't pass audio per se, it shifts bias. So that change made at C17 to me would be a fruitless one. I would put it back to the original value.

Not saying this is what I would do, but If you don't care for Tremolo at all, removing C17 will rid the circuit of use and if that solves your problem, you could simply leave it that way? The schematics addition of having voltages is really helpful. You can test against that and probably nail down where in the Tremolo circuit the problem is located. Again, I suspect a failed capacitor, but that is just a hunch. I would weed out all other possibilities first before I shotgun a full circuit capacitor change.

Ok i changed C17 AND C18 back to it's original value and caps. The tremelo works better now.. not really loud but better(maybe the caps have to reload?:wink. The crackling is still there.. i'll first try cleaning/lube the pots with deoxit F5 this weekend and see if that will help.

Thanks for the help so far!

 

[dudiluty]

Ok deoxit didn't do the trick. I replaced all the tubes again to see if it get worse or better but it didn't. So all the tubes are fine. Every EL84 tube measures 304/305V when measuring plate voltage so looks fine right ?

wiggling caps etc doesn't make a change. So i quess it's just a bad pot somewhere? Or it's just the amp.....o

 

[Ewizard]

Try removing C17 and leaving it out. Does the problem go away then? If removing C17 eliminates the problem, you know for a fact that the issue is on your tremolo circuit. Which I suspect it is anyway.

The schematic has voltages for almost every part of the amp. You need to check all those too. This will help you nail down where the problem part may be.

 

[dudiluty]

Try removing C17 and leaving it out. Does the problem go away then? If removing C17 eliminates the problem, you know for a fact that the issue is on your tremolo circuit. Which I suspect it is anyway.

The schematic has voltages for almost every part of the amp. You need to check all those too. This will help you nail down where the problem part may be.

I'll try that tomorrow. I'll have to get a new fuse tomorrow first.. touched the other tube pin accidentally when measuring plate voltage fix it tomorrow and will remove C17 and see what happens..

Tnx again

 

[Ewizard]

Objectively we are trying to weed out the potential source of the issue. So removing the Tremolo from the circuit is one such way. C17 is the Tremolo's path into the audio portion of the circuit, no C17, no Tremolo. Since the noise still exists with V1 and V2 removed, this must mean that the noise is coming in from the PI circuit or even possibly later somewhere in the power amp section. We only want to do one thing at a time to ensure we don't shotgun the source of the problem. if you do multiple things at once, you never quite know what actually fixed it.

So, if C17 doesn't fix the issue, I would be looking at the coupling caps between the PI and the power tubes and caps on the HT supply, and the bias supply if there are any.

I know you said you checked the board for cold solder joints, but this noise is semi symptomatic of a resistor that has split or has a bad connection. Another way to test solder joints and broken resistors is to use a can of compressed air for cleaning keyboards turned upside down. The liquid that comes out is very cold and will help expose bad resistors and joints. The downside is cleaning the white residue off. I think you can find other freezing agents for this purpose ( like wart removing spray ) specifically for electronics? In either case, just another way to weed things out.

Keep plugging, and I look forward to hearing back!

 

[dudiluty]

I have removed C17 but nothing changed. I measured all the caps that are on the board and none of them are leaking. Measuring voltages in the tremelo circuit is difficult because they are swinging. (C28, C27 and C24)

When i look at the schematic is should measure 155volts between c28 and C27(D leg of MF3) but am i measuring wrong or does is this voltage rating not correct? Because i'm not measuring 155v.. almost nothing. Measuring with my DMM on 200v. I measure a swinging voltage between 02.0 and 20.0v on both C27 and C28. C24 measures between 0.00 and 05.0v

I can hear the tremelo working when a guitar is plugged in but it's just not that loud as is should be.

The amp it self sounds awesome now.. only the tremelo that isn't loud enough and still a low crackling noise(hear it best when no cable is plugged in).

 

[Ewizard]

Ok. So there is still the possibility that the power amp section is mad. There are power supply capacitors there as well as the coupling caps from the PI. The master volume and tone cut both go through a multi-cable via two connectors! That is a possibility. C21 and C22 both feed the PI. I don't have the power section of the schematic. It is sounding very much at this point as if the problem is in the power section between the PI and the power tubes. If removing the PI solves the problem ( the crackling ) it must be something around that. This means the volume, tone, tremolo ( which still isn't working right, but doesn't change when removed from the circuit ). Another set of resistors to check are R40, 41, and 42.

Something else that just caught my attention was the reverb circuit. It doesn't show where it comes and or goes from on the circuit board. If you even have that? This is another potential source of the issue if you have reverb?

Report back. keep plugging. it is something that is around the PI as best as I can tell. FX returns, and just bad connections in general can cause issues.

 

[dudiluty]

Ok i measured between R29, R 28 and R30 and got a voltage of 274v. So i quess that C21 and C22 are working fine ? (DMM set at 1000v)

I also measured R40, 41 and 42 but there is no voltage. If i measure with my DMM at 200k i got a value of 94.0 on R41 and 42. R40 measures 0.40 when DMM set on 20M. (These measurements with the amp off)

But you can't really measure resistors correct when there in the pcb right..

 

[Ewizard]

Sometimes checking resistors in circuits can be problematic, but you should be able to get the values correct across those particular ones. The R40 1Meg that reads .40 is a little concerning. That resistor has two pots that go around it ( depth and speed ), so see how that value changes when those pots are moved. Or simply lift one side of R40 and measure it out of the circuit. the voltage you read on R28-R30 is off a little but not entirely alarming. 20v off is not great, but not so bad that there is immediate worry.

Going back and listening to the crackle you mention, it isn't that bad, and to me, it sounds like resistor noise more than anything. being that it doesn't diminish when the V1 and V2 stage are removed, it could just be the Tremolo circuit is noisy. The fact that it doesn't work right is still a reason to believe that the tremolo circuit is making the noise. You can try lifting R40, C17, and C24 to 100% take it out of the circuit, that may expose it some more?

 

[dudiluty]

Ok i measured all 3 resistors 2 times. With the pots on max and on zero. DMM set on 20M. 220k shoukd normally measure 0.22 and 1M 1.00 out of the circuit. I already replaced the 1M before because i thought that was the bad one but that didn't help.

Max:
R40 0.36
R41 0.06
R42 0.10

Zero:
R40 0.00
R41 0.10
R42 0.04

my quess it's causing by something else the resistors are being out of value ?

 

[dudiluty]

See link for photo of measurements i did just now.

https://www.dropbox.com/s/7zzx0qcp8aj6ieb/20210324_052654.jpg?dl=0

 

[dudiluty]

Here is the schematic again. I took some time to measure everything on the board.

Note that every value with 20M after it is measured with the DMM on 20M.

All resistors measured with the amp off. And some(see notes) with amp on. All measurements with pot controls on 0/min.

Link to photo schematic: https://www.dropbox.com/s/on2fhsld52...70314.jpg?dl=0



P.s: I'm still alive

 

[Ewizard]

Ok, so some things in the circuit are giving you a challenge for measurement. So do this:

1. Lift one leg of R40, R41, and R42. And see if the measurements come up correctly then. Also, try and measure the components with the closest ohm range above the component value. If you have a 200k range with a 220k resistor, it will measure more accurately than if you are in the 20m range. A 100k resistor should be measured in the 20k range of the DMM, etc. etc.

2. Lift the legs of C17 and C24 since you already have R40 lifted. This is just to see if the tremolo is actually making the noise. This will completely delete it from the signal path.

3. As best as I can tell, the tremolo circuit is running too low on voltage. Most of your other voltages look good or are in an acceptable range. So if removing the noise goes away when the Tremolo is completely out of the signal path, I would look to the resistors and caps that are in the tremolo power supply. The switching circuit seems fine, but you are showing 0v where you should have about 155v.

The Tremolo is getting its HT power from R37 and C31. You have good voltage at R37, but nothing on the other side of it? C28 is the cap that keeps DC off of the Speed pot. You should have 274v on the HT side of R37 ( as you have already measured ) and you should have a little under 155v on the other side of R37. If not, it is possible that your culprit is C28 and or possibly even C31. Another possibility is that the Speed pot took a dump. Try and see if you can measure it for resistance ( may be tough in the circuit ), but should give you an idea of the pot is at least showing continuity through its sweep. Right now the suspect part to me is not having 140ish volts on the other side of R37.

 

[dudiluty]

Ok, so some things in the circuit are giving you a challenge for measurement. So do this:

1. Lift one leg of R40, R41, and R42. And see if the measurements come up correctly then. Also, try and measure the components with the closest ohm range above the component value. If you have a 200k range with a 220k resistor, it will measure more accurately than if you are in the 20m range. A 100k resistor should be measured in the 20k range of the DMM, etc. etc.

2. Lift the legs of C17 and C24 since you already have R40 lifted. This is just to see if the tremolo is actually making the noise. This will completely delete it from the signal path.

3. As best as I can tell, the tremolo circuit is running too low on voltage. Most of your other voltages look good or are in an acceptable range. So if removing the noise goes away when the Tremolo is completely out of the signal path, I would look to the resistors and caps that are in the tremolo power supply. The switching circuit seems fine, but you are showing 0v where you should have about 155v.

The Tremolo is getting its HT power from R37 and C31. You have good voltage at R37, but nothing on the other side of it? C28 is the cap that keeps DC off of the Speed pot. You should have 274v on the HT side of R37 ( as you have already measured ) and you should have a little under 155v on the other side of R37. If not, it is possible that your culprit is C28 and or possibly even C31. Another possibility is that the Speed pot took a dump. Try and see if you can measure it for resistance ( may be tough in the circuit ), but should give you an idea of the pot is at least showing continuity through its sweep. Right now the suspect part to me is not having 140ish volts on the other side of R37.

Ok R40, 41 and 42 measures good when legs are lifted.

When i have those resistors and C17 + C24 lifted there is still the noise.

 

[dudiluty]

Ok, so some things in the circuit are giving you a challenge for measurement. So do this:

1. Lift one leg of R40, R41, and R42. And see if the measurements come up correctly then. Also, try and measure the components with the closest ohm range above the component value. If you have a 200k range with a 220k resistor, it will measure more accurately than if you are in the 20m range. A 100k resistor should be measured in the 20k range of the DMM, etc. etc.

2. Lift the legs of C17 and C24 since you already have R40 lifted. This is just to see if the tremolo is actually making the noise. This will completely delete it from the signal path.

3. As best as I can tell, the tremolo circuit is running too low on voltage. Most of your other voltages look good or are in an acceptable range. So if removing the noise goes away when the Tremolo is completely out of the signal path, I would look to the resistors and caps that are in the tremolo power supply. The switching circuit seems fine, but you are showing 0v where you should have about 155v.

The Tremolo is getting its HT power from R37 and C31. You have good voltage at R37, but nothing on the other side of it? C28 is the cap that keeps DC off of the Speed pot. You should have 274v on the HT side of R37 ( as you have already measured ) and you should have a little under 155v on the other side of R37. If not, it is possible that your culprit is C28 and or possibly even C31. Another possibility is that the Speed pot took a dump. Try and see if you can measure it for resistance ( may be tough in the circuit ), but should give you an idea of the pot is at least showing continuity through its sweep. Right now the suspect part to me is not having 140ish volts on the other side of R37.

I just now replaced C31 and C28. Didn't help. Still 0 volts. The pot is working.. hard to measure but works.. there is sweep range.

 

[dudiluty]

I just replaced R37 with a 100k resistor and R38 with a 33k. Tremelo works again! But the voltage after R37 is swinging between 20 and 130 volts.

The crackling sound is still there. Not loud but when master is past 3, and normal + TB is on half you can hear it trough the speakers. But really low sounding so i quess maybe that's just hiss or tubes that are getting voltage then ? Is not loud...

 

[Ewizard]

Awesome!!!! In the recording you provided, the noise wasn't that bad. Now granted at substantial volume, any hiss or noise you hear is much more apparent. The big question is if it normal, or if it is a problem. I designed and built an amp that is so quiet at normal operating levels, you would be hard-pressed to know it was on at all. And that is even with single-coil pickups Point being if it makes a noise that isn't from the guitar, it will very evident that something is up. My Peavey triple XXX is not so quiet. It has a low level hum and a bit of hiss at normal levels. It is probably time for a cap job on that one...

The voltage swinging up and down on the other side of R37 is normal. That is the tremolo swinging the voltage up and down to bias the PI. That is what creates the Tremolo sound. The bias swing pulls the audio signal up and down with the voltage swing. My guess is that as you adjust the speed pot, the rate of swing in voltage would increase or decrease respectively. My bet is that R37 was bad or had a cold solder joint, but seeing as how it works now, doesn't matter.

I would still like to confirm that the R40 - R42 are measuring correctly out of circuit. And since you have pretty much replaced every cap in the amp so far, it wouldn't hurt to change the coupling caps between the PI and the power tubes. Although you don't have to.

Also since the noise does appear to be coming from the PI area, it is possible that the PI's plate resistors are just going south? They measure fine, but if you have some 1 watt 100K resistors to replace them with, the larger wattage resistor will help reduce hiss, and of course, fresh resistors rule out the plate resistors as being the source of the noise. Usually, the frying bacon sound is because of failing plate resistors.

It may just be entirely the tremolo circuit adding noise? The more resistors and things you have in the signal path, the more noise you get. Either way, I'm glad things are working again!

 

[dudiluty]

Awesome!!!! In the recording you provided, the noise wasn't that bad. Now granted at substantial volume, any hiss or noise you hear is much more apparent. The big question is if it normal, or if it is a problem. I designed and built an amp that is so quiet at normal operating levels, you would be hard-pressed to know it was on at all. And that is even with single-coil pickups Point being if it makes a noise that isn't from the guitar, it will very evident that something is up. My Peavey triple XXX is not so quiet. It has a low level hum and a bit of hiss at normal levels. It is probably time for a cap job on that one...

The voltage swinging up and down on the other side of R37 is normal. That is the tremolo swinging the voltage up and down to bias the PI. That is what creates the Tremolo sound. The bias swing pulls the audio signal up and down with the voltage swing. My guess is that as you adjust the speed pot, the rate of swing in voltage would increase or decrease respectively. My bet is that R37 was bad or had a cold solder joint, but seeing as how it works now, doesn't matter.

I would still like to confirm that the R40 - R42 are measuring correctly out of circuit. And since you have pretty much replaced every cap in the amp so far, it wouldn't hurt to change the coupling caps between the PI and the power tubes. Although you don't have to.

Also since the noise does appear to be coming from the PI area, it is possible that the PI's plate resistors are just going south? They measure fine, but if you have some 1 watt 100K resistors to replace them with, the larger wattage resistor will help reduce hiss, and of course, fresh resistors rule out the plate resistors as being the source of the noise. Usually, the frying bacon sound is because of failing plate resistors.

It may just be entirely the tremolo circuit adding noise? The more resistors and things you have in the signal path, the more noise you get. Either way, I'm glad things are working again!

Thanks for all your great help !

You mean R28 and R29 ?

I think i still have a couple of 1 watt 100k carbon film resistors laying around.

 

[Ewizard]

Yes, R28 and R29. Metal film resistors will be quieter and are more robust. Carbon film resistors are noisier and are more suspectable to heat ( less stable ). Metal film is flameproof, much more stable over temperature ranges we care about, and is quieter than both carbon composition or carbon film.