AC30CC2 crackling noise

[dudiluty]

Ok i looked at the photos on my phone at work today and i've seen something that makes me nervous. I think the 2 resistors that are marked on the photo in the attachment are 80k instead of 10k ! Look at the colors.... 10k should be brown, black, orange. NOT Grey, black, orange !

I'll open the amp again this afternoon to check to be sure ! Maybe this could be the problem of the crackling and the tremelo........



Update: checked them and they are 10k so there fine. I measured all the resistors and the values are good.

[dudiluty]

So when you unplug the V1 tube you have no issues at all? You should not hear any sound that is amplified from the V1 tube if it is removed. So if the problem goes away when you unplug V1, the problem is around V1.

A 12AX7 tube has two triodes in it. This means it has two stages of amplification. This means there are two plate resistors and two coupling caps that surround that tube. The first stage in V1 does the most amplification, meaning if there is noise there at all, it will make it VERY LOUD! The second triode in V1 does some fair amplification as well as it is generally used to make up the gain lost from the tone stack placed after the first stage. I don't have an AC30 schematic in front of me, but In many amps, there is a tone stack right after the first gain stage.

Report back.

Sorry was a little confused after looking and searching so much. But when V3(PI) removed everything is quiet(nothing to hear.. should be normal right?). When V1 and V2 are removed the crackling is still there.

I quess it's just a dirty/damaged volume pot of the TB or Normal channel. When only the master is on full i can't hear it now and when i turn the other 2 volumes there is the same crackling noise(when turning). So i think it is making this noise constantly. I'll try some Deoxit F5 this weekend and if that doesn't help i'll have to replace those 2 pots. Still don't now how because those damn chrome nut collars of the toggle switches are glued on

[Ewizard]

Ok, we are getting somewhere. So with V3 removed the problem goes away, then it is likely around V3. With V3 being the PI it can be a multitude of things. The coupling caps going to the power tubes could be bad. It may also be any one of the caps from C17 through C23. Any capacitor that is going into the PI or out of the PI basically. What may be suspect is the Cap you changed coming from IC1. That IC has a warning not to use any other type of device. You changed the capacitor coming from it, but that IC is the actual tremolo. The other transistor you have marked is just the switch that turns the Tremolo on and off. Being that your tremolo doesn't work, I would guess that your problem is there.

The downside to most tremolo circuits is that they are always on even when they are off. It is still connected to the PI after all. You can try removing the capacitor C17 to see if that eliminates the noise. Also, be very sure that all your grounds and screws that have ground connections are tight and well terminated to ground. In either case, I do feel your problem lies in the tremolo circuit, which can be a PITA to figure out ( so many components ). The schematic does give you voltages to check against, so that is helpful. You get the fun joy of testing voltages. When you find something that is off, see if you can figure out what component in that area is not working. Often times it is something as simple as a bad connection from a ribbon cable or to ground.

[dudiluty]

Ok, we are getting somewhere. So with V3 removed the problem goes away, then it is likely around V3. With V3 being the PI it can be a multitude of things. The coupling caps going to the power tubes could be bad. It may also be any one of the caps from C17 through C23. Any capacitor that is going into the PI or out of the PI basically. What may be suspect is the Cap you changed coming from IC1. That IC has a warning not to use any other type of device. You changed the capacitor coming from it, but that IC is the actual tremolo. The other transistor you have marked is just the switch that turns the Tremolo on and off. Being that your tremolo doesn't work, I would guess that your problem is there.

The downside to most tremolo circuits is that they are always on even when they are off. It is still connected to the PI after all. You can try removing the capacitor C17 to see if that eliminates the noise. Also, be very sure that all your grounds and screws that have ground connections are tight and well terminated to ground. In either case, I do feel your problem lies in the tremolo circuit, which can be a PITA to figure out ( so many components ). The schematic does give you voltages to check against, so that is helpful. You get the fun joy of testing voltages. When you find something that is off, see if you can figure out what component in that area is not working. Often times it is something as simple as a bad connection from a ribbon cable or to ground.

Thanks for the reply. So C17 could be the bad one in here you mean ? Coming from the IC1B. This one is stock 100nF and i replaced it with a Mallory 150's 0.047uF(mods)

I CAN hear the tremelo a little bit in the background when i put the volume really high. But it's really hard to hear through all the hiss when the volume is that high.

[ehdwuld]

Say you dont hear any crackle with the guitar plugged in?

So its only with nothing plugged into the input that he amp crackles or buzzes?

Shame on you with the amp on with nothing plugged in

I wouldn't worry it it doesn't do it with the guitar plugged in.

all my amps buzz and crackle if i turn them on with nothing in the input

I noticed that the other day when setting up my shelving unit of doom

[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?). 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.....

[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?