AC30CC2 crackling noise

[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.

[dudiluty]

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.

Ok i think i will change all the replaced carbon comps to 1watt carbon films today. I'll let you know

I only have 1/4 watt metal films in stock.

[dudiluty]

I changed all the resistors and reheated all the solder joints. The crackling got worse and the tremolo stopped working again. i brought the amp to my amp tech 'cause i'm out of patience. I think he will know how to deal with it.

Thanks for all the great help ! I'll let you know what the problem was.

[Mincer]

I'd love to know what your amp tech says. This kind of thing is super frustrating.

[dudiluty]

I'd love to know what your amp tech says. This kind of thing is super frustrating.

Yeah i told him.. and no matter what the costs are... just fix it! Lol

[Ewizard]

that sucks!!! That is the major downside to PCB based amp designs. they really aren't that serviceable. It especially is maddening when you get it working and then it breaks again. It could be a bad trace, another loose part you fixed inadvertently and then made mad again. The good thing is that you found the problem again before you were on a mission-critical stage.