Breezio

Knob cracking noise on AMC CVT3030
« on: February 20, 2019, 01:53:09 PM »
Dear all,

After some extensive fixing of my hybrid amp AMC CVT3030 I am now facing the final challenge.

When I turn the knob Direct/Normal/Phones(Mute) I hear a rather loud cracking noise which eventually fades out after around 10mins of operation.

The knob is on the solid state preamp section (see schematic attached).

I have already replaced all the elcos on the signal path (C309/C311/C313 and C310/C312/C314) with Elna Silmic II.
In parallel to these caps there are 0.2uF Wima film caps.

In addition, I also replaced other elcos such as C305/C306, C325, C324 (all Elna Silmic).

I also measured the voltage over time at the switch (top of R323/R324) and here are the results. Values are in mV.

Measure Point R323 (L)
30sec: 230
1min: 110
3min: 70
5min: 55

Measure Point R324 (R)
30sec: -290
1min: -240
3min: -65
5min: -7

Any suggestions where else to look?

Thanks again

Fabrizio


Breezio

Re: Knob cracking noise on AMC CVT3030
« Reply #1 on: February 23, 2019, 01:07:26 PM »
Hi everyone, any feedback on this?

JohnRoberts

Re: Knob cracking noise on AMC CVT3030
« Reply #2 on: February 23, 2019, 01:48:00 PM »
The obvious suspects are leaky electrolytic caps  C313 C311...?

JR
John Roberts
http://circularscience.com
Tune it, or don't play it...

Breezio

Re: Knob cracking noise on AMC CVT3030
« Reply #3 on: February 23, 2019, 03:08:26 PM »
Thanks JR but as mentioned above I already substituted those caps

JohnRoberts

Re: Knob cracking noise on AMC CVT3030
« Reply #4 on: February 23, 2019, 03:12:15 PM »
Of course you checked the polarity?

Old capacitors may need to be formed in... leakage may reduce over time.

JR
John Roberts
http://circularscience.com
Tune it, or don't play it...

Breezio

Re: Knob cracking noise on AMC CVT3030
« Reply #5 on: February 23, 2019, 03:15:00 PM »
Yes indeed

Breezio

Re: Knob cracking noise on AMC CVT3030
« Reply #6 on: February 24, 2019, 04:16:20 PM »
If the problem is too complicated I’m happy to eliminate the Normal position, provided no cracking is present when switching between phones (mute) and direct.

What would you advise to do in that case?

Re: Knob cracking noise on AMC CVT3030
« Reply #7 on: March 03, 2019, 03:34:55 AM »
If the problem is too complicated I’m happy to eliminate the Normal position, provided no cracking is present when switching between phones (mute) and direct.

What would you advise to do in that case?

Uh, well let's slow down here... for starts, did you alter any capacitance values? I would take the WIMA's off, make it stock and see what happens first.

nashkato

Re: Knob cracking noise on AMC CVT3030
« Reply #8 on: March 03, 2019, 02:49:28 PM »
you looked at the Jfets Q315/316 ?
i´m not sure though looking at the schematics , but they´ve been a culprit at my 3030 .
you built in tube sockets , too ?

Breezio

Re: Knob cracking noise on AMC CVT3030
« Reply #9 on: March 03, 2019, 11:29:34 PM »
you looked at the Jfets Q315/316 ?
i´m not sure though looking at the schematics , but they´ve been a culprit at my 3030 .
you built in tube sockets , too ?


Hi Nashkato,

Do you mean q309/313 and q310/314? Good to hear that you had the same problem and managed to solve it. I wonder where you can still find the bss92 fets at reasonable prices.  Where did you get them?

And yes, I modified the amp to accomodate sockets. Now using golden lion kt77...very very nice!

Grüße

Fabrizio


Breezio

Re: Knob cracking noise on AMC CVT3030
« Reply #10 on: March 03, 2019, 11:37:19 PM »
Uh, well let's slow down here... for starts, did you alter any capacitance values? I would take the WIMA's off, make it stock and see what happens first.


Thanks, that’s another option to check

Re: Knob cracking noise on AMC CVT3030
« Reply #11 on: March 04, 2019, 01:13:59 AM »

Thanks, that’s another option to check

I mean, did you raise the capacitance values on the electrolytics in the signal chain (309-314, ignoring the fact that you introduced the WIMA's)?

Breezio

Re: Knob cracking noise on AMC CVT3030
« Reply #12 on: March 04, 2019, 01:17:03 AM »
Hi, the .2uF wimas are in parallel to the new electrolytes whose values are 22uF just as in the schematic

Re: Knob cracking noise on AMC CVT3030
« Reply #13 on: March 04, 2019, 01:55:52 AM »
Hi, the .2uF wimas are in parallel to the new electrolytes whose values are 22uF just as in the schematic

Right ok, then that really shouldn't do much.

So any way that you turn the switch the crackle happens? Like if you go from normal to mute, you'll hear crackle in the headphones, or if you go from normal to direct, you'll also hear crackle?

I'm presuming by your list of voltages that this is in both speakers as well... also presuming that the headphone out and tape out are likely from the same source (L-CH, R-CH).

If it's in the headphones when you switch to mute is it possible that it has to do with the treble/bass filter? You cleaned all switches and pots heavily with deoxit and checked them for oxidation on the contacts (if possible to dissassemble without harming) right?

Literally, the switch being dirty could cause this if it's the kinda 'crackle' i'm thinking of, but I'm sure you know that. I know you said 'cracking' and not 'crackle' but that term does leave a vague idea of what the sound is really like to the imagination.
« Last Edit: March 04, 2019, 02:10:28 AM by davemascera »

nashkato

Re: Knob cracking noise on AMC CVT3030
« Reply #14 on: March 04, 2019, 10:10:26 AM »
Oh man , that SM sucks without the unit in front of you ... sorry but it´s been some time i was in there .
Where is CN601 ?
Quote
Do you mean q309/313 and q310/314?
no , i really meant those Muting Jfets 315/316

Quote
Good to hear that you had the same problem and managed to solve it. I wonder where you can still find the bss92 fets at reasonable prices.  Where did you get them?
Didn´t have the same Problem , mine had completely burnt Connectors (PS to poweramp ,Do yourself a favour , throw them out immediately and hardwire  , they will fail )
and dead  muting fets .

Breezio

Re: Knob cracking noise on AMC CVT3030
« Reply #15 on: March 04, 2019, 12:43:48 PM »
Thanks Nashkato,

Now I see the fets that you mentioned, I could not find them ;-)
I already changed the connectors with new ones and everything works fine.

Btw, did you make any mods?
In the thread below you can read about a few things I recently reviewed on mine and now I am very pleased with it (except for the cracking which is not a big issue as I never change the direct position...unless one of my small kids do ;-).

https://www.diyaudio.com/forums/tubes-valves/334194-amc-cvt3030.html

Cheers

Fabrizio

Breezio

Re: Knob cracking noise on AMC CVT3030
« Reply #16 on: March 04, 2019, 01:06:28 PM »
Right ok, then that really shouldn't do much.

So any way that you turn the switch the crackle happens? Like if you go from normal to mute, you'll hear crackle in the headphones, or if you go from normal to direct, you'll also hear crackle?

I'm presuming by your list of voltages that this is in both speakers as well... also presuming that the headphone out and tape out are likely from the same source (L-CH, R-CH).

If it's in the headphones when you switch to mute is it possible that it has to do with the treble/bass filter? You cleaned all switches and pots heavily with deoxit and checked them for oxidation on the contacts (if possible to dissassemble without harming) right?

Literally, the switch being dirty could cause this if it's the kinda 'crackle' i'm thinking of, but I'm sure you know that. I know you said 'cracking' and not 'crackle' but that term does leave a vague idea of what the sound is really like to the imagination.

Hi,

I hear cracking (or crackle??) L and R whenever I change any position and I can hear it also on the headphones (and yes, the source is the same as per schematic).
I had heavily soaked the related knob switch with contact cleaner but the problem persists. I'll give it another soak and see if anything changes.

Re: Knob cracking noise on AMC CVT3030
« Reply #17 on: March 05, 2019, 02:01:17 AM »
Hi,

I hear cracking (or crackle??) L and R whenever I change any position and I can hear it also on the headphones (and yes, the source is the same as per schematic).
I had heavily soaked the related knob switch with contact cleaner but the problem persists. I'll give it another soak and see if anything changes.

I would think that 'cracking' refers to an isolated instance, where crackle might refer to an ongoing sound. Maybe I'm the weirdo though who knows.

So if you switch it to MUTE and you hear it in both the speakers and the headphones simultaneously, it must be the switch right?

Because switching to MUTE is supposed to isolate the power amp input from the preamp output by grounding the input. So the only thing touching both parts of the circuit at that point that could be causing a common noise are the 4 separate poles of the switch (unless you've modified something you haven't made us aware of, which is why I always say return to stock).

So even if that JFET is bad, you shouldn't hear it in the headphones when set to MUTE because that section should then be isolated from the line that goes to the headphones.

Since all 4 poles of the switch are isolated, you might expect if it's dirty or corroded that the 'cracking' noise is a bit different in each speaker and headphone. Meaning they shouldn't be identical to one another. And I mean literally identical. They may still sound very similar.

EDIT: By the way, often bathing or spraying a switch with Deoxit doesn't really do the trick. While it's wet or soaked or sprayed or whatever, you often have to rapidly flip it to help break up whatever corrosion there might be. It might sound a bit weird that dirty contact could cause this for 10 minutes straight, but I've actually had pots that took way longer than that to stabilize and stop making noise each time they were moved.

Of course, always check the solder points of all the things you removed and reassembled... if you rap on the chassis a bit does the noise happen? This could help maybe isolate the issue as a soldering problem.
« Last Edit: March 05, 2019, 02:21:39 AM by davemascera »

Breezio

Re: Knob cracking noise on AMC CVT3030
« Reply #18 on: March 07, 2019, 02:39:14 PM »
Thanks Dave for the detailed analisys.

As per your explanation I can confirm that what I hear is cracking because it happens only when I switch.
So I will work more on the potentiometer with the help of contact cleaner and see what happens.
Btw, I did not actually soaked it. I meant that I used a lot of cleaner while switching left and right.

I will let you know how that goes

Cheers


 

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