AU-8500 Noise From Tone Board

[LBPete]

I've got an AU-8500 that has loud, random crackling noises in the left channel. The right channel is clean. The noise does not appear to be affected by volume and occurs at zero volume. At times it's severe enough to kick it into protection.

I've isolated it to the tone board F-2019c. Removing the pre-out jumpers eliminates the amp side and It's present at pin 2 and at the volume control wiper. I've disconnected the wire that runs from the volume control wiper to pin 2. The noise is still present at pin 2 and without the wire connected the output from the volume control is clean.

Now here's the kicker, I've replaced all the transistors on the tone board. The only semiconductors remaining are the input FETs and some power supply related diodes. It's a common power supply for both sides of the board so I don't think the diodes could only affect one channel.

Can the FETs be making the noise?

- Pete

 

[LBPete]

Here's the schematic.

- Pete

 

[Robisme]

Is it easy to pull the FETs and swap them?

Rob

 

[LBPete]

No, the the damn tone board is buried. The component side is accessible from the bottom, so it's not too bad for testing. The foil side is the problem. Like most tone board it has a shitload of shielded wires attached to it and access is very limited. You have to remove the power supply board and even then access is very limited. Changing out the transistors took hours.

The FET is a 2sk30. The current production seems to be all surface mount. The ones in through hole cases are either incredibly cheap from questionable suppliers or incredibly expensive NOS units. It crosses to an NTE 459 which is in a TO72 package. Do I dare?

- Pete

 

[ManyMoonsAudio]

Hi Pete,

Yes, the FET can absolutely make this noise, and it sounds like you have it very well isolated. Have you tried teasing it with freeze spray? The NTE part is likely a 2SK30 with their markings on it, but you never know. I have a better option.....

I can IDSS match you a pair of 2SK117 GR that will surely do the job. Be happy to send them to you.

 

[LBPete]

Thanks Paul, I'll send you a PM with my address.

- Pete

 

[LBPete]

Hi Pete,

Yes, the FET can absolutely make this noise, and it sounds like you have it very well isolated. Have you tried teasing it with freeze spray?

I have hit it with freeze spray no direct affect during the application. After sitting for awhile afterwards, it did quite down. Hard to say if it was the freeze spray or just the randomness of the noise.

- Pete

 

[LBPete]

The FETs did the trick! It's nice and quiet now. This unit had the worst transistor noise I've ever heard. Sometimes the spikes were so high it would actually kick it into protection. These 2SK30s need to be put on the list of known noisy transistors.

Thanks for your help Paul.

- Pete

 

[Oldsansui441]

Good to hear you got it sorted .

 

[ManyMoonsAudio]

No problem Pete! Glad that cured it. Those nice new low noise FET's should sound quite lovely in that position.
I wouldn't put the 2SK30 on the shit list just yet. FET's are very reliable typically. I'd chalk this one up to sometimes things just fail.

 

[kevzep]

Nice one, yeah the old FET's are pretty robust, but they do fail occasionally...