Can anyone ID this rather unhealthy noise coming from my amp?

[SatinMill]

noise.wav - 0.88MB


It comes through one channel only on my amp when the volume is altered, I recorded this noise from the pre-amp section out which rocking the volume knob back and forth. I started a thread assuming that it was noise from a worn out pot but it persists despite having replaced it. I also replaced the two 5.6k resistors on the board the pot is on with a better matched pair, this has not helped.

Categorized Schematics and Service Manuals for free download The schematics for my amp can be found there.

Any advice would be greatly appreciated, I'm incredibly curious as to what exactly is causing the issue, it seems so bizarre that an issue of this type wouldn't have been solved by replacing the pot.

Thanks!

[currentflow]

Originally Posted by SatinMill

noise.wav - 0.88MB


It comes through one channel only on my amp when the volume is altered, I recorded this noise from the pre-amp section out which rocking the volume knob back and forth. I started a thread assuming that it was noise from a worn out pot but it persists despite having replaced it. I also replaced the two 5.6k resistors on the board the pot is on with a better matched pair, this has not helped.

Categorized Schematics and Service Manuals for free download The schematics for my amp can be found there.

Any advice would be greatly appreciated, I'm incredibly curious as to what exactly is causing the issue, it seems so bizarre that an issue of this type wouldn't have been solved by replacing the pot.

Thanks!

It sounds as if you may have DC on the pot. Check for leaky electrolytic capacitors around the pot as well as placing a voltmeter across it. You can then compare left and right channels to see if they are identical. Does this symptom appear without any input leads connected to the preamp input?

Regards, currentflow

[currentflow]

I would initially check C501 (Left channel) & C502 (Right channel), both are 4u7 / 25V. In any case, there should be no DC at the preamp output sockets - test with tone defeat switch in both positions.

Regards, currentflow

[SatinMill]

The issue is before the volume board, thus it's definitely not those caps. I elimited that possibility first by swapping the output channels, the noise then presented on the right channel. I then switched the inputs to the board then the noise returned to the left channel.

This indicates the issue comes before the volume board yet is triggered by altering the volume.

The issue comes through both channels regardless of tone defeat and when it's set to mono predictably the noise comes through both channels.

[currentflow]

Originally Posted by SatinMill

The issue is before the volume board, thus it's definitely not those caps. I elimited that possibility first by swapping the output channels, the noise then presented on the right channel. I then switched the inputs to the board then the noise returned to the left channel.

This indicates the issue comes before the volume board yet is triggered by altering the volume.

The issue comes through both channels regardless of tone defeat and when it's set to mono predictably the noise comes through both channels.

IC801 is the only device in the signal path between the input sockets and the volume control which is why I suggested looking for DC offsets at the volume pot. S501 should be in the 'stereo' position.

Please confirm that the noise occurs with the input sources ALL unplugged. One of your source inputs may have DC on its output and this would be passed directly to the volume control via IC801.

Does this happen with any input selected, or just one?

Regards, currentflow

[SatinMill]

Originally Posted by currentflow

IC801 is the only device in the signal path between the input sockets and the volume control which is why I suggested looking for DC offsets at the volume pot. S501 should be in the 'stereo' position.

Please confirm that the noise occurs with the input sources ALL unplugged. One of your source inputs may have DC on its output and this would be passed directly to the volume control via IC801.

Does this happen with any input selected, or just one?

Regards, currentflow

I have no inputs plugged in and the noise presents no matter what input is selected. S501 is indeed set to stereo, I merely toggled it to establish that the issue persisted on either setting.

The IC seems to be available still, but it's not particularly cheap so if it needs replacing it's doable.

[currentflow]

Originally Posted by SatinMill

I have no inputs plugged in and the noise presents no matter what input is selected. S501 is indeed set to stereo, I merely toggled it to establish that the issue persisted on either setting.

Have you measured across the volume control yet (both channels)? DC will make such a sound.

[SatinMill]

Originally Posted by currentflow

Have you measured across the volume control yet (both channels)? DC will make such a sound.

Yeah, there's like 15V on one channel! This is troubling. I measured this between 1 and E on TX-457.

[currentflow]

Originally Posted by SatinMill

Yeah, there's like 15V on one channel! This is troubling.

Then it seems highly likely that you will have to replace IC801. However, it would be a good idea to check the supply rails are correct on pins 9 and 22 (+18.2V and -18.2V respectively) of this chip before condemning it to the bin.

Regards, currentflow

[Mooly]

As curentflow says, it could well be a faulty IC but first measure on EVERY pin of the IC before making a decision.

Too many years spent as a bench tech and the saying "it's never the big IC" come to mind.

Could it be spillage ?