Pioneer C 90a hiss issue

[zorba1960]

I bought a Pioneer C-90A and when I turn it on, there's a constant hiss in both channels that gradually decreases over time until it completely disappears after about 20 minutes. If I turn it off and back on, the hiss returns and again disappears completely after about 20 minutes. Where should I look to fix this issue? What do you recommend doing?

 

[Nigel Goodwin]

You don't give much information, is it on all inputs?, and does it disappear when the volume is turned right down?, and do other controls affect it?.

 

[zorba1960]

Hi

• Independence from volume: The noise level is not affected by how loud or soft you set the volume.
• Affected by tone controls and loudness: Adjusting the bass, treble, or loudness feature causes the noise level to change.
• Consistent across inputs: The noise is present regardless of whether you're using a CD player, turntable, or other input.
• Gradual decrease over time: The noise gradually diminishes and eventually disappears after about 20 minutes.

 

[Nigel Goodwin]

The noise level is not affected by how loud or soft you set the volume.

Still not very clear, with the volume control turned right down, do you still have any noise? - if you do, then the fault has to be after the volume control. Likewise, with the volume turned right down, the Loudness control should make no difference. The Tone controls can be switched out, so that eliminates those - so if volume right down, tone controls switched out, and you still get the noise then it only leaves the filter circuit - IC404 and IC401.

However, another issue is that it's on both channels - so it's got to be something common to both channels.

 

[zorba1960]

with the volume control turned right down, do you still have any noise? Yes the volume does not affect the noise(hisss).

Do you refer to this ones?

 

[zorba1960]

All the switch are done trough relays. Can be a faulty one with bad contacts?

 

[zorba1960]

The IC401

This IC401 its common to both channels

 

[rjenkinsgb]

The first thing I'd try is to remove and refit every plug-in connection a few times, to clean off any tarnish - tarnished connections and switches are a very common problem in audio gear.

Other than that, could it be something like a failing and leaky capacitor on one of the power supply rails, breaking down and adding noise?

Looking at the circuits, it could also be noise from the 5V logic & microprocessor supply area.

Try replacing C208 - C211 in the main supply, and/or C417 - C420, or temporarily add an extra cap across each in turn and see if the noise is affected?

At around 40 years old, some electrolytic caps may well be failing.