Sansui AU-8500 Scratchy Background Noise on Preamp Section

[avantbored]

Hi all,

Picked up a Sansui AU8500 few months ago for an extremely nice price during my self-imposed no-buy drought. A price too good to pass up!

Unmolested in terms of upgrades, but dirty as hell on the inside. I did my best to clean the interior of the chassis and controls with many Q-tips and soft brushes without completely dismantling the machine.

Was working with no noise for the first two weeks I had it in regular use. But now, after about two minutes of playing, scratchy, spitting background noise appears with music.

Couple things I've isolated:

1) Only with music. With no input signal from a source, the background is quiet (though with some hiss).
2) Only on the preamp section. I hooked it up to another preamp, and no background noise.

Question: Before I ask my tech to open it up, is there anything else to clean and hence document for him?

Just want to make sure I've done my own basic due diligence before I send it to him. The signal PCB boards plug into the chassis like computer cards, so how best to clean those? I've gotten at the controls already, manipulated them several times while the problem presents, with no audible change.

Thanks all -

Ryan

EDIT: Nevermind. Distortion on the right channel. Off to the tech! I know this thing has some known noisy transistors.

 

[Sui-Yogi]

Noise transistors on the pre-amp 2SC1313 and the compliment .

Potmeters on the back that controls the level of the input from the AUX

Oxidation on the Tone control switch..

I can keep on going but I assume when you said “Never mind” means your tech will short it out right ?

 

[avantbored]

Noise transistors on the pre-amp 2SC1313 and the compliment .

Potmeters on the back that controls the level of the input from the AUX

Oxidation on the Tone control switch..

I can keep on going but I assume when you said “Never mind” means your tech will short it out right ?

I'd cleaned the pots and switches when I opened it up to do my own cleaning -- I will look again before I call my tech. But yes, dealing with the noisy parts, I will ask him to investigate. Just wanted to do what I trusted myself to do to help him narrow down the issue. I will look at the AUX pot and the tone control switch. Thank you!

 

[avantbored]

I don't if it was the AUX pot or cleaning all the pots and switches, but that noise is gone!! Totally overlooked the AUX pot the first time I cleaned it. Thanks for the reminder, @ARGOU .

I read here at this page that the 9500 doesn't have a speaker protection feature. Is this true? Would that mean the 8500 doesn't have it either?

 

[inzite]

I don't if it was the AUX pot or cleaning all the pots and switches, but that noise is gone!! Totally overlooked the AUX pot the first time I cleaned it. Thanks for the reminder, @ARGOU .

I read here at this page that the 9500 doesn't have a speaker protection feature. Is this true? Would that mean the 8500 doesn't have it either?

It's almost certain it was the pots and switches all together - a slight noise floor type hiss is probably normal if you have your ears right beside the speakers. Congrats! the 8500 is lovely - i love mine both as an integrated and as a preamp to my monoblocks. Pretty sure both have speaker protection relay, my 8500 has a distinct click about 4 secs after turning it on. You should also get a multimeter to check your DC-offset and feel by hand of it's running overly warm also - for reference, my 8500 runs barely warm (about 25c in indoor temp of 22c)

 

[Sui-Yogi]

I don't if it was the AUX pot or cleaning all the pots and switches, but that noise is gone!! Totally overlooked the AUX pot the first time I cleaned it. Thanks for the reminder, @ARGOU .

I read here at this page that the 9500 doesn't have a speaker protection feature. Is this true? Would that mean the 8500 doesn't have it either?

Nice to hear that you solved your issue!!
The AU-9500/8500 have a protection circuit for sure, see the schematics and follow the drawing from the speakers terminal backwards and there you ll see the relay
(F-2030).
In my opinion and critical listening the amp it will need more that a typical re-caping and it will benefit if you apply the “Leestereo” upgrades an other extra “treatment” with auditable results is the replacement of the input J-Fets at the
pre-amp , metal film resistors at the
pre-amp and metal film resistors at the feedback loop on the power-amp.
In my ears the AU-8500 has a sublime
pre-amp section and a so-so power amp…
when in the AU-717 is the other way around . Combine the two models as a pre-amp and power-amp …I think… is maybe one of the best
“Sansui sound results”.

 

[avantbored]

Thanks, everyone, for your enthusiasm and attention to this.

A quick update:

The noise returned shortly after the most recent replies; I could mitigate it after turning the volume knob past 4 or 5, but it would return again. I did not chime back in to my thread since I was going to bring it to my tech. He reported that he could not see me until Feb since his bench was full.

All good. So, I thought I'd maybe run it once a week just to exercise the AU8500.

Then I remembered I neglected the speaker relay. "I should try that," I thought. The input here made me think I had not yet explored every avenue of cleaning that I could without desoldering anything.

It took some minutes to prise off the cover, but I did end up doing so yesterday, and cleaned all the contact points on the relay. (Common method explained in many posts on AK -- strips of card stock soaked in contact cleaner).


Soundwise: Smooth sailing yesterday. The definition in the low, mid, and high seems to have improved. I am compiling a list of things I'd like my tech to investigate and/or replace when I bring it to him. So far they include a relay replacement (preventative), @ARGOU's post about "Noise transistors on the pre-amp 2SC1313 and the compliment," offset, and bias.

I will continue to research on AK other gremlins people have found.