Sansui S-X900 noticeable white noise on left channel

[davidslv]

Hello all,

I've been noticing a white noise coming from the left side on this receiver.
I was wondering if anyone had experienced this sort of thing and if you did fixed it would you say it was capacitors, resistors, transistors or something else?

I have opened it yesterday to have a look in case I could see something obvious.
I checked the capacitors on the power supply area, they looked ok to me, resistors also looked ok, nothing broken or toasted.

I did notice that someone before me had marked the capacitors with a pink or green pen, what does it mean I don't really know, but I'm starting to think that someone was preparing to do a recap and then decided to sell it instead.

I've also checked for bad soldering points with a magnifier and I have not found anything in there either.

Now, this is obviously time consuming and I'm not sure that buying a new array of capacitors would solve the problem, so I was hoping that before I go down that rabbit hole I could get some hints on what to look for from someone with more experience doing this than me.

Before someone suggest to leave it alone and take it to a professional, I've got in contact with a local person to me, but it's been 2 months without any answer so it's time to learn how to do it myself.

I know you all love some photos, and there hasn't been anyone talking about this receiver, so here they are:

 

[davidslv]

I also want to add something else, maybe one of you have this receiver and can tell me why is this hanging around, and was this suppose to be screwed somewhere?

 

[CrSens]

Maybe dirty potentiometer, or loose connection at the potentiometer. Also work out the balance control a couple of times.
If not that, a transistor somewhere developed a noise issue, could be in preamp section, or at first stage of power amp, prior to output driver section. Might be hard to find which one of you don't have experience.

 

[davidslv]

Thank you, I will work my way through it (potentiometers and transistors then)

 

[davidslv]

I've cleaned the potentiometers, I never touch them, had no realised they were so stuck now... wow, it's night and day on the movement of those.

Cleaning didn't resolve the issue so I'll move to the next phases as soon as I can.

 

[davidslv]

I have cleaned the potentiometers (volume, balance, loudness, bass and treble) which were very sticky.

I've looked inside, I can't see anything obvious but while looking at the service manual and schematics I think I may have narrow down on the following transistors that are responsible for low frequency:

Board F-5764 (Power Amp & Power Supply Board)

Transistor

mQ1 - 2SD357
mQ3 - 2SB527
mQ5 - 2SB507
mQ6, mQ7- 2SD313HP

I also don't know if this could be related but I noticed rust around the STK3082 II IC unit.

 

[CrSens]

That STK is a small power amp on a chip used as a driver for output devices. It could be the source of your problems, the other transistors are not known to be problematic with developing noise over time. Unfortunately, finding the replacement is not easy, there are a lot of fakes floating around.

 

[davidslv]

That STK is a small power amp on a chip used as a driver for output devices. It could be the source of your problems, the other transistors are not known to be problematic with developing noise over time. Unfortunately, finding the replacement is not easy, there are a lot of fakes floating around.

I have replaced the STK (mostly because I killed the original one trying to find the source of the problem... silly me), the humming still occurs so the problem is definitely somewhere else. I don't have the time at the moment to chase this, I may end up sending it to a technician.

 

[Hyperion]

I also want to add something else, maybe one of you have this receiver and can tell me why is this hanging around, and was this suppose to be screwed somewhere?

That looks like a grounding tab - and should be screwed down to the metal chassis somewhere I think. Notice that it has ceramic capacitors attached to it which are often used to filter noise from parts of the circuitry. If you carefully examine the surrounding area for 'witness marks' you might be able to discover where it should be secured. Wherever it goes, it will be a 'natural fit' with little or no straining of the wires attached to it. It is possible that what it was originally attached to has been removed and lost, like an additional chassis screen/bracket or something like that.

You are right to think that maybe only someone who has one of these could tell you where it goes, and if there are any missing internal parts.