Sansui 9090 noise dead end

[EricRa]

Hello everyone,


First of all, thanks everyone for all this knowledge shared in this forum.

I am restoring a Sansui 9090 early production that was not working.
It had plenty of issues that I had to fix (some output transistors failed, driver board, protection, service bulletin not applied).

Now the unit is working, but I still get a noise. I would like to continue fixing before recaping.

• Controls cleaned (deoxit)
• Dim bulb is ok
• Noise is present with zero gain, increases as the gain increases
• Noise on both channels, whatever speaker the selection is
• No noise before relay comes out of protection
• Noise is independant from the source
• Noise acts the same with jumpers pre-out main-in removed
• If I tune the treble it changes the intensity of the noise

I replaced the transistors on the power board (2546), as I discovered an old heat trace on TR02.

• 2SC1211 with KSC2690 that is overkill but works (KSC2334 creates oscillation due to its frequency, creating vu meter needles to bounce)
• 2SD313 with MJE15030

I checked the ground connection on 2546 and it seems ok.
I changed the transistors on the driver board, and all 2SC1313 that are known to become noisy over time.
I changed all the capacitors (as most little ones had very high ESR or were dead) on :

• Power
• Driver
• Tone (both)
• Equalizer
• Protection

I still get the noise…

I feel like I am in a dead end now…
Also, I do have an oscilloscope with built-in AWG, but I do not know how to see the noise on it.

Thank you for your help !!

 

[Robisme]

What is the sound? A low hiss? Hum? Hiss may be normal.

You state that it gets louder as gain increases. Do you mean as you turn up the volume? Also that it does the same thing when the pre/main jumpers are removed. This is confusing.


Rob

 

[EricRa]

Hello Rob, thank you for your help.

The sound is a hiss, but not that low.
Yes, it gets louder when I turn up the volume.
I just checked again with pre/main jumpers removed and I have no hiss anymore.

Thank you !

 

[EricRa]

Hello again,

I still have a hiss.
Does anyone have any idea how to see this on an oscilloscope ?
Or where might the hiss come from ?

Thank you.

 

[Hyperion]

Noise acts the same with jumpers pre-out main-in removed

I just checked again with pre/main jumpers removed and I have no hiss anymore.

The most likely cause of noise would probably be the BA-312 transistor arrays which have a habit of going noisy so I understand. The BA-312's are all located in the signal path before the 'pre-out / main-in' jumpers. However your first quoted statement above seemed to indicate that the noise was still present coming from the power amp with the pre-amp disconnected, hence the confusion, thank you for clarifying.

Replacement BA-312's are no longer available - however there are discrete component replacements (on a tiny PCB), which solve the problem completely. Do an AK search for BA312 replacement - and you should find the information you require, if you can't find it, let us know, and we'll find a link for you - link added below.

Here is a link
https://audiokarma.org/forums/index.php?threads/ba312-replacement.973016/page-6#post-15868299

Several AK members have made their own versions, one of which I have used with great success.
https://audiokarma.org/forums/index.php?threads/ba312-replacement.973016/

 

[EricRa]

Hi Hyperion,

Thank you for your answer.

Quick questions :
- are BA312 to change on the tone board (F-2544) or equalizer (F2541) ? Both have the IC.
- Can I see the noise with an oscilloscope and if so, how ?

Thank you !

 

[Hyperion]

Quick questions :
- are BA312 to change on the tone board (F-2544) or equalizer (F2541) ? Both have the IC.

The BA-312's on the F-2544 (qty=2) and F-2541 (qty=2) boards should be replaced, there is another one the F-2543 board for the Mic amp which you may want to change also.

- Can I see the noise with an oscilloscope and if so, how ?

You will see this noise as 'grass' on the oscilloscope trace, (so called for its appearance) - if you monitor the output of the power amp, and adjust the sensitivity of the oscilloscope to say 10mV/cm or better, and then disconnect the power amp from the pre-amp you should then see the height of the 'grass' decrease accordingly.

 

[EricRa]

Thank you !
I will change all of them. I will post back when BA312s replaced.

I will take a look at the "grass" also.

Thank you.

 

[EricRa]

Hello !

I received BA312 replacement (eBay).
It totally fixed the hiss.
The receiver is almost dead silent now.

Thank you very much @Hyperion !