Sansui AU4900 incredibly loud crackling distortion on right channel

[slimecity]

So if you have no fluctuating DC in the channel with noise - I would be looking at other small signal transistors in that channel.

Does changing the volume make the sound louder/quieter - or does the sound stay at the same level when you do this?

 

[mcgreg66]

No volume doesn’t matter....
Even with volume shut down statical noise and incredibly loud pops.
I’ll check dc....

 

[theophile]

I used to have an amp that got so hot that you could fry bacon and eggs on it. It always had a crackling sound to it.

 

[mcgreg66]

No fluctuating DC on both channels.....
Can somebody give me advice how and where to start!

 

[mcgreg66]

I got the similar LCR Tester EZM328.....
I think I read I can test transistors in circuit?

 

[mcgreg66]

Wrong schematic!

 

[slimecity]

Ok - there are plenty of 2sa726 & 2sc1313 small transistors in this amp. They get leaky & noisy with age.

To fix this issue - and to prevent it occurring again in the future - I would recommend that you locate and pull all of these from the tone control board (F2555). Try doing this first.

There are modern substitutes: European BC series. Although if you didn't want to use these, I recommend you search on Google to find good subs. KSA992 for example can be used to sub 2sa726. See this:

http://audiokarma.org/forums/index.php?threads/sansui-au3900-noise-problem.712035/

 

[mcgreg66]

I already changed all 2sa726 and 2sc1313 in F2555....
I also changed diodes rectifier......
- fuse resistors
- electrolytic capacitors
Step by step and the Amp was running nice and quiet....

The pops and crackles don’t change with volume, input or any controls....

So imho it’s a problem on F2554 driver board?!

Or can it be the PSU?!

Maybe I should separate Preamp and Driver board to make sure where is the problem!

 

[restorer-john]

Absolutely lift the preamp end of C01/C02 and separate the power amp from the preamp stage.

The AU-4900 has the volume control back before the bass/treble gain stage so its position (turned up or down) will not affect the cracks you are getting.

Don't shotgun any more components in the power amp. You may have just replaced a whole bunch of stuff for no reason.

Once you have separated the pre and power, fire it up and report back. You can inject signal into the power stages and listen to the pre stages.

 

[mcgreg66]

I separated power stage from preamp by lifting wires coming from ToneControl to C01/C02.
Firing up I still hear the same kind of noise but much quieter!

So the problem is in the preamp section,right?!

Thanks for advice.....

 

[Reliving.]

Any more Gents I was just digging this problem ...am still learning more please .. Cheers Matt UK

 

[mcgreg66]

Please gentleman......
I need help how to continue......

I had alook for cold solder joints on preamp board....found some suspect but still the same problem.

 

[Reliving.]

A,m just a Starter in this Game .....but for me Caps make Bad Hums....? and Transistors make Bad Crackles. is this right................

Please help this Gent with his piece of HiFi History. and me...............

Heating or cooling Transistors can Help find the problem....

 

[mcgreg66]

As far as I know .....Yes!

 

[Reliving.]

whats the lastest on your great amp......

matt from slightly snowing UK.

 

[mcgreg66]

Ok...
I tried to bring it down to a few suspect transistors.....
The operation block diagram shows the left channel,so the other of each transistor pair should operate the right channel!
TR02 + TR04 belong to the phono stage right channel!
TR601+TR602 to mic amp!
I assume that they are ok, as the problem is not on phono stage or mic amp!
So I think I should look for TR06,08+10

I anybody would like to comment my thoughts....i would be very pleased!

 

[mcgreg66]

 

[mcgreg66]

I think I fixed it......
I checked the suspect transistors as far as possible in circuit - nothing!
Checked all voltages from the diagram around the transistors - everything in tolerance!
Checked all caps around - nothing!
Way before I had the feeling that it’s maybe coming from the volume pot......so at the very end I flooded the volume pot with tuner spray......and there is silence (as far as now)

So again ......knowlege is powerful....intuition sometimes more!!

 

[Reliving.]

well done that man....many many times its a simple fix just by taking something apart can give you One more Fault.......

 

[tube-a-lou]

I just picked up a AU-3900 and it had the dreaded static/crackling in the right channel so I changed both of the 2SA798's and no more noise and the amps sounding great so far. Thanks to all for added to this thread.