Sansui AU 4900 problem

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chiunana

Member
Joined 2012
#1
Hi there,
It's been a year now i got this Sansui AU 4900, i made a video of the problem i have..

https://www.youtube.com/watch?feature=youtu.be&v=0wEJfoN5FwQ


Crackling all the time. Actually it's strange 'cause for periods (days) the amp works good. But then, suddenly, it appears again and never ceases.
I got the service manual.
I searched for dry joints, couldn't find anything.

Do you have some hints of where to look at, which stage i should consider first?

Thanks to all of you.

Chiu
 
#2
Look at transistor legs....ones that have turned black particularly power amp input diff pair transistors can become noisy just like in your YouTube recording.
Heating/cooling individual transistors can help to track which ones are bad/worst.
Is the noise in one channel, or both ?.

Dan.
 
#3
Both, and doesn't matter which input i select.
I have several transistors with blackened leg.. But i remember pickin some out and measuring the voltage drops.. They were fine. Does the blackened leg suggest me something, like an overcurrent?

Btw, the bulb limiter shows activity in the same manner of the cracklin..

Thanks Dan!
 
#6
This is a typical behavior of a 2SA798 faulty in the input stage of the main amplifier

I will have to presume that even if you separate pre and main the problem will still exist

2SA798 is expected to be unobtainable so you may replace it with almost anything that starts with a like A970 and so on

Difference is that these are transistors that operate about 50 v and using parts that operate like 120 v will not produce the same sound

Look at things like 2sa844 normally easy to find

caution watch pinout some older 2SA models have the base in the left side

Kind regards
Sakis
 
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#7
Thanks to ll od you.
I checked Tr603, its voltage drops, and it's fine. Have to measure on board the supplies yet. Will do now.

I got the 2sa798 out, one of them (Ic2) and i do have a strange behaviour when i measure the voltage drops. It's not a steady value, but i might start at 0.7 v, then goes to 1.2 v, then again to 0.7 v, then to OL. I mean, it oscillates.. I think this is not good, right?

Edit

I can't tell for sure, but now the drops are stable at 0.7v circa, i cleaned the legs which were blackened. Same for Ic1, the other 798, the drops are fine..

@ Orit:

Should i test all the voltage drops of the transistors of the pre, in your opinion?
 
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#11
Cleaning the transistor legs won't help, the parts are faulty internally. This happens to all old Sansui amps, and the only way to fix them is to replace them. Checking the voltage drop isn't enough to see whether or not they work.

You can use KSA992 for the 2SA798 and most other PNP transistors in there, and KSC1845 for the NPNs. Pretty much no other transistor is suitable.
 
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#13
ok, so guys you are suggesting me to change every transistor of the pre and power board? except the out and driver transistors, i would say.
though, considering the noise is present indipendently of the volume knob, i would think the problem should be in the power amp section..
 
#14
Just replace the A798 for starts it will be a 5min fix ...then observe for other problems

Sansui amplifiers can make similar problems in the preamp sections but NOT this model
Small capacitors all around are to be replaced as a standard service procedure .The A798 though fails in this and many other amplifiers

As about the comment all Sansui amplifiers is an overestimation

we have are currently repairing 1000 audio devices per year take my advice ....
 
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#16
Hey, i got a question..
I found some 798s, new, for 10€ each. I've read different opinions about replacing a 5 pin transistor with 2 3-pin transistors glued togheter. There are also different options, i've read.

For instance, someone says you could replace the 798 with two ksa992, which could be replaced with 2sa733 (among others). I do have some 2sa733... Or obviously i would prefer not spend 20€ for just two components.. But only if possible!
 
#17
There is no point ...
-----check A798 specs easy to find anywhere
-----mind not only operating voltage but ft and cob
-----If dirt cheap A733 is close ( expected to be but higher voltage ) get 10 of them match them if possible ( not mandatory but will help to a better sound quality )
-----Place them on the pcb and strap them together so the share the same temperature ....

done

To ensure that your problem lays there just drop the 733 if specs match or close then you proceed to refine your work


Kind regards
sakis
 
#18
chiunana said:
ok, so guys you are suggesting me to change every transistor of the pre and power board? except the out and driver transistors, i would say.
though, considering the noise is present indipendently of the volume knob, i would think the problem should be in the power amp section..
Click to expand...

Since your Sansui is very old amp First thing you have to do is to:
Replace with fresh & quality new ones All electrolytic capacitors on power amp section !, PSU section ,
and later on pre. amp section to,
since dry and leaky old electrolytic caps usually tend to shifts the transistors operating points and can cause similar negative crackling effects as is presented in your YT video .
Further Check on power amp boards section for correct values of All trim pots .,
and finally check for oxidation particles between the pins of All small transistors on power amp section ,
and in the same time check for dry solder points around elements which suffered heating , on transistor pins and power resistors pins .

Best Regards
 
#19
chiunana said:
Hey, i got a question..
I found some 798s, new, for 10€ each. I've read different opinions about replacing a 5 pin transistor with 2 3-pin transistors glued togheter. There are also different options, i've read.

For instance, someone says you could replace the 798 with two ksa992, which could be replaced with 2sa733 (among others). I do have some 2sa733... Or obviously i would prefer not spend 20€ for just two components.. But only if possible!
Click to expand...

Never buy 2SA798 transistors! If they're new, they're fake, and if they're old, they will have problems just like all the others - and they're far, far too expensive.

Just two KSC992 or, indeed, 2SA733 will work just fine. No need to glue them together, either. Pretty much all differential pairs are simply close to each other, and that is enough. 2SA798 and 2SA979 (same thing) was an experiment - and a failed one at that.
 
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#20
It works!
I replaced the 798 with two 733's... Until now it works well, i keep it going warm a couple of hours to be sure.. And i'm testin it on some bad speakers, i got to test it with good ones to be sure.. Great job, thanks a lot all of you!

What if i would change its power chord? It's really old.. Would like to put a 3 terminal cable, but where do i put the ground terminal? Can i screw it on the chassi of the amp?
 
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