Sansui Au 4900 problem

If your fridge, A/C, freezer or whatever is on the same electrical circuit (breaker) you mean?

I do have some news.

There is a capacitor which causes a big hum noise coming out from the amp when i touch its "head". It's actually the first in the power amp section, an orange elctrolytic.
I will replace all the elctrolytics on that board. Maybe someone could tell me why it happens? I mean... It's like creating a ground loop by touching the metal plate of the cap, right?
I measured it, and it seems to be ok. 0.9 uF instead of 1 uF. Considering the tolerance, it's ok. But i am wondering why it happens....

Hi,

The orange 1uf electro on the input on the power amp is a signal coupling cap. The negative will be connected to the metal case of the cap, and hence it will be susceptible to noise/hum if you put your finger in contact with the case.

Re the crackle you are getting on the right channel, the 4900 has 2SA726 and 2SC1313 tr ask stores in the pre amp section ( and the phono amp as well). These are known to go noisy, particularly the 2SA726. It is likely at one of these in the right channel has gone noisy.it could also be the main amp input differential pair transistors - 2SA798 - this is a dual transistor in a 5 pin SIL package. These are also known to cause noise problems.

Cheers

John

Thanks for the explanation, John.

But... If the crackling is costant, independently of tone controls, shouldn't i think that the problem is in the power amp section?:scratch2:

Edit:
I've checked the 798, they work properly, i.e. the voltage drops are fine. Don't know if that measure would reveal noise problems though.

There are two transistors -TR08 2SA726 and TR10 2SC1313 on the F-2555 preamp board that are in the right channel. They are after the volume and balance controls, and before the tone controls. Typically, the crackle and/or hiss will not be noticeably affected by the tone controls if either of these transistors has gone noisy. As previously stated, it could also be the input pair (2SA798) on the main amp board F-2554.

Cheers

John

Ok, i'll try to replace them, i'll keep you all informed. Thanks again, a lot.

If you can get hold of a can of freezer spray you can try finding the exact transistor that is causing the noise. A short spray on each transistor and see if the noise gets better/worse after each, one is sprayed.

freezer spray....... you said in february you were going to get some! could work out easier than solder/desolder unnecessarily

Just wanted to say i'm back. Now i wanna repair it. I might post considerations or questions, you don't have to answer, i know being absent for so long is really not nice.
Life is not a linear system, though.

Hi, i solved by replacing the 2sa798's with a couple of 733's bounded togheter.. Just wanted to tell you, in case of similar problems.

Bye!