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Recommendations on professional restoration of an AU-919

marsellas3 said:
I'm sure that this top amp must be definetely dead silent.
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Sorry, that was the end of my post. I asked to help me with the following:
I've just performed restoration and the total recap of my 919 (except ovals which were tested with good results). The sound is great (as it was predictable), but I noticed the issue with the preamp power transformer hum (50hz noise) which goes thoughout to the spearers. The same noise -mechanital from the transformer and audio noise from the speakers (begins after 1-2 mins after switching on). It does not depend of the volume level. The ground is definetely unsuspected. All screws tightened good. Is it the issue with the transformer or just the matter to replace ovals by new ones? Have you any idea how to fix this issue? I'm sure that this top amp must be definetely dead silent.
Thank you veru much!
 
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I am understanding from your post that you are experiencing two kinds of noise from your Sansui AU-919, transformer noise and audible noise from the speakers. Is this correct?

Transformer Noise: Have you put your ear near the transformer and you can physically hear it humming? If it is physical transformer hum caused by vibration of the transformer it can be because the glue inside the transformer windings is coming loose with age or it could be because the idle voltage is set incorrectly, causing the transformer to hum (or some combination of these things). Sometimes tightening the mounting screws on the transformer can lessen the vibration but may not fix the underlying problem of why it is vibrating.

Audible Noise from Speakers: What does the noise coming out of the speakers sound like?
 
americannigh said:
I am understanding from your post that you are experiencing two kinds of noise from your Sansui AU-919, transformer noise and audible noise from the speakers. Is this correct?

Transformer Noise: Have you put your ear near the transformer and you can physically hear it humming? If it is physical transformer hum caused by vibration of the transformer it can be because the glue inside the transformer windings is coming loose with age or it could be because the idle voltage is set incorrectly, causing the transformer to hum (or some combination of these things). Sometimes tightening the mounting screws on the transformer can lessen the vibration but may not fix the underlying problem of why it is vibrating.

Audible Noise from Speakers: What does the noise coming out of the speakers sound like?
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Hi! Thank you very much for your comment.
Yes, you're definetely right. I hear the physical noise 50hz from the preamp transformer and the slight audible 50hz noise in the speakers. The audible noise rising withinn 1-2 mins after switching the amp on.(Once the amp is on i can't hear anything).
The thing is that I've bought this amp as damaged (it had been fallen down from the table) with the burned transistors in the preamp stage. Probably the transformer was overheated due to this accident.
 
marsellas3 said:
The thing is that I've bought this amp as damaged (it had been fallen down from the table) with the burned transistors in the preamp stage.
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I'd be looking really hard for physical damage to the unit then since it is known to have taken a big fall. Look really hard at the ground points where the circuit boards solder and connect on to chassis ground. Press on those points with something non-conductive to see if there are any cracked or bad ground joints. With AC hum like that, a bad ground connection somewhere can be the fault, especially now knowing the history of the unit.
 
americannigh said:
I'd be looking really hard for physical damage to the unit then since it is known to have taken a big fall. Look really hard at the ground points where the circuit boards solder and connect on to chassis ground. Press on those points with something non-conductive to see if there are any cracked or bad ground joints. With AC hum like that, a bad ground connection somewhere can be the fault, especially now knowing the history of the unit.
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Thank you very much for your attention! Let me some time to investigate all the ground-to-chassis points and I'll come back with the results. Also please advise would be JBL4311(L100) a nice pair to 919 (my current speakers are the vintage SABA Pro1300)?
Thanks again.
 
@marsellas3 I have not owned an AU-919 or L100 pair so can't comment on how that pairing will sound. However, the AU-919 looks to be pretty high power at 100 watts per channel into 8ohms so should sufficiently drive the L100 speakers. Beyond that, these things usually come down to the listener's tastes.
 
Yes, You're right!
You know.. regarding the humming - you've routed me to the ground points. This is probably the cause of the hum. Furthemore, each detail of the chassis was stripped and repainted by powder paint. This may loose the electrical contact of the joints (I remember some restoration projects throught the internet with the chassis details joints by the copper screws. What do you think?
 
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