Sansui AU-717 Buzzing in Both Channels at all Volume Level

[MCM_Fan]

I have a Sansui AU-717 that I bought about 4 years ago. It was my daily driver for about a year and I liked it very much. That is, until one morning about a year after I bought it, it started making a buzzing noise out of both channels. I shut it down immediately and pulled it from my system. This buzzing is present at all levels, and only increases very slight as the volume is turned up.

It is present with all sources and in both the left and right channels with both the Speaker A and Speaker B outputs. The amp still produces music and other than the buzzing, sounds and functions fine.

I finally got around to popping the cover this morning and can see some of the dreaded glue damage, but at first glance it doesn't look as bad as some of the photos I've seen. I would like to attempt a compete recap and cleaning of the glue residue and replacement of any damaged components.

There are several threads on that process, but unfortunately many have been rendered nearly useless by the Photobucket 3rd party hosting fiasco.

In the meantime, before I get around to the recapping and complete cleaning are there any quick suggestions as to what might be the source of this buzzing? Since it is in both channels and nearly constant volume and the amp is otherwise functional, is the power supply board the likely culprit and where I should start my investigation?

 

[slimecity]

yes I would be looking at the powersupply board as its common to both channels. Perhaps the main caps have dried out so the amp is running out of power? Do you get sound distortion as well, especially if you turn the amp up?

 

[MCM_Fan]

A couple more observations...

The buzzing starts as soon as the amp comes out of protection. It's in both channels, with the volume completely turned down.

It disappears when I slide the pre-power amp switch from connected to separated, but so does the music.

Other than the buzzing, I don't notice any other audible distortion or defects when playing music. In fact, other than the buzzing, I'd forgotten how good this amp sounds.

 

[woodj]

Do the repair before you recap. If you recap and the problem would persist you may never find it.

 

[MCM_Fan]

Do the repair before you recap. If you recap and the problem would persist you may never find it.

What would you recommend as a first step? Pull the PS board and look for any obvious glue damage?

 

[woodj]

Some others may know better than I.

 

[jim249]

So by separating the preamp from the power amp you have isolated the problem to the preamp. I would start by looking at the grounding connections for the preamp mostly looking for bad solder joints.

 

[slimecity]

Absolutely agree to focus on finding the fault first.

The best way to isolate the problem is by connecting another poweramp to the "pre out" jacks and switch the sansui to "separate."

Then connect a very low-level sound source (ipod, etc) to the "power amp in" RCA's while still in separated mode.

By doing this you will isolate the issue to either stage of the amp. I would suspect you have an issue perhaps in the powersupply to either the pre or poweramp section.

 

[MCM_Fan]

Absolutely agree to focus on finding the fault first.

The best way to isolate the problem is by connecting another poweramp to the "pre out" jacks and switch the sansui to "separate."

Thanks for the recommendation. I actually did this yesterday before I read your post. I used a little TPS3116 amp I built a few years ago.

Then connect a very low-level sound source (ipod, etc) to the "power amp in" RCA's while still in separated mode.

However, I did not do this. I fed the output of an Airport Exress into the AUX input of the AU-717. The noise was still present and of the same character, but not quite as loud.

By doing this you will isolate the issue to either stage of the amp. I would suspect you have an issue perhaps in the powersupply to either the pre or poweramp section.

How would running the output of the Airport Exress into the power amp in RCAs differ from running it into the AUX inputs? Since the noise is still present with the TPA3116 in place of the internal amp, it would seem the fault lies in the preamp section. Will repeating the experiment by moving the Airport Express output to the power amp in provide any additional information?

 

[jdurbin1]

Running signal into the Aux in doesn't take the preamp out of the signal path. Running it into the "power amp in" jacks does. That would have confirmed that the power amp is not noisy. Your last test just confirmed again that the buzz is being injected somewhere in the preamp.

John

 

[slimecity]

I would start by finding and downloading the schematic/service manual and check that the voltages from the preamplifier powersupply, to the preamplifier section are correct as per schematic.

Just my thought - others may have better ideas.

 

[MCM_Fan]

Running signal into the Aux in doesn't take the preamp out of the signal path. Running it into the "power amp in" jacks does. That would have confirmed that the power amp is not noisy. Your last test just confirmed again that the buzz is being injected somewhere in the preamp.

John

OK, I just repeated the experiment using the external TPA3116 amp and the output of the Airport Express connected to the "power amp in" RCAs on the AU-717. I'm controlling the volume using my computer. The noise is still there, but it's only about half as loud. I can hear the difference in noise levels when I switch back and forth between the separated and connected settings (it's about twice as loud when "connected").

 

[MCM_Fan]

I would start by finding and downloading the schematic/service manual and check that the voltages from the preamplifier powersupply, to the preamplifier section are correct as per schematic.

Just my thought - others may have better ideas.

Yes, I definitely need to get a schematic and service manual to take this further.