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Sansui AU-517 humming

HansVenlo

HansVenlo

Member
HI AK-members,
I'm new on this forum and not very experienced in restoring old Sansui equipment to it's full glory. I successfully did a recap and restoration of my Sansui AU-666 (1970) that I gave to my oldest son 15 years ago. With lots of help from the many threads on this topic.
Learned a lot and got the taste of it. So I ventured on a restoration of a Sansui AU-517 that my youngest son inherited from my father. I found most of the information here on AK and after a long wrestling with the Power Supply Board it was sounding very wel, but...
I managed to get a hum in the amplifier right channel, that was not there when I started.
It sounds like a 50Hz hum and it is audible when no inputs are connected. Musics masks the hum. The hum increases to annoying level when I switch the tone control on with the defeat/on switch. At this point I'm stuck. Checked my work over and over again, and even pulled the Power Supply Board for the second time.
I replaces all the capacitors in the amplifier following the suggestions of Hyperion and Lee-stereo, plus I replaced the zeners on the Driver Blocks, a few fusible resistors and the varistors, and of course the dreadful glue was removed. Did not yet touch the transistors.
Is there anyone that is willing help with this problem? If the hum gets fixed it may be one of the best amps ever heard. But for now I'm to ashamed to return it to my son.
 
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When you replaced the capacitors, did you reinstall them the same way the originals were installed, or per the polarity markings on the boards?

Rob
 
This being your first posting.
Welkom to AudioKarma!
And specifically "Exclusively Sansui" !
That are two nice Sansui's you have worked on.
Although I have never encountered any hum problems on my amplifiers, what I have seen here in threads on AudioKarma, it's mostly related to grounding.
You have the benefit of it being just right channel and being worse with tone defeat off switch engaged.
So we know it's in that channel and pretty sure related to your tone/ control boards F-2673/F-2674 and there connections to the driverboard and chassis.

Screenshot_20211111-093446_Moon+ Reader.jpg

Take the schematic and check for any bad ground connections.
You can do this first with power off and just compare the left and right channels for differences in resistance.
If that doesn't give any leads you could try it with power on. But be really carefull. Preferable with mini-grabbers and making connections with power off first, then power on, read measurements, power off again, move grabbers, etc . .
Don't forget board F-2670, the pre-power amp switch board.
Happy hunting.

Ps
I don't know if you replaced the main filter capacitors, because the AU-517 is dual power supply you should check if the chassis is connected to your ground in between the two capacitors for the right channel.
See next message the ground sign between the two capacitors.
 
I have an idea it might be some broken connections in the black ribbon cables connecting the rear panel sockets to the EQ (Phono) Board. For the right channel connections at '06' '12' and '22' being the ones to look at first. A simple multimeter check on the F-2673 Tone Control Board ground connection might reveal a lost ground.

These:-
https://audiokarma.org/forums/index...u-517-needs-recap.974218/page-6#post-15154556

With thanks in advance to @Alex22 for the use of his picture.
 
Last edited:
Does the unit get really hot? Did you check the bias?

I have an AU-7900 and one of the trimmer pots failed open, feeding 500mv to the outputs instead of 18mv (I think?). Symptoms were a burning hot heat sink and an audible hum from the unit itself, not the speakers. Surprisingly the speakers sounded fine...

Re-reading your post, I think you're referring to a hum from the speakers. If so then never mind.
 
Robisme said:
When you replaced the capacitors, did you reinstall them the same way the originals were installed, or per the polarity markings on the boards?

Rob
Click to expand...
Hi Rob, thank you for your suggestion. I used the polarity markings on the boards, but I will check with the pictures of the old situation. I am aware of discrepancies between the manual, the silk screen prints on the board and the actual orientation in AU-666, but do not know of any problems like that in the AU-517. I’ll check and let you know. In the mean time if there is such a fault in the AU-517, please let me know.
 
GrisPato said:
This being your first posting.
Welkom to AudioKarma!
And specifically "Exclusively Sansui" !
That are two nice Sansui's you have worked on.
Although I have never encountered any hum problems on my amplifiers, what I have seen here in threads on AudioKarma, it's mostly related to grounding.
You have the benefit of it being just right channel and being worse with tone defeat off switch engaged.
So we know it's in that channel and pretty sure related to your tone/ control boards F-2673/F-2674 and there connections to the driverboard and chassis.

View attachment 2391984

Take the schematic and check for any bad ground connections.
You can do this first with power off and just compare the left and right channels for differences in resistance.
If that doesn't give any leads you could try it with power on. But be really carefull. Preferable with mini-grabbers and making connections with power off first, then power on, read measurements, power off again, move grabbers, etc . .
Don't forget board F-2670, the pre-power amp switch board.
Happy hunting.

Ps
I don't know if you replaced the main filter capacitors, because the AU-517 is dual power supply you should check if the chassis is connected to your ground in between the two capacitors for the right channel.
See next message the ground sign between the two capacitors.
Click to expand...

Hi GrisPato, I see you live in the neighborhood!
Yes I replaced the main caps with KEMET 15.000 uF capacitors. I checked the grounding between the left and right caps. I did not touch that solder joint but it looks ok. I will reflow it just to make sure. It has one black lead to the metal bridge between the caps and one lead to the oposite grounding post on the other side. Furthermore there is a white lead to the metal bridge between the left main caps. And I will check the tone control boards as well. I’ll let you know!
 
percivilla said:
Does the unit get really hot? Did you check the bias?

I have an AU-7900 and one of the trimmer pots failed open, feeding 500mv to the outputs instead of 18mv (I think?). Symptoms were a burning hot heat sink and an audible hum from the unit itself, not the speakers. Surprisingly the speakers sounded fine...

Re-reading your post, I think you're referring to a hum from the speakers. If so then never mind.
Click to expand...
Hi Percivilla, the hum is indeed in the soeakerchannel. Checked DC (zero) and bias (both channels set to 21mV) there is no temperature rise in the amp.
 
HansVenlo said:
Hi GrisPato, I see you live in the neighborhood!
Yes I replaced the main caps with KEMET 15.000 uF capacitors. I checked the grounding between the left and right caps. I did not touch that solder joint but it looks ok. I will reflow it just to make sure. It has one black lead to the metal bridge between the caps and one lead to the oposite grounding post on the other side. Furthermore there is a white lead to the metal bridge between the left main caps. And I will check the tone control boards as well. I’ll let you know!
Click to expand...
Yes I do!
With the metal bridge you mean from the chassis?
Like here on a picture of an AU-719,
You see a wire from the ground from each capacitor pair found to the middle of the chassis.

20210502_113337.jpg

And Hyperion his advice would be good to start there, the ribbon cables can cause all kind of problems.
 
Hyperion said:
I have an idea it might be some broken connections in the black ribbon cables connecting the rear panel sockets to the EQ (Phono) Board. For the right channel connections at '06' '12' and '22' being the ones to look at first. A simple multimeter check on the F-2673 Tone Control Board ground connection might reveal a lost ground.

These:-
https://audiokarma.org/forums/index...u-517-needs-recap.974218/page-6#post-15154556

With thanks in advance to @Alex22 for the use of his picture.
Click to expand...
 
Hi Hyperion,
Thank you for helping out. I checked the ribbon cable. All positions you mentioned seemed to be ok. Furthermore I checked wether the tonecontrol has good ground, and it has. I checked every connection to the groundpoint on the board. Absolutely fine. I also checked the connection to the bass/treble board. There was a big solder blob on the ground connection. I removed the cable and wil clean the board tomorrow.
Thanks to everybody who is trying to help!
I’ll be back.
 
GrisPato said:
Yes I do!
With the metal bridge you mean from the chassis?
Like here on a picture of an AU-719,
You see a wire from the ground from each capacitor pair found to the middle of the chassis.

View attachment 2392362

And Hyperion his advice would be good to start there, the ribbon cables can cause all kind of problems.
Click to expand...
Yes indeed. I reflowed the connection, but it did not help.
Now working out Hyperion’s and Robisme’s advice.
 
Good day,
Are all the metal covers attached?
I noticed that sometimes when the cable(in reality just 2 wires red and white) leading to power diode indicator in certain position you can hear buzz/hum. So try to move that cable.
 
TupolevFan said:
Good day,
Are all the metal covers attached?
I noticed that sometimes when the cable(in reality just 2 wires red and white) leading to power diode indicator in certain position you can hear buzz/hum. So try to move that cable.
Click to expand...
Thanks for the suggestion, I will try that too.
 
I agree with some responders here - Probably a grounding issue.

As there are a lot of PCB to PCB cable connectors in your amp (Ive worked on one, the power supply board can be. bitch to get to), focus on those. I am working on an X1 at the moment and I have tightened all the sockets that connect to pins in these cables - a good connection is critical. Check and double check all these cables, connectors, pins, sockets.

Some of PCB soldering in these Sansuis left a bit to be desired, check the PCB connections of the cable fittings, reflow solder where these look a bit dodgy
 
Thanks for this suggestion, I found some sloppy soldering which I will correct too.
I will go over al my recap work and all connections to see if they are ok.
Thanks to all the great folks on AK for all the help. I’m confident it can be done.
 
I'm sorry that you are still having problems, I just finished a restoration on my first AU-517 ( and my first restoration in general) so I don't have the experience to help you. Having said that have you tried swapping the wiring between F-2721 and F-2722, easy to do and you don't have to remove any parts. Disconnect the right channel and connect it's cables to the left channel see it you still have a hum running one channel, at least you can rule out the driver boards from your fault finding.
 
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