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Sansui Au 717 - Recapped but issue with High Frequency on one channel

W

wanguskohn

New Member
Hi all,

So I recently took on the task of recapping and replacing any visibly bad parts on my Sansui AU 717. Prior to doing the work, the amp was plugged in, would come out of protection, but no sound. I went through the boards, starting with the driver boards, working my way to the power supply board, followed by the other boards. All capacitors, resistors, visibly bad diodes (diodes touched by the glue residue) along with the relay, were all replaced in all boards (I used a recap kit available on eBay). The unit was then plugged in the unit once the cabling was back to normal to run the system through some test prior to buttoning everything up. Once plugged in everything worked. System came out of protection, and audio was playing beautifully, except one channel had muffled high end.

I can narrow it down using the balance. As a note, when I had the cables back together I did accidentally plug in the signal cables backwards. the cables coming from the Pre-Power amp switch going to the driver boards, were switched. After correcting this, and swapping them back to the balance worked properly, I did find the muffled highs followed. So to me, this would mean that the issue is not on the driver boards, but the signal path prior to the boards.

Since this amp has been pretty well gone over on the forums, and many of these unit having been rebuild from the ground up, I wanted to see if anyone could help me narrow this down to possible culprits to the muffled highs on one of the channels, and could offer any tests to run/ components to test to try and find the issue. I would like to avoid dropping the unit off to a shop for final fixing.

Any help would be greatly appreciated! If pictures would assist I would be more than happy to take some and upload.
 
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wanguskohn said:
Since this amp has been pretty well gone over on the forums, and many of these unit having been rebuild from the ground up, I wanted to see if anyone could help me narrow this down to possible culprits to the muffled highs on one of the channels
Click to expand...
Well, that is true, but we aren't psychic, replacement parts - you are surely not serious? - you say 'all capacitors and resistors' - really?

You don't mention checking all the power supply voltages (the first thing you should do), so I assume you have no skills in that area?

I think this issue needs someone who knows what he is doing to look at it - using a signal generator and oscilloscope. Otherwise you're going to end up replacing all the pre-amp components in the faulty channel to try and find it, and you may not be successful, even after that.
 
Hy Hyperion,

Thanks for getting back to me. Measurements as followed (after running for 5 min)-

between point 21 and 23: -33.17V
between point 21 and 19 : 34.15
between point 22 and 24: -32.86
between point 22 and 20: 34.07

I guess I should have said anything that was touching the glue or showed any signs of corrosion were replaced. While I am not an electronics engineer, or anything of the like, I do understand schemactics and have a basic understanding of electrical work. Or as much as a college education teaches a Mechanical Engineer. So yes, someone with expertise would be better suited at debugging, but I find joy in trying to fix things/learn about them. This is my way of doing that. I bought this amp 6 years ago, it worked for a few months then cut out on me and has been put away ever since. But with the whole COVID lock down I have been looking for projects and have always wanted to attempt fixing this. So here I am. Thanks for any additional feedback.
 
You could start by checking your work very carefully, but also importantly clean all the switches, (something I forgot to begin with) in particular the connected/separated switch on the rear panel. Read up on Caig DeOxit and how to use it.

Failing that
You'll need the signal generator and an oscilloscope. You'll also need to learn how to use them. And then learn how to use them to diagnose this particular fault.
 
A friend of mine had his rebuilt. After a while he lost the highs in one channel and took it back to the shop for repair.

It was the connected / separated switch. Moved it back and forth about 20 times and it was fine.

You should exercise all your switches every now and then. Hopefully that helps.
 
Well I do have some good news about the amp. I went through and was checking all my solders and trying to find any caps I may have put in backwards. And I did find that one of my leads did not have a good connection, and it happened to be one connecting the large filter cap.Went ahead and fixed that and poof, the amp seems to have been fixed! I do still have it open on my bench running it for a while before I button everything up just to make sure. I also have the Detox it on order so I will be sure to apply that before everything it closed back up as well.

Kevin D said:
It was the connected / separated switch. Moved it back and forth about 20 times and it was fine.

You should exercise all your switches every now and then. Hopefully that helps.
Click to expand...

Kevin I will be sure to Detox the switches. I did give the switch on the back quite a few flips back and forth. That was my first thought when all of this started.....
 
Hello all. Quite new to this forum as a member, although have read a lot here.
I have on my bench a Sansui AU-717 for recap´ing and restoration. It was tested before I started working on it, and it worked. After recap´ing the treble/ top is completely gone, I guess everything over 5KHz is filtered. I have done 2 of these before without problem. The customer bought a recap kit from HifiAudio on ebay, with the two others I bought Nichicon Gold and Muse from Mouser. I have another 717 in my shop, untouched, and the problem is in the preamp. When splitting the amp and use the preamp from the other 717, everything sounds as it should, but when switching around the filtered sound appears again. Every function works, sound on both channels work. I know this could be a electrolytic cap mounted the wrong way, but as far as I can see all of them are mounted the right way according to +/- on the pcb. Every switch and potmeter is thoroughly cleaned, and the pre/ power switch on the back is cleaned as well. I have a scope and a signal generator if someone can point me in the right direction on where to measure. Any advice is welcome. And please advice if this is the wrong thread. Thank you very much in advance. Thomas
 
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