AU-717 Mylar and ceramic caps etc

afroaudio

Active Member
Hi gang

Been silent for a long while but have recently found some time to start the resto of my AU-717. Cant believe its taken me almost 3 years to get to! I have been reading through everything I can find on this site and feel pretty confident diving in thanks to the brilliantly documented threads here. I am following LeeStereos blueprint so far to the letter as well as some tips gleaned from Skippy124 and others..

My unit is an earlyish model (July '77) I received the amp with the power chord cut off but all insides original if dirty. In my haste after purchasing the amp in this state 3 years back, I immediately pulled out the 2721 and 2722 main amp modules and ordered parts etc... then life got busy! Of course I should have done some testing to see where I was at before disassembling anything.

My plan is now to complete these 2 boards and reinstall and then perform the tests with dim bulb

Quick questions for the experienced Sansui techs here:

* Are the green mylar caps in the 717 worth upgrading to Polyprops at all? Even just in some key positions?

* Are the KCK ceramic caps worth replacing wholesale with COG on the main amp boards or elsewhere or are they not critical?

* Are the original output devices worth holding onto (mine test good) or would anything be gained from dropping in OnSemi subs given the choice? looking at longevity here but also hope to keep the sonic signature intact.

*Lastly, I have opted to replace the 2SA750 and 2SC1400 with KSA992/KSC1845 as advised I believe by Skippy124. Are there any other of the small signal transistors that are advisable to replace?

Thanks again to those who have gone before me and provided so much great info here, an invaluable resource and dont know where I would be without it!

cheers,

Jonathan
 
Quick questions for the experienced Sansui techs here:

* Are the green mylar caps in the 717 worth upgrading to Polyprops at all? Even just in some key positions?
If you can find suitably sized Polypropylenes to replace the Mylars then this is a worthwhile upgrade - especially for those on the F-2720 Tone Control PCB.
* Are the KCK ceramic caps worth replacing wholesale with COG on the main amp boards or elsewhere or are they not critical?
Not critical in my opinion.
* Are the original output devices worth holding onto (mine test good) or would anything be gained from dropping in OnSemi subs given the choice? looking at longevity here but also hope to keep the sonic signature intact.
The only reason for replacing the OP transistors would be if you have one or more damaged, if yours are OK then leave them alone. The only other reason could be that the correct Onsemi replacements are more robust than the originals. So if you are going to be really exploring just how powerful the AU-717 is, and are not concerned about originality this might be a consideration - or just an indication that you have the wrong amplifier. :)
*Lastly, I have opted to replace the 2SA750 and 2SC1400 with KSA992/KSC1845 as advised I believe by Skippy124. Are there any other of the small signal transistors that are advisable to replace?
The only other one to consider in my opinion is the relay driver transistor TR603, which is not in the signal path, however you may already be aware it has a hard life, so replacing it can improve speaker relay operation.
 
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Hi John

Thanks for the quick reply, was hoping you might chime in!
There are a lot of tiny ceramics littered through the amp so I am somewhat relieved that you deem only the ones on the Tone board good candidates for upgrade. I will see what I can fit in there. :)
Re the Outputs, I think I will indeed stay with the originals.. doubt very much I'll ever get to turn the wick up too much with this as my speakers are fairly efficient old Tannoy HPD 12" alnico dual concentrics (which are being powered by my recapped AU555 at present - and that little amp is performing quite admirably!)
Thanks for the reminder re TR603 - 2SC945- I did have it noted to replace that. I have some new KSC945(CGTA) and assuming these will be appropriate here rather than the KSC1845(FTA)?

Jonathan
 
There are a lot of tiny ceramics littered through the amp so I am somewhat relieved that you deem only the ones on the Tone board good candidates for upgrade.
Only the green or red Mylar/Polyester types (for potential upgrade to Polypropylene). The main ones being C05/06, C17/18 and C25/26, these should be easy to find and I think you should limit yourself to changing only those. Also don't bother to change them if you can only find Polyester replacements. Don't worry about any of the ceramic disk capacitors unless they are physically damaged.
I have some new KSC945(CGTA) and assuming these will be appropriate here rather than the KSC1845(FTA)?
Yep, correct.
 
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