AU-999 Bias and current adjustment ??

Hey kids,

I am looking at my service manual on page 28 where the instructions to test my balance adjustment. I'm ASSuming that is the same as the BIAS adjustment, correct? It tells me to "Connect an 8 to 16 ohm load resister to the left channel SYSTEM A speaker terminal". I've set the bias on my other Sansui 771 and all I did was hook up the DMM to the positive and negative speaker terminals and adjust the BIAS. Is this the proceedure I should use here as well, or is there something I'm missing? Also setting the current seems to be the same thing as on my 771 cept for the current value set to 28-32ma. I have a PDF version of the manual and have included the page in question.

Any help??

Thanks,

Bob Brown

Bob,

The "balance" adjustment using the 8 ohm load resistor is to set the DC balance. You adjust VR-801 for as close to 0.0 volt reading as you can get.
Note that the mistake in the manual is that both the left/right driver pcbs are
marked the same. So, you adjust VR-801 on both boards.

The "current" adjustment is to set the output bias. Again, note that both boards are marked VR-803. Adjust for 28-32ma using your multimeter.

Good luck, these are old enough that many have problems now. I just finished restoring 5 of them back to back.........and it is probably my favorite AU series amp!:thmbsp:

QB

QuadBob said:

Bob

Good luck, these are old enough that many have problems now. I just finished restoring 5 of them back to back.........and it is probably my favorite AU series amp!:thmbsp:

QB

Click to expand...

Don't leave us hanging there, Bob. Give us a clue and tell us what you had to do to the bulk of those '999's to bring them back to their glorious potential? :yes:

Complete recap?
Just a few caps that turn out to be problematic in that model?
Any particular caps that seem to work out best in that model?

Throw us a bone. :tresbon:

QuadBob said:

(speaking about the AU-999)

<snip>

Good luck, these are old enough that many have problems now. I just finished restoring 5 of them back to back.........and it is probably my favorite AU series amp!:thmbsp:

QB

Click to expand...

Me too! My AU-999 is an astounding amp and everyone should own a copy just so that can appreciate the pinnacle of Sansui design and build quality.

And no nasty protection circuits to cause unnecessary shutdowns :thmbsp: .

Paul

Morden2004 said:

Me too! My AU-999 is an astounding amp and everyone should own a copy just so that can appreciate the pinnacle of Sansui design and build quality.

And no nasty protection circuits to cause unnecessary shutdowns :thmbsp: .

Paul

Click to expand...

Actually Paul.........depending on the unit THERE IS a protection circuit! I've seen three variations thus far:

1) No protection circuit at all (depends on internal and speaker fuses)
2) Protection PCB F-1313.....utilizing two SCRs
3) Protection PCB F-1334......also utilizes two SCRs in a different layout.

And the bad news is that these early "protection" circuits don't utilize a relay......they simply short the output to ground. Which can be VERY BAD for the driver pcb and output trannys!

The good news is that the entire circuit in both "protection" variations simply sits parallel to the speaker fuse. So, a simple but effective "cure" is to remove it completely and rely completely on the fusing.

And, it is pretty amazing that they designed this circuit the way they did...if the protection circuit fails (SCR shorts).......the output is directed straight to ground and won't even blow the speaker fuse BECAUSE it is parallel!!:tears:

QB

AU-999 Quintuple Restorations

MarkMeyer said:

Don't leave us hanging there, Bob. Give us a clue and tell us what you had to do to the bulk of those '999's to bring them back to their glorious potential? :yes:

Complete recap?
Just a few caps that turn out to be problematic in that model?
Any particular caps that seem to work out best in that model?

Throw us a bone. :tresbon:

Click to expand...

Ok Mark........you're just lucky that right now I'm just totally enthralled by my "new" AU-999 driving my PC audio system now, so I might as well type while listening!

First of all.......all five units were completely "re-capped" with new electrolytics. Main filter caps were upgraded to 50V 6800uf from the stock
50V 4700uf and a film bypass cap added across each of them.

That's why it is QuadBob's restorations AND NOT QuadBob's repairs!

So here's the breakdown:

#1 My second personal AU-999 (My first lives as the center channel amp paired with my QRX-999 for my Sansui 5.1 system) this one now resides as the driver for my PC system.

#2 A local restoration for a poor guy who had just had it repaired by a "reputable" local audio repair shop........it lasted less than an hour AFTER they repaired it........he called me when he saw smoke rising from it!:tears:

#3 Restored for a QB QRX-9001 owner to add center channel capability.

#4 Restored for another quaddie who runs 3 QB restored AU-D11 II's who wants to try it for center channel use instead of one of the AU-D11 IIs or will run it in stereo to drive some ALTEC VOT model 19's.:thmbsp:

#5 Restored for another quaddie prior to restoring his QRX-8001...so he will have decent "stereo" capability while waiting for his QRX-8001 restoration.

Ok...........here's how they went:

#1 Protection circuit (F-1334) had both SCRs shorted. Luckily the bias fuse blew and stock output trans. were good. Although it did take out one 15ohm resistor on the driver pcb as well.

F-1334 was removed (I'll trust the fuses!). Replaced the open resistor. Total recap as noted above. Once finished.....calibration dialed right in. And, I'm in love all over again!! It will be in heavy duty on the PC driving some small BIC V-52 Venturi speakers!

#2 This one was the toughest. The local shop misdiagnosed it completely, replacing the left channel Hitachi output trans. with some incorrect SK replacements. What they missed was again shorted SCRs on the F-1334 pcb and the smoke was from the 5watt ceramic emittor resistor flaming. Bias fuses blown as well (they probably replaced them.....but they didn't last long). Luckily the shop gave the owner the original Hitachi outputs back with the unit.........so I asked him to bring them over for verification.

So, as before.....F-1334 was removed. Replaced the burned up emittor resistor then check the Hitachi's........which were fine! So, they went back in! After that.........straight calibration, and she sings like new again. Owner picked it up tonight and I had it setup driving some small Wharfdales in the shop. He left with a big grin after hearing it!!

#3 An Ebay purchase for the owner that went VERY well. Upon inspection if this unit were a car..........it couldn't have more than 10,000 miles on it. It just looked "new"! And after restoration by a very slight margin it would rank as the #1 best sounding of the five. (I'll post pictures of this one in a separate post). If he doesn't love it........I'd buy it and put it in my bedroom!

#4 In fairly "hard run" condition. But, it cleaned up nicely. No real faults found other than a blown bias fuse, and someone installed 20amp fuses in the speaker fuse holders!

#5 In good condition other than a broken tape dub toggle switch which I replaced from my "parts" AU-999. This one had F-1313 installed and one SCR shorted.......so F-1313 was removed. After that.......it was straight forward.

And, for those of you who might care........I debated whether to repair these early protection pcbs (which i could easily have done) or remove them as I ended up doing. From the schematic and wiring........it was obvious these circuits simply are waiting to "blow"........Sansui might have thought the SCR would conduct to ground....and blow the bias fuse, then reset once powered down........so you replace the fuse and you're back in business. But, the reality is that the SCRs short hard to ground..........and then all kinds of bad things "can" happen to the amp stages. The bias fuse does open, but once a replacement is installed.....it just immediately fails again as those SCRs are just sitting there shorted now. I can only guess this is what made the local shop think the output tranny's were bad in #2.

So, with more than a little trepidation.......I determined it was better to simply depend on the correct fusing to do the job.

QB

QuadBob said:

Actually Paul.........depending on the unit THERE IS a protection circuit! I've seen three variations thus far:

1) No protection circuit at all (depends on internal and speaker fuses)
2) Protection PCB F-1313.....utilizing two SCRs
3) Protection PCB F-1334......also utilizes two SCRs in a different layout.

And the bad news is that these early "protection" circuits don't utilize a relay......they simply short the output to ground. Which can be VERY BAD for the driver pcb and output trannys!

The good news is that the entire circuit in both "protection" variations simply sits parallel to the speaker fuse. So, a simple but effective "cure" is to remove it completely and rely completely on the fusing.

And, it is pretty amazing that they designed this circuit the way they did...if the protection circuit fails (SCR shorts).......the output is directed straight to ground and won't even blow the speaker fuse BECAUSE it is parallel!!:tears:

QB

Click to expand...

Dang -- I'll have to open mine up. It's one of the later models - January 1972.

The problem is that when I pop the top on that AU-999 I don't want to put it back on -- it's such a treat to look at it naked

I wonder if I could add some small lamps to the inside so that .... :scratch2:

Paul

AU-999 Restoration

Here's #3 which is about the best looking AU-999 I've seen.....just immaculate!

Ready to go!



Top uncovered:



Underside:

Morden2004 said:

Dang -- I'll have to open mine up. It's one of the later models - January 1972.

The problem is that when I pop the top on that AU-999 I don't want to put it back on -- it's such a treat to look at it naked

I wonder if I could add some small lamps to the inside so that .... :scratch2:

Paul

Click to expand...

If it has one of the pcbs (F-1313/1334) it will be mounted on the underside parallel to the speaker connectors just to the bottom left of the transformer in my picture of the underside (unfortunately I don't have a picture of it installed).

There are only 3 wires to it........one hardwired to the left speaker fuse holder, one to the right speaker fuse holder, and one to the ground stud on the chassis. I simply snipped the wires at the fuse holders and ground stud.

If you do decide to remove it.......be sure to re-cal the offset and bias!!

And it will never be an issue again................:thmbsp:

QB

Cool pics! :thmbsp:

QuadBob said:

If it has one of the pcbs (F-1313/1334) it will be mounted on the underside parallel to the speaker connectors just to the bottom left of the transformer in my picture of the underside (unfortunately I don't have a picture of it installed).

There are only 3 wires to it........one hardwired to the left speaker fuse holder, one to the right speaker fuse holder, and one to the ground stud on the chassis. I simply snipped the wires at the fuse holders and ground stud.

If you do decide to remove it.......be sure to re-cal the offset and bias!!

And it will never be an issue again................:thmbsp:

QB

Click to expand...

Thanks, Bob. I'll pop the hood and have a look.

Mine is virtually pristine as well -- it still had the manufacturers plastic film on the bezel when I got it. Paid a fairly hefty price for it at the time ($135 in 2004) but I was so impressed with the unit I forgot all about the cost :thmbsp: .

Paul

Wow, that is a very clean 999 but it doesn't look at all like a Sansui wood case. Was it custom made?

- Pete

BTW, Bob, do I have the serial numbers for those AU-999's? :scratch2:

Paul

that case

I have one that came with the same case, it appears there may have been 2 options on cases, the pictured one seems to be the later model, earlier had the case you usually see common to a 5000 or 2000 receiver, with the lateral cut-outs for cooling airflow.
That later case I have is warped at the top...:sigh:
Casey

Finished the project I was working on!

Pictures @ http://audiokarma.org/forums/showthread.php?t=144862

Thanks for all the inspiration. I used the protection board idea among others, QB.