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AU-999 bias woes

xoaphexox

xoaphexox

Supernatural Anesthetist
Are there errors in the bias/offset procedures for the AU-999? I have read them about 10 times and they dont make any sense.

1. Remove F003 and F004 (this one seems easy enough: "remove the fuse closest to each respective driver board")
2. Set VR803 and VR804 to minimum clockwise position. (this is where it begins to go wrong. not only is there no VR804 on the diagram, but the wording is not very clear. Do they mean "turn the pot counterclockwise as far as it goes"??)
3. Turn the power switch ON
4. Set ammeter in place of F003. Connect its + terminal to 27, and its - terminal to B1 in schematic diagram. (Is it just me or is the first part of this sentence enough: "connect an ammeter in place of the fuse": it's that simple, right? This talk about B1 and 27 is pointless?
5. Turn VR803 and adjust to 28-32mA
6. Turn power OFF and reset F003 to its original position (so here we just put ONE of the two fuses we took out back in??)
7. Turn power ON and set ammeter in place of F002. Connect its + terminal to 27, and its - terminal to B2. (WTF- you never told me to take out F002, which is not only a speaker fuse, but has NO terminal 27 or B2 in the schematic - i can only assume you mean F004, right?!)
8. Turn VR804 and adjust to 28-32mA. (There is no VR804 in the diagram or schematic!)
9. Turn power switch OFF and replace F003 and F004.


Can someone please shed some light on this before I royally screw my new AU-999 up?

THANKS!!
 
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Sucks huh? You gettin' some of that real Sansui Looooove goin' now, huh?:smlove:

Simplify it. The boards on the underneath mounted next to the card slot that holds the driver board has the emitter resistors on it. Connect your meter across R853 (or R854 for the other side), and adjust VR803 (the trimpot near the edge of the board, close to where the remote-mounted bias transistor wires go) for about 13mV.

 
Hot damn, Echo. That is as easy as pie. Why cant they just SAY that in the SM?!

Thanks!

p.s. you have any tricks up your sleeves for the offset adjustment? I dont have any 8 ohm resistors.
 
I tried to perform this adjustment this morning and the fuse F004 blew. I had a 4A in place of a 5A fuse, though.

I was reading about 3mV before I started twisting VR803. Soon as the VR started to give the fuse popped.

Should I have removed the fuse before performing this adjustment?
 
I sat down and thought for a while and came up with the hypothesis that corrosion or dirt from non-use of the VR on the driver boards probably resulted in an area of the path that would have had infinite resistance between it and the wiper.

To test this, I turned the unit off, replaced the 4A fuse I had in there and gave the VR several complete back-and-forth wipes. I then turned the unit on and started the bias adjustment all over again.

This time with success. I have a perfect 13mV in the channel after 10 minutes of power-on time.
 
EchoWars said:
Sucks huh? You gettin' some of that real Sansui Looooove goin' now, huh?:smlove:

Simplify it. The boards on the underneath mounted next to the card slot that holds the driver board has the emitter resistors on it. Connect your meter across R853 (or R854 for the other side), and adjust VR803 (the trimpot near the edge of the board, close to where the remote-mounted bias transistor wires go) for about 13mV.

Click to expand...


echo,

Im a newbe...i just got done recapping the AU999, doing testing right now..


I dont need to take the fuses out like the procedure says, right? bec im measuring V instead of mA?..
 
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audio0412 said:
echo,

Im a newbe...i just got done recapping the AU999, doing testing right now..


I dont need to take the fuses out like the procedure says, right? bec im measuring V instead of mA?..
Click to expand...

No you do not need to take the fuses out because you are measuring the voltage drop across the resistor......
 
uploaded or re-uploaded a picture that can help

index.php
 
So all you do is hook up the multimeter to those points on the above said resistor 853 and then turn the VR801 to set the bias?
 
That'll work, but:
1. Make sure DC offset is already set to an appropriate low value, and
2. There should be NO load connected to the amp (no speaker or dummy load).
 
rromeo said:
So all you do is hook up the multimeter to those points on the above said resistor 853 and then turn the VR801 to set the bias?
Click to expand...

VR803 is BIAS, VR801 is for DC-offset - just look at the driver board. The pot closest to the bias feedback transistor (the one in a sleeve attached to the heat sink) is for the bias.
 
Definitely better to measure voltage drop across the resistors- less chance of clip lead disasters. It also seems like any time a meter gets put in current mode, one forgets to put it back and shorts out the next thing measured. I don't know why manufacturers don't always choose the easiest method- I've got a power amp where they want you to measure the AC line current, and then turn up the bias so it increases by a certain amount. And then a certain amount more for the next channel. I just measure across the emitter resistors for the right mV.
 
ConradH said:
Definitely better to measure voltage drop across the resistors- less chance of clip lead disasters. It also seems like any time a meter gets put in current mode, one forgets to put it back and shorts out the next thing measured. I don't know why manufacturers don't always choose the easiest method- I've got a power amp where they want you to measure the AC line current, and then turn up the bias so it increases by a certain amount. And then a certain amount more for the next channel. I just measure across the emitter resistors for the right mV.
Click to expand...

That is extraordinary and sounds really dangerous :yikes: on the face of it, but maybe they facilitate this measurement method - test points?

Regarding the AU-999 bias adjustment - the OP should make sure to identify the correct trimmer before adjusting (as noted by stereofun) as turning the wrong trimmer could cause sufficient confusion to lead to a disaster.
 
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Conrad must be working on a solid state McIntosh 2200 or such, I never could get that adjustment to work either, at least I was never comfortable with it.

Craig
 
Hyperion said:
That is extraordinary and sounds really dangerous :yikes: on the face of it, but maybe they facilitate this measurement method - test points?

Regarding the AU-999 bias adjustment - the OP should make sure to identify the correct trimmer before adjusting (as noted by stereofun) as turning the wrong trimmer could cause sufficient confusion to lead to a disaster.
Click to expand...

You can generally use a Killawatt meter to do that measurement which is simple and safe, and yes, its likely a McIntosh amplifier, I have had to use this method also on an Altec amplifier as well......
 
Thanks everybody. This thread along with learning how to use my multimeter correctly allowed me to set the dc balance and dc bias on my au999
 
No fancy-pants Mac stuff here, the amp with the goofy bias method is the Marantz 250. They look for 10 watts increase for one channel compared to the no bias condition on either, then 10 watts more than that number when you do the other channel. It's a method but I wouldn't call it a good one.

Big hint for grabbing emitter resistors and other risky test points- they make mini-grabbers in very long versions. I think mine are 5" long or so. Even then I suggest powering down when making or breaking connections.
 
ConradH said:
No fancy-pants Mac stuff here, the amp with the goofy bias method is the Marantz 250. They look for 10 watts increase for one channel compared to the no bias condition on either, then 10 watts more than that number when you do the other channel. It's a method but I wouldn't call it a good one.

Big hint for grabbing emitter resistors and other risky test points- they make mini-grabbers in very long versions. I think mine are 5" long or so. Even then I suggest powering down when making or breaking connections.
Click to expand...

Good advice!!

Mini Grabbers!! Always!! Long ones!!

Yeah the McIntosh use this method for setting bias because there's nowhere to actually access the emitter resistors!!
 
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