Sansui AU-9900 bias help

[LordJim]

I just acquired a au-9900 in very nice shape. I was a little suspicious, cause I got it for a fairly good price (300$) and the owner had a lot of good audio gear, and it surprised me that he would let this nice piece go.
Before hooking it up i opened it and everything looked very nice, and from the looks it seems that nobody has worked on this unit before. When I hooked it up, i was a bit disappointed, cause the music felt a bit lifeless and there seemed to be very little bottom end too. When i measured the bias on the speaker outlets i got -120mv and -60mv, so i opened here to adjust the bias.

I'm not sure if I'm doing it right. In order to get the measurement, i have to unplug the red cable and hook up my multimeter between the red cable plug and 06 (or 01 for right side), as seen in this picture, right ?

the measurement reads 46.5v on both sides, but I'm unable to adjust anything by moving vr03 and vr04. I'm also not able to adjust the bias at the speaker terminals with vr01 and vr02.

I have read dr*audio's post and unless i'm doing something wrong i suspect it's the transistors on the amplifier board 2583 (in my case). If that is the case, i could uninstall the amplifier board and take it to my tech, who probably has some replacement transistors and can install them. I found a pic on the web, and it would be very helpful if somebody who has done the job to point out which transistors to change.

cheers, ingo

 

[bktheking]

For BIAS you need to measure in MILLIAMPS, not MV or volts. DC offset and idle current are 2 different things. IDLE current in this case is being measured using your meter in series with the driver board with the meter set to milliamps, you probably need to move your DVM leads to a different spot on the meter.

You need to 0 the dc offset on the speaker terminals using vr01/02. VR03/04 is for idle current, you should be able to set to 50ma if there is no issue.

 

[LordJim]

Thanks for your reply bktheking. I suspect that my cheapo multimeter's ma setting isn't working properly. So i was measuring voltage i guess. but regarding the Dc offset, i put my voltmeter to Dc and 200m, and reading are left -109.5 and right -55. That method of measuring is correct, yeah ? Those numbers remain static and don't change when i turn vr01 or vr02.

 

[bktheking]

That could be a blown fuse in the meter itself. The way you are measuring DC offset is correct. Get the bias set properly then try dc offset. Dc offset should adjust though, something may be wrong.

 

[dr*audio]

In the first place if your read my thread, here:
http://www.audiokarma.org/forums/showthread.php?p=5966591#post5966591
You will see that you should not put your meter in series with the fuse, as in the manual. Connect your meter to the Output transistor emitters and put the meter on volts, then divide by the total value of the emitter resistors to measure the bias current.
You are confusing bias current with DC offset voltage, which is what you measure at the speaker terminals and adjust for 0V.

 

[bktheking]

Does the manual procedure for MA not work?

 

[LordJim]

Thanks for chiming in dr*audio. Yea i read your post. I should have just taken the measurements as suggested. The output transistors are the ones between the heatsinks, correct ? i took some pictures.

 

[LordJim]

Does the manual procedure for MA not work?

not sure, i think my multimeter's ma settings are fried

 

[Hyperion]

Does the manual procedure for MA not work?

Yes it does, but a bit more likely to blow the meter fuse if you are not careful.

The "across the emitter resistors" method is best, reading mV as per Doc's post on the subject :thmbsp:

not sure, i think my multimeter's ma settings are fried

You might just have a blown 'current range' fuse, replace the fuse and you should be good to go.

 

[bktheking]

Agreed- i've had great success setting bias using sansui's fuse pull methods in the past however if you "screw up" you pop fuses in the meter.

 

[dr*audio]

Agreed- i've had great success setting bias using sansui's fuse pull methods in the past however if you "screw up" you pop fuses in the meter.

The other reasons not to do it that way:
1. It is more difficult and un-necessary, since you can measure across the emitter resistors and get the current that way, much easier.
2. On the AU-9900 and AU-11000 the bias diodes are on the driver transistors rather than on the output transistors and because of this you can't get any thermal feedback from the output transistors. The outputs can and do get warmer and draw more current.
You have to set the bias and monitor it with the case closed to hold in heat, and monitor it for 20 minutes or so, re-adjusting it as needed until it is stable. If you do it this way, the top of the unit will be around body temperature after many hours of idling. If you do it the way the manual states, the unit will run hot.

 

[LordJim]

Hyperion, I checked the meter, and it has a fuse but the ma part still doesn't works. Eventhought it's a cheap meter, it has worked pretty good in the past. Hard to find good multimeters here for a reasonable price

dr*audio, are the output transistor the once i posted in the pics ? Sorry to sound like a noob.

 

[dr*audio]

Hyperion, I checked the meter, and it has a fuse but the ma part still doesn't works. Eventhought it's a cheap meter, it has worked pretty good in the past. Hard to find good multimeters here for a reasonable price

dr*audio, are the output transistor the once i posted in the pics ? Sorry to sound like a noob.

Yes. Here is a picture of the pinout for those transistors. Make sure you make your connections with the power off! Otherwise you may slip and short something out. Use mini - clips or alligator clips and make sure the alligator clips are on there securely and not shorting to anything else.

 

[LordJim]

Thanks a lot for taking the time and posting the pic with the pinout. I will find a decent multimeter today, and check the bias and keep you guys updated.

 

[LordJim]

Just bought a multimeter. Cheap one, but it seems a bit better then the last one here's a pic.

In order to learn a little more how to measure ma, i put a laptop charger in series with the multimeter at these settings, to power a small pc ventilator and got a reading of 50ma. Would this be the correct setting for measuring between the emitters ?

Since I didn't have a hemostat with small alligator clips, and couldn't find one in the local shops, i unplugged the heatsink triple-checked the emitter pinout and soldered some small cables to each emitter. One NPN and one PNP. Does this seem correct so far ?

 

[Hyperion]

No, the 'other' SM measurement method uses the mA current range of your meter. Doc's method is to measure mV across those emitter resistors.

Whatever you do DO NOT use your meter on mA range to measure across the emitter resistors - use the mV range.

To be perfectly honest I have forgotten the mV reading you should see across one of those resistors - I am sure Doc will be able to confirm the exact figure to be used.

For example
For an emitter current of 20mA, (0.020A) you should see 6.6mV across 1 emitter resistor. Note that you have 2 PAIRS of output transistors per channel - so the reading across ONE emitter resistor is only half the total current drawn by the output transistors for one channel.

R x I = V

so, 0.33Ω x 0.020A = 0.0066 V = 6.6mV

 

[LordJim]

Ok, so i just hooked it up. let it warm up for 5 minutes and measured 3.3mv on the right channel. I wouldn't adjust when i turned on vr03 on the amplifier board.

btw, Hyperion you where one of the guys that helped me fix a problem with the phono stage on a au-717 and it still runs very nice :thmbsp:

 

[Hyperion]

Ok, so i just hooked it up. let it warm up for 5 minutes and measured 3.3mv on the right channel. I wouldn't adjust when i turned on vr03 on the amplifier board.

btw, Hyperion you where one of the guys that helped me fix a problem with the phono stage on a au-717 and it still runs very nice :thmbsp:

That sounds a lot like the problem I had with mine, couldn't get the bias to adjust - it stayed stubbornly at about 25mA, and moved about +/- 2 or 3 mA with adjustment.

Is the other channel the same? - if so, you might need to change out those green and grey transistors on the driver board (not the ones on the heatsink the others). My thread with what I did is out there somewhere, have a look around - I think I tagged it for easier searching.

Glad the AU-717 is still good :thmbsp:

 

[LordJim]

I read thru your thread regarding the problems setting the bias. Sound just like what i'm experimenting. The amplifier is very clean, everything seems to work, only the music sounds lifeless, when compared to the 717. I will talk to my tech this afternoon and will check the transistors and see if he has some replacements. Was there a big difference in sound quality once you upgraded to the nichicon super through 10k uf caps ?

 

[Hyperion]

It was certainly noticeable - but not a night and day difference.

Note also that the value was bumped from 6,800µF - to 10,000µF.