au-9900 bias setting issues

D

darai22jp

Active Member
Hello and regards from Japan.
Thank you to many members of this forum I was able to rescue an AU-9900 out of protection mode and be able to reproduce beautiful tunes. I am very happy with the sound I `m getting from this beast BUT I am having issues with biasing. I can`t get any stable reading... it just keeps jumping up and down every time I rotate the volumes. I found out that it seems the service manual is messed up in this area and the vrs are all flipped... I found a picture (corrected) in wiki but still getting unstable readings...
I read a post by Mr audio about setting bias in this amp but I found it too complicated.....is there a better way to set up bias in this amp or maybe is something still wrong with this amp? btw I changed the volumes.. (5k and 10k) the original were all rusted.
Thanks
 
Bias is supposed to change with change in volume. Measure it with volume at zero (no volume) and no input signal with all tone and filter switches out of the circuit. As a side note, as you increase volume bias will increase from the initial setting, a best practice is to monitor both channel bias settings at the same time observing linearity of bias increase (which will show you how well the volume potentiometer is tracking channel to channel. Note that you need to have a single frequency test tone to both channels to do this as source music material will give relatively meaningless results. Initial bias should be set to 50ma, Let me look and see if I can find the other way to set bias that is easier to do---
 
Dr Audio posted this method a few years back and makes things much easier, thanks Warren :)
Quote---
When adjusting the bias, don't use the method in the manual. Instead, do the following:
The bottom panel and feet must be in place for this to work properly.
1. From the top, with a hemostat, you attach alligator clip jumper wires to the emitters of 2 output transistors; one NPN, one PNP on each channel. You can access the emitters on the transistor sockets on the heatsink.
2. Drape the wires over the back of the amp, out of the way, and make sure they can't short to each other or the chassis. Connect your meter to one set of wires for one channel. I use 2 meters so I can do both channels at once.
3. Place the top cover onto the unit with no screws. If you are careful you will not crimp or damage the testing wires.
4. remove the cover over the driver board.
5. Turn the amp on and allow it to warm up for a few minutes, then adjust the idle current for 20mA across the 2 emitter resistors. I don't remember the values of the resistors, but you multiply 20mA X the total resistance to get the voltage reading you want.
6. Take a piece of electrical tape and stick it to the top of the driver cover, to use as a handle. Place the cover over the drive board and attach with one screw.
7. Monitor the voltage and adjust after 10 minutes. Always replace the cover over the driver board.
8. After it has been stable for 30 minutes you are done.
 
Further notes referencing the above method---
Use mini grabbers on the emitter leads, one probe slip and bad thing will happen.
Set DMM to MV scale not MA scale, stay below 8mv as the bias will climb as the unit warms up and yes it will take anywhere from 15-30 minutes to settle down. As I recall the final setting will be around 13mv.
 
After a lot of patience at last I was able to set bias... but now another problem..... again the phono.... right channel lack of bass..... left channel just fine... the funny thing is that is just the phono... all other inputs are fine... obviously is not the balance dial since it works with all other inputs.... I think I'll have to go back to f-2570... started to hate this amp...
 
***update****
Problem solved!!
C12&C24 in F2570 were not working properly. Replaced them and the bass was back!
Thank you everybody!!
 
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