Sansui AU-9900A

[SanthoshA]

Dear All,

A friend of mine has recently sourced the Sansui AU-9900A, it kind of runs very very hot that the heat sink is hot to touch, and he is using a cooling fan to remove the heat .

And this is regarding the bias adjustments, i know this has been discussed already.
Basically we do the calculation based on milli volts and then adjust bias accordingly based on the emitter resistors used, and since the 50ma value is for the entire driver board, we go for 20ma per side.

And as per the schematics the emitter resistors are 0.33ohm, this leads to 6.6mv across the left and right channels.

There are couple of points to check, firstly if the bias is adjustable or not(which i am unsure) , and next can these points be used as test points when adjusting bias, instead of probing the emitters directly?




As per the above picture, the voltage across the pins 09 and pins 11 should give us the bias voltage(i have outlined in red) ?
Pin 9 is before the emitters of TR01(2SD386A), and pin 11 is before the emitters of TR01(2SB541A).
The bias voltage should be the same across the pairs as per understanding , please correct me in case i am wrong.

In case the bias test points which i have picked are wrong, please let me know and i will go by probing the transistors emitters instead.

Thanks a lot for your humble guidance and support.

 

[Hyperion]

Yes - that's fine, those connections go back to the protection circuitry which monitors the voltage at those points.

You are correct to say that measuring across one pair of emitter resistors should give ~20mA.

(50mA per channel comprising 20 + 20mA + ~10mA for the driver transistors).

 

[SanthoshA]

Yes - that's fine, those connections go back to the protection circuitry which monitors the voltage at those points.

Thanks Hyperion !

 

[salvo79]

Salve a tutti amici sono nuovo del forum spero possedere aiutarmi, ho lo stesso problema con lo stesso amplificatore, scalda molto, un quanto capisco i punti indicati sopra
bene per il test ma quali trimmer occorre toccare per ottenere 6,6mv?
la misura deve essere tarata anche su TR02?
grazie a tutti


Hello to all friends are a problem with my amplifier, warms a lot, as I understand I said above
compartment well for the test but which trimmer need to touch to get 6.6mv?
should the measurement also be calibrated on TR02?
thank you all

 

[llwhtt]

What?

Craig

 

[salvo79]

What?

Craig

Hello to all friends are a problem with my amplifier, warms a lot, as I understand I said above
compartment well for the test but which trimmer need to touch to get 6.6mv?
should the measurement also be calibrated on TR02?
thank you all

 

[Hyperion]

but which trimmer need to touch to get 6.6mv?

VR01 for one channel and VR02 for the other channel.

Bias adjustment procedures are in section 5 of the Service Manual - but it is better to use the method outlined by Santhosh in post #1

 

[salvo79]

Thanks Hyperion, I read the manual, I would like to proceed as in Santhosh's post but my question is?
the measurements as in post1 from image are to be done on transistors TR01 ok, must they also be repeated on transistors TR02? Yes or no?
thanks to all friends

 

[Hyperion]

must they also be repeated on transistors TR02? Yes or no?

Short answer = no.

However if you have had any problems with the output stage you might want to check just in case.

 

[salvo79]

Thanks again for the Hyperion help, I'm about to do this, I have one last question thanks
No there were problems with the output phase, the amplifier works well, only that it works very hot 20 minutes even in the absence of a signal
and with volume 0 so I think the problem may depend on the Bias, last two questions guys ..

on the photo Post1 and depicting only one channel, then on the other channel the measurement must always be made on transistors TR01 and Tarare at 6.6mv yes or no?
a wrong configuration could damage the appliance yes or no?
thank you very much for your attention and patience excellent forum

 

[Hyperion]

only that it works very hot 20 minutes even in the absence of a signal

That very strongly suggests a bias problem, i.e. too much.

on the photo Post1 and depicting only one channel, then on the other channel the measurement must always be made on transistors TR01 and Tarare at 6.6mv yes or no?

Referring to the diagram in post #1
You must measure between the emitters of a). TR01 (NPN) & TR01 (PNP) and/or b). TR02 (NPN) and TR02 (PNP) - these should both be 6.6mV

a wrong configuration could damage the appliance yes or no?

Yes, very much so, bias is extremely important, you MUST make sure you are setting it correctly - if the amplifier gets too hot when set to the correct level using your meter - then suspect that you have a faulty meter. (or double check that you are actually setting bias correctly).

 

[salvo79]

Hi Guys, I have just measured my bias, on both sides I read 132mv at the ignition which then after half an hour become 150mv and tend to rise again
and the amplifier in the meantime is hot to touch, I have not touched the trimmers yet .. what do you think I have to do with the calibration?
my meter is ok set on volt DC scale 200m..help

happens on both channels

 

[Hyperion]

So you have 132 - 150mV and you should have 6.6mV - time to adjust the bias trimmers I think.

 

[salvo79]

Quindi devo spostare entrambi i trimmer per provare a riportarlo a 6.6mv?
Aspetta 10 minuti per? ogni regolazione

Thank you very cordial

 

[salvo79]

Hello to all friends..calibration perfect bias ........ great amplifier..great sound .. thanks to the next

 

[smurfer77]

great new. I, Dr Audio, and others have repeatedly said that AU-9900 and variant models should not normally run too hot to touch. The manual is sadly not that clear on the adjustment and there are some errors in the manual (which are detailed in my AU-777 vs all contenders thread) and as Dr Audio pointed out many times, the adjustment of these models is best done with lid/cover in place, or almost in place because it makes real difference on this model.

 

[slimecity]

Second other views here. I had a 9900A and it never even ran warm