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sta2080 need help to adjust bias

I have an old realistic stereo model STA-2080 and it works ok but is running very hot even at idol. The bias voltage at the output terminal is less than .3v. I can move the bias control all the way from one end to the other with no change. This is for both channels. Maybe I am using the wrong test point/s to calibrate it by. I would really appreciate it someone could tell me the bias adjust procedure for this particular amp. Thanks ,Doug



sta2080 need help to adjust bias

Thanks for your quick return on some advise. I tried it the way you said across the resistors and that made a huge difference in the results. The bias was not within range though and was drawing way too much current. After checking deeper into the circuit I found the 100 ohm resistor right behind the bias control was open and after replacing it, the bias adjusted beautifully. What is weird though is that this resistor was open on both channels without the amp being blown up. Just running very hot. Thnks agian for the help, Doug

karodimitrov's picture

sta2080 need help to adjust bias

Another way...On the resistors R519A and R520A(0.5 Ohms) on Main Amp you must measure voltage.Then divide it on 0.5.This is your bias.Try adjust it with VR501A.Normaly-30...50mA.What is the impedance of your speakers?

karodimitrov's picture

sta2080 need help to adjust bias

For most Amps bias adjust procedure is identical.Input of main amp-grounded.No loudspeaker-open output.
Disconect + & - of power supply (points 3 and 7 of Main Amp in your case)and conect them by ammeters again.Swich ON power and adjust current about 50 mA.In your Amp-with VR 501 A-220 ohms.Will be good to wait for,that the Amp warm up normaly and adjus fine.If you cant adjust it-check transistors.At point 5 on Main Amp you must have zero volts(+/-10mV).Thats all from me.Hope help you.

sta2080 need help to adjust bias

There is no 'identical procedure' for most amps, and I have never heard of adjusting bias with the method outlined above. Be careful, and use the manufacturer's recommended procedure.

karodimitrov's picture

sta2080 need help to adjust bias

Even when i repare my Technics SA 5600X i use this method for adjusting.No schematic...And still works perfect."Identical"-because "BIAS"is identical(dependеnt power and class of the Amp).

sta2080 need help to adjust bias

For bias adjustment you need to put a DC millivoltmeter across the emitter resistors (one at a time for each adjustment); you should get 10mv. Start with the controls in the center position. The emitter resistors are R509A and B. If you don't get around 10mv you have a problem in the circuit.
The DC balance is the level of DC on the speaker line; it should be 50mv approximately. If you are getting .3 volt, that is 300 mv, which is too high. 50mv is 1/20 of a volt.
Note: do the bias adjustment with the volume control turned all the way down. The service information is here in the library but the adjustment procedure is not very clear.