Hi, I have a problem with adjusting bias on my Aiwa AA8700 amplifier,
Service manual says that you should connect a probe to TP6 ( it is on emitter of Q8) and reach 23mV turning SFR2 potentiometer. The value of that pot. is 100R.
Actually, after warming up, I have like 66mV and turning SFR2 gives nothing ( maybe +- 1mV).
This is on the right channel. On the left channel which is the same I have like only 1mV and also turning a potentiometer has no reaction.
I thought that maybe the pot is broken so i replaced them but no change.
Maybe I should put a bigger value potentiometer ?
This pot is between transistors Q4=2SB648A and Q5=2SD668A maybe they should be replaced but for what ?
There’s also diode D6=STV-4H acting as temperature sensor ( it is attached to heat sink) the diode looks ok, passes the diode test but now that diode can be replaced by a transistor e.g. BC550 + 3K3 and 1k resistors.
What do you guys think I should do ?
Service manual says that you should connect a probe to TP6 ( it is on emitter of Q8) and reach 23mV turning SFR2 potentiometer. The value of that pot. is 100R.
Actually, after warming up, I have like 66mV and turning SFR2 gives nothing ( maybe +- 1mV).
This is on the right channel. On the left channel which is the same I have like only 1mV and also turning a potentiometer has no reaction.
I thought that maybe the pot is broken so i replaced them but no change.
Maybe I should put a bigger value potentiometer ?
There’s also diode D6=STV-4H acting as temperature sensor ( it is attached to heat sink) the diode looks ok, passes the diode test but now that diode can be replaced by a transistor e.g. BC550 + 3K3 and 1k resistors.
What do you guys think I should do ?
To measure idle current you need to place probes across one (usually) or both (in some amplifiers) emitter resistors.
I.e. TP4/6 and 3/5 in the other channel. I read the SM - the wording there is not one of the most exact ones.
Also 23mV across 0,47R translates to ca 50 mA in my calculator which seems a bit too "hot" for my liking.
I.e. TP4/6 and 3/5 in the other channel. I read the SM - the wording there is not one of the most exact ones.
Also 23mV across 0,47R translates to ca 50 mA in my calculator which seems a bit too "hot" for my liking.
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Madis64 you were right and I should have known that ... anyway I measured bias voltage between TP4/6 and 3/5 and on left channel I could easily set the voltage to those 23mv. While on the right channel I have 57mv and no reaction on pot turning. Any suggestion is welcome.
Start the legwork - measure voltages and currents (voltage drops across resistors) and compare the failing channel to the good channel.
Thnks, actually i am going to replace transistors one after another and see if I find a foulty one. The diode SV4h was also replaced with no success. Then I will check the voltages. I looked to the pcb and checked transistors and diodes with diode test on my meter but all looks ok. There are three zener diodes so they will have to be measured on working circuit. I will come back once I have some new results
Please don't. Randomly changing parts is the surefire way to disaster.
You need to measure the voltage between the base's of the two drivers and record the range of voltage as you turn the pot.
You have four base/emitter junctions to bias and that means you need around 0.6 * 4 (so 2.4 volts) in order for the stage to bias up. Less than that and the bias current should reduce to zero.
If you have a DC offset and a load connected that can greatly skew any bias measurement result.
You have four base/emitter junctions to bias and that means you need around 0.6 * 4 (so 2.4 volts) in order for the stage to bias up. Less than that and the bias current should reduce to zero.
If you have a DC offset and a load connected that can greatly skew any bias measurement result.
I was exactly in the same position a year ago - now I have 10 DMM-s (cost of each around 6 EUR). Gives you better understanding about what is going on ... 😉
Hi guys,
please see attached the results of my measurings.
My voltage is +- 56 VDC.
In a good channel I get bias of 23 mV and DC offset of ca 2 mV
in a bad one I get bias of 52 mV and dc offset of ca +- 2 mV as well
When I measure voltage between bases of driver transistors
in a good channel I get 2,4V while in a bad one I can go down to min 2,6V
Does it mean that the value of SFR2 potentiometer should be increased ?
Below there are measurings against ground, SM means service manual (schematics)
please see attached the results of my measurings.
My voltage is +- 56 VDC.
In a good channel I get bias of 23 mV and DC offset of ca 2 mV
in a bad one I get bias of 52 mV and dc offset of ca +- 2 mV as well
When I measure voltage between bases of driver transistors
in a good channel I get 2,4V while in a bad one I can go down to min 2,6V
Does it mean that the value of SFR2 potentiometer should be increased ?
Below there are measurings against ground, SM means service manual (schematics)
No 🙂 It is already on minimum resistance if 2.6 volts is the lowest you can achieve. 2.6 is a bit to high and is why the output stage is turning on when it should not.
Look at the circuit diagram you posted earlier. You now need to measure the voltage across R22 and compare to the good channel. I would expect about 1.35 volts give or take. This is the constant current generator to feed the bias network.
Next measure the voltage across the STV4 diode D6. This should be around 2.4 volts but compare to the good channel.
Quoting myself here: you do not need to measure resistance, the voltage drop across the trimpot in its limit position has to be zero.