The procedure of adjust idle current in service manual is as follow :
aa5.jpg - Google Drive
For the right channel, there are 5 test points (5,6,7,8,9) and the final voltage should be adjusted to 50mV.
I've tried to adjust 5-7 to 50mV, then measure 6-7(also 50mV) and then 7-8(102mV), 7-9(102mV).
ie. 5-7 and 6-7 are 50mV, 7-8 and 7-9 are 102mV
Then I changed to adjust 7-8 to 50mV, then it comes out
5-7 and 6-7 are 23mV, 7-8 and 7-9 are 50mV
It seems that 5,6 are in one group and then 8,9 in another group.
But it can't adjusted to all 4 points to 50mV because there always exist difference between the 2 groups.
Can anyone tell me that for the right channel which 2 points should be connected to the voltmeter to adjust to 50mV ?
Thanks very much
aa5.jpg - Google Drive
For the right channel, there are 5 test points (5,6,7,8,9) and the final voltage should be adjusted to 50mV.
I've tried to adjust 5-7 to 50mV, then measure 6-7(also 50mV) and then 7-8(102mV), 7-9(102mV).
ie. 5-7 and 6-7 are 50mV, 7-8 and 7-9 are 102mV
Then I changed to adjust 7-8 to 50mV, then it comes out
5-7 and 6-7 are 23mV, 7-8 and 7-9 are 50mV
It seems that 5,6 are in one group and then 8,9 in another group.
But it can't adjusted to all 4 points to 50mV because there always exist difference between the 2 groups.
Can anyone tell me that for the right channel which 2 points should be connected to the voltmeter to adjust to 50mV ?
Thanks very much
Last edited:
Look at the circuit diagram. All you are doing is using points 19 and 7 as the common terminal for each channel and then you measure the voltage between there and any of the other four. You adjust for the required voltage.
It doesn't matter which of the four you use and all may vary a little.
All you are doing is setting a voltage across the 0.47 ohm resistors and it is is assumed that whichever you choose, the others will all give a similar result.
It doesn't matter which of the four you use and all may vary a little.
All you are doing is setting a voltage across the 0.47 ohm resistors and it is is assumed that whichever you choose, the others will all give a similar result.
I'd always check each voltage (on each channel) and then average that value to then set your bias level as they won't be selected devices.
Hello Mooly, thanks for your information. However, I've tried to adjust 5-7 to 50mV, then measure 6-7(also 50mV) and then 7-8(102mV), 7-9(102mV).
ie. 5-7 and 6-7 are 50mV, 7-8 and 7-9 are 102mV
Then I changed to adjust 7-8 to 50mV, then it comes out
5-7 and 6-7 are 23mV, 7-8 and 7-9 are 50mV
It seems that 5,6 are in one group and then 8,9 in another group.
But it can't adjusted to all 4 points to 50mV because there always exist difference between the 2 groups.
Can you give further advice?
Thanks
ie. 5-7 and 6-7 are 50mV, 7-8 and 7-9 are 102mV
Then I changed to adjust 7-8 to 50mV, then it comes out
5-7 and 6-7 are 23mV, 7-8 and 7-9 are 50mV
It seems that 5,6 are in one group and then 8,9 in another group.
But it can't adjusted to all 4 points to 50mV because there always exist difference between the 2 groups.
Can you give further advice?
Thanks
Are you setting this with no speakers attached? That makes a difference if the amplifier should happen to have a small DC offset voltage.
Try it with no speakers attached.
Does the other channel also have this imbalance?
Try it with no speakers attached.
Does the other channel also have this imbalance?
Hello Mooly, there is an important question, do the common terminal(central points) 19 and 7 be connect -ve or +ve pin of the voltmeter ?
Look at the circuit and relate it to the pins, you're simply measuring millivolts across the 0.47r emitter resistors
Doesn't matter which way round you connect the meter to the common (output), it'll just show neg in the display
Doesn't matter which way round you connect the meter to the common (output), it'll just show neg in the display
It doesn't matter. If you get the meter polarity wrong you just see a negative value which you just ignore the sign of. So -50 mv or +50mv you just read as '50'
Hello Mooly and MikePP, I currently adjust to 5-7 and 6-7 to 35mV, and 7-8, 7-9 to 65mV.
Both groups differ 15mV from the ideal 50mV voltage.
Is it OK to keep this status? Any further action can I do to improve the situation?
Thanks
Both groups differ 15mV from the ideal 50mV voltage.
Is it OK to keep this status? Any further action can I do to improve the situation?
Thanks
I would say always aim for lower rather than higher. Lower will mean cooler running and less tendency for thermal runaway.
In practice you will not hear any difference in the audio performance even at very low levels of current. Just set the highest in each group to no more than 50 and leave it at that. Make sure the amp is quite warm for the final adjustment.
You can not easily balance all the currents, it would need detailed testing of all the currents flowing around those transistors and also a check on the matching of all the transistors. There is unlikely to be any real problem as such.
The above assumes all the transistors and resistors are original and have never been replaced.
In practice you will not hear any difference in the audio performance even at very low levels of current. Just set the highest in each group to no more than 50 and leave it at that. Make sure the amp is quite warm for the final adjustment.
You can not easily balance all the currents, it would need detailed testing of all the currents flowing around those transistors and also a check on the matching of all the transistors. There is unlikely to be any real problem as such.
The above assumes all the transistors and resistors are original and have never been replaced.
The emitter resistors you are measuring will all have a 5-10% tolerance, so add into the voltage variation.
The only other real element adding to the variation in voltage you are measuring are the output devices themselves (which odds/on won't be matched ) and the 5r6 base resistors (which have a 5-10% tolerance.)
Both pairs are fed by the same driver , which is where the error more or less ends (unless you have more on the upper or lower pair)
The only other real element adding to the variation in voltage you are measuring are the output devices themselves (which odds/on won't be matched ) and the 5r6 base resistors (which have a 5-10% tolerance.)
Both pairs are fed by the same driver , which is where the error more or less ends (unless you have more on the upper or lower pair)