I'd say trial and error for this. Perfectly matched devices gave a significant offset in the simulation.
I would set the preset to the midpoint and then fit one of the middle ranking devices for one transistor and a higher ranking for the other. See what the offset and range of adjustment is and then depending on what you get either retain them or swap them around and retest.
I would just very lightly solder these into place and not trim the leads at this point because you may well be swapping and changing.
I would set the preset to the midpoint and then fit one of the middle ranking devices for one transistor and a higher ranking for the other. See what the offset and range of adjustment is and then depending on what you get either retain them or swap them around and retest.
I would just very lightly solder these into place and not trim the leads at this point because you may well be swapping and changing.
I agree. But I must said that leave the transistor get the thermal equilibrium before adjusting the set or make any measurement(s). Temperature is a very important factor in determining the quiescent point in BJT's.
Hello !
The amplifier is buttoned up and sounding fine. It was not for the faint of heart.
When I removed the screws holding the transistors on the radiator, one of them took a flight in the shop. I spend an hour searching for it but did not find it. It took me some time to search into the amp that it was not making a short ... So I spend some time searching for a suitable replacement in my box of remains...
Then, when I removed one of the 2SA942, my sucking pump removed the solder and the copper trace .... Fortunately, it is the trace joining the two emitters. So I had no problem making a new one with the wires of the Fairchild KSA992 ....
Then I put the circuit back on the radiator, the radiator in the case, and powered the thing up.
I was surprised to find that the idle current had not changed for both channels. But the output voltage had on both channels.
Of course, the left channel was far from good and the right channel had moved a bit (20 mV).
I was able to set both channels to 0.0 mV +/- 0.2 mV ! Yes ! And this setting holds for a couple of hours. During that time, the radiator increased temperature from the 23.4 °C ambient to 33.0 °C. I heated the radiator with a hot gun to 60 °C and the bias did not change a lot. It decreased a bit. So I bet the Q403 is doing it's job.
As this was quite fine, I put a set of speakers on, a little CD player hooked, and some New Orleans Jazz band disc on. The amp sound very good in the shop. And I was pleased to see that the speakers did not suffer when the fuse blow ! So everything's fine and I'm happy !
Thank you for your help !
P.S. : I've put the best matched transistors : ß 430 and Uf 665 mV for one and 664 mV for it's brother.
The amplifier is buttoned up and sounding fine. It was not for the faint of heart.
When I removed the screws holding the transistors on the radiator, one of them took a flight in the shop. I spend an hour searching for it but did not find it. It took me some time to search into the amp that it was not making a short ... So I spend some time searching for a suitable replacement in my box of remains...
Then, when I removed one of the 2SA942, my sucking pump removed the solder and the copper trace .... Fortunately, it is the trace joining the two emitters. So I had no problem making a new one with the wires of the Fairchild KSA992 ....
Then I put the circuit back on the radiator, the radiator in the case, and powered the thing up.
I was surprised to find that the idle current had not changed for both channels. But the output voltage had on both channels.
Of course, the left channel was far from good and the right channel had moved a bit (20 mV).
I was able to set both channels to 0.0 mV +/- 0.2 mV ! Yes ! And this setting holds for a couple of hours. During that time, the radiator increased temperature from the 23.4 °C ambient to 33.0 °C. I heated the radiator with a hot gun to 60 °C and the bias did not change a lot. It decreased a bit. So I bet the Q403 is doing it's job.
As this was quite fine, I put a set of speakers on, a little CD player hooked, and some New Orleans Jazz band disc on. The amp sound very good in the shop. And I was pleased to see that the speakers did not suffer when the fuse blow ! So everything's fine and I'm happy !
Thank you for your help !
P.S. : I've put the best matched transistors : ß 430 and Uf 665 mV for one and 664 mV for it's brother.
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Congratulations! Good job‼
Next time have a deep look to find the dropped piece(s), because they can do a big disaster. You had lots of luck, but not always it happen.
Next time have a deep look to find the dropped piece(s), because they can do a big disaster. You had lots of luck, but not always it happen.
Perfect‼
It completely logical that current decrease with heating of the heatsink. Surely it will be more noticeable (Better regulation) if the transistor resides in the heatsink (Refering to the Vbe multiplier)
Congratulations again. I suggest to recap the unit, because some old caps may leak electrolyte and corrode other parts. I work repairing electrical and electronic industrial equipment and recapping is the first step in the repairing job.
well done. I'm pleased to hear its all working well.Thank you both for the congratulations.
It was not my best job ever, and by a large margin.
I did a lot of mistakes showing that !i did not use my electronic skills often. Add to this that I lost a screw and you are near catastrophe.
I will refrain from recapping because I've done that on an L-2 a couple of months ago and found that all electrolytic condenser I removed where actually perfect. I mostly work on old tube radios and I've found that quite all German capacitors made after WWII still survive and are good still. I won't say the same thing with French made radios with French made components.
I will use this amp instead of the L-2 I re-capped earlier and keep an eye on it being prepared to change all the electrolytic in case they need it ;-) but will keep my soldering iron cold.
Anyway, I THANK YOU very much for your help, advice and support.
It was not my best job ever, and by a large margin.
I did a lot of mistakes showing that !i did not use my electronic skills often. Add to this that I lost a screw and you are near catastrophe.
I will refrain from recapping because I've done that on an L-2 a couple of months ago and found that all electrolytic condenser I removed where actually perfect. I mostly work on old tube radios and I've found that quite all German capacitors made after WWII still survive and are good still. I won't say the same thing with French made radios with French made components.
I will use this amp instead of the L-2 I re-capped earlier and keep an eye on it being prepared to change all the electrolytic in case they need it ;-) but will keep my soldering iron cold.
Anyway, I THANK YOU very much for your help, advice and support.
Good luck. I insisted in the recap, because cap leak is very corrosive not only for the components, also for the PCB itself. If you note a small sign of leak, the earlier that you replace them, the less the injury to the entire set. In any case I understand your point and I respect it.
Hello!
Sorry to resurrect this old thread, however I am having a bit of a problem in identifying the placement of this image in the Luxman Service Manual for the L3 amplifier;
There's nothing even remotely similar in the power amp board, or around it. No "blue" wire either, and no jumpered board. Argh
i think there must be multiple revisions of the manual and of the hardware.
Can anybody help me in performing the alignment procedure?
this is what the board actually looks like, from the same POV:
Sorry to resurrect this old thread, however I am having a bit of a problem in identifying the placement of this image in the Luxman Service Manual for the L3 amplifier;
There's nothing even remotely similar in the power amp board, or around it. No "blue" wire either, and no jumpered board. Argh
i think there must be multiple revisions of the manual and of the hardware.
Can anybody help me in performing the alignment procedure?
this is what the board actually looks like, from the same POV:
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There is only DC offset and bias current to adjust. Trim the offset to zero as measured at the speaker terminals and set the bias current to the recommended value by measuring the volt drop across the 0.33 ohm emitter resistors. Calculate the voltage needed using ohms law. No speakers and no signal for the adjustments.
The manual should say what the current should be.
The manual should say what the current should be.