Alas, it appears with the pot on the minimum stop.
So I bet I'm going to put the BC639/BC640 in place. Should I replace the drivers in the working channel too ?
So I bet I'm going to put the BC639/BC640 in place. Should I replace the drivers in the working channel too ?
By Murphy's laws, I don't touch the working channel. I suggest no to do such a thing. Only I would work in the faulty one. If, once both are working OK you want to change the component in the good channel to get experience or match both, then OK.
If the pot is in an extreme, it suggests me that thing are better, but not completely good. Any case, a re-search can help. Perhaps now it is good, but few time latter it may fail.
If the pot is in an extreme, it suggests me that thing are better, but not completely good. Any case, a re-search can help. Perhaps now it is good, but few time latter it may fail.
OK.
I've another question, though. The idle current is measured on one power line, th +B. But what about the other one, i.e. the -B ?
Just to clarify something, what transistors do you recommend changing ? Q401/Q402 (2SA942 both) or the driver of the output transistors, i.e. Q405/Q406 (2SC2235/2SA965) ?
I've another question, though. The idle current is measured on one power line, th +B. But what about the other one, i.e. the -B ?
Just to clarify something, what transistors do you recommend changing ? Q401/Q402 (2SA942 both) or the driver of the output transistors, i.e. Q405/Q406 (2SC2235/2SA965) ?
Last edited:
Idle current must be measured without load. Then, both currents MUST be equal unless something is deviating excessive current, and it is an abnormal condition.
Yes, both transistors. And the base resistors too.
Yes, both transistors. And the base resistors too.
Last edited:
It sounds like you are having some success 🙂
I would make a point that the final set up will be slightly different once full mains is applied (no bulb). The important things initially are that the offset is close to zero (and it is ) and that the quiescent current adjusts correctly and with plenty of adjustment range at the low end... in other words you want to be able to bring the current right down.
The idle adjustment is best performed by monitoring the voltage across one of the 0.33 ohm resistors and calculating the required voltage.
Do we know what the recommended current is?
If not then I would suggest playing safe and aiming for around 30 milliamps which would be close to 10 millivolts DC across the 0.33 ohm.
You could always check the good channel in the same way first.
Once you are happy it adjusts OK then be sure to recheck once on full mains, in fact its best to turn it back down, remove the bulb and reset from scratch.
I would replace the drivers given the fact the outputs had failed, even though the cause was the 1k5.
I would make a point that the final set up will be slightly different once full mains is applied (no bulb). The important things initially are that the offset is close to zero (and it is ) and that the quiescent current adjusts correctly and with plenty of adjustment range at the low end... in other words you want to be able to bring the current right down.
The idle adjustment is best performed by monitoring the voltage across one of the 0.33 ohm resistors and calculating the required voltage.
Do we know what the recommended current is?
If not then I would suggest playing safe and aiming for around 30 milliamps which would be close to 10 millivolts DC across the 0.33 ohm.
You could always check the good channel in the same way first.
Once you are happy it adjusts OK then be sure to recheck once on full mains, in fact its best to turn it back down, remove the bulb and reset from scratch.
I would replace the drivers given the fact the outputs had failed, even though the cause was the 1k5.
Yes, the rail voltages are lower than they should be at 35 V instead of the required 37.5V.
I do plan to make the adjustment again without the bulb but prefer to fix as much as I can before putting the full load to the circuit !
Luxman says that you should have 50 mA idle current with only one channel powered (the other one fed via a 10 Ohm resistor ) then measure the idle current on the other side. The adjustment is done with VR402.
Then to set 0V on the output using VR401. Which I can't do on the faulty channel.
Once the set is properly working, and as it apperas to have a couple of decades from manufacturing, I suggets a complete recap of the unit, changing 2 or 3 by time and powering on the device WITH THE BULB IN SERIES AGAIN. So if something goes wrong (inverted cap or faulty new unit), it is easy to check between the last two o three recently changed. Under no circumstance change lots of them, an error will take hours to detect and correct or may cause catastrophic general failures.
Last edited:
The 25mv offset you are seeing won't affect the quiescent current setting. 50ma sounds reasonable but be sure to check this finally once the amp is up to temperature.
You could try swapping Q401 and Q402 around and see if that corrects the offset adjustment issue. The front end of the amp is fairly basic and has no real designs on DC precision. Years ago an offset of -/+100mv was deemed acceptable and if you calculate the current flow into 8 ohms that such an offset causes you will see it is minimal at just over 12 milliamps. The power dissipated in the speaker is a mere 1.25 milliwatts. And that is for 100 millivolt offset.
Dear Mooly, respectfully I do not agree. Perhaps the 25mV is shadowing a defective component still in the amp, with the circuit above mentioned he must be able to get the 0.00V although momentarily.
We'll have to disagree on that one 🙂
I would doubt you will find a failed or out of tolerance part for the slight offset error... I suspect the slight discrepancy is quite possibly down to the fact the rail voltage is slightly low (the bulb) and also that the transformer tappings were mentioned as being altered (post #1).
The offset will vary a little with changes in supply voltage because the circuit isn't designed with DC precision in mind. Even the zener voltage will shift slightly with variations in supply and that will effect the balance of the stage.
I would just swap those two transistors around and see if the offset either goes worse (so swap them back) or shifts in the other direction (so leave them swapped).
I would doubt you will find a failed or out of tolerance part for the slight offset error... I suspect the slight discrepancy is quite possibly down to the fact the rail voltage is slightly low (the bulb) and also that the transformer tappings were mentioned as being altered (post #1).
The offset will vary a little with changes in supply voltage because the circuit isn't designed with DC precision in mind. Even the zener voltage will shift slightly with variations in supply and that will effect the balance of the stage.
I would just swap those two transistors around and see if the offset either goes worse (so swap them back) or shifts in the other direction (so leave them swapped).
In his place, I want to be sure that it is the natural offset and that it is not caused by a degraded or partially faulted device in the amp.
The lamp in the mains must affect both channels more or less in the same amount given both work properly.
The lamp in the mains must affect both channels more or less in the same amount given both work properly.
Well,
I'll sleep on your comments and see tomorrow where we stand.
Thanks for all the help and advice !
I'll sleep on your comments and see tomorrow where we stand.
Thanks for all the help and advice !
Hello, here am I again.
I figured that I've to start making real measurements, so removed the 60 W bulb and put the required 1.5 A fuse.
With this in place, voltages are as follows :
-B -37.9 V
+B +37.6V Both specified at 37.5V
-A -44V. this one is a bit low. Specified 46V
The zener voltages are 19.1 V for the "faulty channel" and 19.9 V for the good one.
I adjusted idle currents per the Luxman procedure having left the amplifier run for 10 to 12 minutes.
The "faulty channel" is adjusted at 50.0 mA and is stable. The other channel is also adjusted at 50.0 mA and fluctuate -0.2 mA to +0.3 mA....
The output voltage, though, is as expected, not good. Good channel stays at 0.0 mV and the "faulty channel" can't be set lower than -26.7 mV (the other channel can be set from +67 mV to - 51 mV)
As I did not want to remove the heat-sink, unscrew the printed circuit board and unsolder the two Q401/Q402 on the faulty channel and swap them, put back the printed circuit board, and then put back the heat sink I thought I had better time measure the VBE voltages for both transistors and both channels. If I'm not mistaken, the voltages should be equal for each pair if everything is fine ? Isn't it ?
Here are the results :
===========Q401==========Q402
Good channel : 0.595 mV and 0.594 mV
Faulty channel 0.592 mV and 0.576 mV
So this proves that these transistors have suffered from the failure of R406 (1.5k).
I've looked into my component drawer and have some ON Semi BC640TA. Will they fit ? Should I change them ?
Needless to say I did not put any load on the amplifier, and I refrained to put some CD and speakers to hear it ;-)
And, by the way, the mains voltage at my home is 235 V so I thought the 240 V tap would be better than the 220 previously used ;-)
As always, I _do_ appreciate your comments and help ! Thanks !
I figured that I've to start making real measurements, so removed the 60 W bulb and put the required 1.5 A fuse.
With this in place, voltages are as follows :
-B -37.9 V
+B +37.6V Both specified at 37.5V
-A -44V. this one is a bit low. Specified 46V
The zener voltages are 19.1 V for the "faulty channel" and 19.9 V for the good one.
I adjusted idle currents per the Luxman procedure having left the amplifier run for 10 to 12 minutes.
The "faulty channel" is adjusted at 50.0 mA and is stable. The other channel is also adjusted at 50.0 mA and fluctuate -0.2 mA to +0.3 mA....
The output voltage, though, is as expected, not good. Good channel stays at 0.0 mV and the "faulty channel" can't be set lower than -26.7 mV (the other channel can be set from +67 mV to - 51 mV)
As I did not want to remove the heat-sink, unscrew the printed circuit board and unsolder the two Q401/Q402 on the faulty channel and swap them, put back the printed circuit board, and then put back the heat sink I thought I had better time measure the VBE voltages for both transistors and both channels. If I'm not mistaken, the voltages should be equal for each pair if everything is fine ? Isn't it ?
Here are the results :
===========Q401==========Q402
Good channel : 0.595 mV and 0.594 mV
Faulty channel 0.592 mV and 0.576 mV
So this proves that these transistors have suffered from the failure of R406 (1.5k).
I've looked into my component drawer and have some ON Semi BC640TA. Will they fit ? Should I change them ?
Needless to say I did not put any load on the amplifier, and I refrained to put some CD and speakers to hear it ;-)
And, by the way, the mains voltage at my home is 235 V so I thought the 240 V tap would be better than the 220 previously used ;-)
As always, I _do_ appreciate your comments and help ! Thanks !
Voltages in the schematics, usually are nominal, and say, ±10% is acceptable, so don't worry about them, in the power buses. The definitive preoccupation is that you are not able to get 0.00V at the output, like the ±60mV of the other channel. So, definitively something is still wrong, and try to fix it until play music with the speakers.
The fluctuation in the current appears to not be too big to take into account, may be noise, caps with a bit of leak or presets noisier. Initially, it is better to have the bad channel working, and then, take the fine tuning.
The fluctuation in the current appears to not be too big to take into account, may be noise, caps with a bit of leak or presets noisier. Initially, it is better to have the bad channel working, and then, take the fine tuning.
Last edited: