I wouldn’t worry too much. Important is that you were able to adjust the the bias current.
But it seems that your readings are a corrupted by oscillations because 8mV on 0.1R is 80mA while 14 V on 100R is 140mA.
So, remove the 100R resistors, set the Bias to its correct value and measure the voltage drop on the base resistors again.
Important is that your heatsinks are electrically connected to the chassis, if not, oscillation will occur.
Hans
This is weird. I’ll come back to it.I removed 100R resistors and set the voltage drop 27mV on R20. Now I have around 3mV on negative side base resistors and around 7mV on the positive side so at least they are conducting. But why there is so big difference?
Hans
With 270mA bias, this means on average 45mA per NPN and 54mA per PNP.
With resp 7mV and 3mV on the base resistors this would mean a Hfe of resp 64 and 180, not impossible but 180 is very high for the PNP.
The other thing that is almost too nice is that all NPN's have 7mV and all PNP's have 3mV on their base resistors.
So before jumping to conclusions, could you specify the following voltages.
1) NPN: All six individual voltage drops on the 10R base resistors plus the corresponding voltage drop on their 0.22R emitter resistors.
2) PNP: All five individual voltage drops on the 10R base resistors plus the corresponding voltage drop on their 0.22R emitter resistors.
3) The absolute voltages on both sides of R44 and R20
4) The absolute voltage on both sides of R50
Hans
With resp 7mV and 3mV on the base resistors this would mean a Hfe of resp 64 and 180, not impossible but 180 is very high for the PNP.
The other thing that is almost too nice is that all NPN's have 7mV and all PNP's have 3mV on their base resistors.
So before jumping to conclusions, could you specify the following voltages.
1) NPN: All six individual voltage drops on the 10R base resistors plus the corresponding voltage drop on their 0.22R emitter resistors.
2) PNP: All five individual voltage drops on the 10R base resistors plus the corresponding voltage drop on their 0.22R emitter resistors.
3) The absolute voltages on both sides of R44 and R20
4) The absolute voltage on both sides of R50
Hans
Thx for measuring all these data.
I'm still puzzled by the base resistor voltages that were yesterday resp 3mV and 7mV, and are now more like 4mV and 11mV, so I do suspect that touching these resistors induces some unwanted effects. To be sure you could connect a Scope to the LS output and see what happens while measuring.
It makes sense anyway to look with a scope how the LS output signal looks like.
But the emitter voltages are roughly o.k.
Adding the current from the positive side results in 346mA instead of the 270mA in R44, but this must could have to with tolerance in the 0.22R resistors, but ... ?
Seemingly the PNP's have very much higher Hfe as the NPN's, 159 versus 61, but I'm still skeptical because this 159 is almost unprecedented.
But the question is, suppose it is true, does it matter ?
In some way this asymmetry is not ideal, but will most likely not result in any noticeable effect.
Hans
I'm still puzzled by the base resistor voltages that were yesterday resp 3mV and 7mV, and are now more like 4mV and 11mV, so I do suspect that touching these resistors induces some unwanted effects. To be sure you could connect a Scope to the LS output and see what happens while measuring.
It makes sense anyway to look with a scope how the LS output signal looks like.
But the emitter voltages are roughly o.k.
Adding the current from the positive side results in 346mA instead of the 270mA in R44, but this must could have to with tolerance in the 0.22R resistors, but ... ?
Seemingly the PNP's have very much higher Hfe as the NPN's, 159 versus 61, but I'm still skeptical because this 159 is almost unprecedented.
But the question is, suppose it is true, does it matter ?
In some way this asymmetry is not ideal, but will most likely not result in any noticeable effect.
Hans
Hello Hans,
I have set the bias lower 230mA as you described. I also checked the LS output with the scope and did not see any ringing or oscillation. I do not have a signal source present but was able to use my phone and freq sweep to the input of the amp and it seems to work! I can make some images but it requires some work...
I have set the bias lower 230mA as you described. I also checked the LS output with the scope and did not see any ringing or oscillation. I do not have a signal source present but was able to use my phone and freq sweep to the input of the amp and it seems to work! I can make some images but it requires some work...
O.k. that’s reassuring.
Putting everything together in its final shape and comparing both channels with the same test signals will give you a reliable way to compare both.
When responding the same, you’re done.
Hans
I suspect the elcos in the overload circuit.
The separate PCB containing this part is located a the back of the chassis.
Those elcos with a brown plastic encapsulation are of bad quality and have the tendency to explode after some time.
Hans