Compare your DC setpoints to the schematic in post 1. They will be a little different due to variations in parts and R131:132 but should not be too far off.
We can immediately see that the voltage of the collector of BC547 should be about 5v and emitter close to 0v. Until that is true, you will jot have the correct bias current. Try changing out your BC547 for a new one. Even a BC550, or BC546 can work. Measure R131 to make sure it’s not open circuit.
Another observation is that the two KSA992’s should have their Bases about 0v. But if there is no bias current, feedback will be -22v. This will unbalance the LTP and it will try to swing the other way to drive output positive since it is acting like an opamp.
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I think the output DC offset is -23v because the PMOS is on but NMOS is off. Raj seems to be showing positive voltages where they should be negative. Q4 on Hugh’s schematic is V121 on JPS64’s schematic which matches the board silkscreen.
I suspect that BC547 is dead. Please swap for a new BC series NPN (546/547/550).
I suspect that BC547 is dead. Please swap for a new BC series NPN (546/547/550).
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Yes X as my house voltage will vary +20v than normal.
DC output from psu some times reaches to 27v.
When tested the voltage from psu are 26.+
As suggested will change the BC547C and update
Thanks X and Hugh
Sorry X, it’s 143.9 mv
Checked now and want to make sure should i change the bc547c which is on the board or any other things to be considered?
Rajesh,
You are experiencing difficulty with this amp. Do you have local people who can help with you? You need to methodically go over every component and check what is in your pcb, then check the soldering, then measure every voltage on your schematic, then post the schematic on this forum. We can then help you.........
Hugh
You are experiencing difficulty with this amp. Do you have local people who can help with you? You need to methodically go over every component and check what is in your pcb, then check the soldering, then measure every voltage on your schematic, then post the schematic on this forum. We can then help you.........
Hugh
The layout was designed for BC547 so the pinout cannot be the issue. Given that Raj had an incorrect 470R instead of 47R at R108, which has since, now fixed the DC offset problem from +23v to 53mV is progress, but the zero bias current still exists. I think is most likely an incorrect resistor value somewhere in Aleph CCS block. A bad cold solder joint, or open circuit on R121/122/123 or a major component like the R131 source resistor is another suspect - especially if using budget bright blue painted 3W Aliexpress metal film resistors. They sometimes act like fusable links and blow out upon first current loading, going open circuit henceforth. Panasonic ERX series (matte finish light blue) are really the only reliable and low distortion resistors to use here. Vishay wirewound green ceramic body ones can work reliably here, but are higher distortion.
I think your offset is OK, Rajesh, but next is WHY is the upper mosfet not turning on?
What is the voltage from COLLECTOR of BC547C active CCS transistor to EMITTER?
It should be around 5V.
If it is that voltage, what is the gate voltage from the upper nmos GATE to output of the amp, the speaker output point? It too should be around 5V. If not, we know that transistor is not turning on, or if yes, it is not passing it's potential to the gate of the nmos.
Hugh
What is the voltage from COLLECTOR of BC547C active CCS transistor to EMITTER?
It should be around 5V.
If it is that voltage, what is the gate voltage from the upper nmos GATE to output of the amp, the speaker output point? It too should be around 5V. If not, we know that transistor is not turning on, or if yes, it is not passing it's potential to the gate of the nmos.
Hugh
Hi Hugh,
The info Raj gave earlier shows that the G on the upper NMOS is the same as the drain or rail at 27.6v. When voltage below R122 is same as upper rail, I think it means there is no current flow through the BC547. The collector of the BC547 was 27.7v and emitter was 23.4v same as the base at 23.4v. If there was a bad open circuit on NMOS, the BC547 would still have the voltage drop and produce 5v at the collector regardless. So it looks like either the BC547 is bad, fake, mislabeled (PNP actually), or cold solder joint on the base or emitter etc.
Although all these numbers were pre-R108 error (470R vs 47R) correction. Now that the LTP is working properly, it changes voltage at emitter of BC547 since that was way off at 22v vs 0v at the output. We will have to wait for the new marked DC setpoints diagram before we can proceed.
The info Raj gave earlier shows that the G on the upper NMOS is the same as the drain or rail at 27.6v. When voltage below R122 is same as upper rail, I think it means there is no current flow through the BC547. The collector of the BC547 was 27.7v and emitter was 23.4v same as the base at 23.4v. If there was a bad open circuit on NMOS, the BC547 would still have the voltage drop and produce 5v at the collector regardless. So it looks like either the BC547 is bad, fake, mislabeled (PNP actually), or cold solder joint on the base or emitter etc.
Although all these numbers were pre-R108 error (470R vs 47R) correction. Now that the LTP is working properly, it changes voltage at emitter of BC547 since that was way off at 22v vs 0v at the output. We will have to wait for the new marked DC setpoints diagram before we can proceed.
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Ok Hugh, 0.47 for 4ohms would work?
One more thing regarding the psu left rail fix, do i need to change the R values (would that have gone?) or any things to check?
Am using the crc psu
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