Remove all the output devices.
Wire up the new output from the driver stage back into the NFB loop.
Try again, but this time with a much smaller fuse.
Wire up the new output from the driver stage back into the NFB loop.
Try again, but this time with a much smaller fuse.
I guess your soft start circuit has a resistor which is shorted out by a relay. This might mean that the output current is high on power up but limited due to the input resistor and shoots up and blows the fuse when the resistor is taken out of circuit.
Alternatively before the soft start circuit times out the supply voltage might be lower and not cause any problem and then burst into oscillation/or cause a high dc bias when the power supply voltage jumps up on 'time out'.
You could check the voltage across the soft start resistor and see if it is high on power up. It should probably peak on power up and start to fall. However if it is high all through it might indicate that the problem is there at any supply voltage.
It might be a good idea to try out all simpler possibilities BEFORE you start to de-solder components !
I never looked at the circuit diagram. Shall do so sometime now. I think you posted it earlier.
By the way, the change in behaviour now, regarding the heatsink and chassis connection, might just mean that something else gave way !
Alternatively before the soft start circuit times out the supply voltage might be lower and not cause any problem and then burst into oscillation/or cause a high dc bias when the power supply voltage jumps up on 'time out'.
You could check the voltage across the soft start resistor and see if it is high on power up. It should probably peak on power up and start to fall. However if it is high all through it might indicate that the problem is there at any supply voltage.
It might be a good idea to try out all simpler possibilities BEFORE you start to de-solder components !
I never looked at the circuit diagram. Shall do so sometime now. I think you posted it earlier.
By the way, the change in behaviour now, regarding the heatsink and chassis connection, might just mean that something else gave way !
@AndrewT -- great suggestion. I will try that tonight. Should I remove just the output devices and leave the two driver stages in (ie up to the inductor in the base circuit of the output transistors)?
@ashok -- actually my term of soft start is a bit of misnomer. It actually is an R C (~10 seconds) that operates on the output device current limiter but the circuit turns off the current sources at the input of the amplifier, thus turning off drive to subsequent stages.
@ashok -- actually my term of soft start is a bit of misnomer. It actually is an R C (~10 seconds) that operates on the output device current limiter but the circuit turns off the current sources at the input of the amplifier, thus turning off drive to subsequent stages.
Hello all --
Success!
Thanks to everyone's suggestions and support, I finally cracked this nut. I desoldered and tested parts from the power devices back to the driver board. I noticed one driver device 2SA1306 having a much higher b-e and b-c diode drops with my meter (0.85V) versus 0.6 or so on the other drivers. I found a couple pairs of 2SB649A/2SD669A that have roughly the same specs and slightly higher Ft and replaced drivers in both channels. I also went and re-capped a few supply decoupling caps and bypassed them with ceramic SMT caps. Re-assembled the unit and it is now back running. So the theory is one of the drivers was breaking down and driving the power devices to full on and tripping the fuses. Luckily the power devices seemed to have survived.
Thanks again.
Success!
Thanks to everyone's suggestions and support, I finally cracked this nut. I desoldered and tested parts from the power devices back to the driver board. I noticed one driver device 2SA1306 having a much higher b-e and b-c diode drops with my meter (0.85V) versus 0.6 or so on the other drivers. I found a couple pairs of 2SB649A/2SD669A that have roughly the same specs and slightly higher Ft and replaced drivers in both channels. I also went and re-capped a few supply decoupling caps and bypassed them with ceramic SMT caps. Re-assembled the unit and it is now back running. So the theory is one of the drivers was breaking down and driving the power devices to full on and tripping the fuses. Luckily the power devices seemed to have survived.
Thanks again.
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