If you don't have a replacement, temporarily substitute a 150k resistor in it's place.
You'll likely have to use a generic sub.
I don't know.
The thermistors are in separate circuits, it appears. Since there is only one red LED, I'd assume that both circuits drive the LED. Since there are two supplies, teh front and rear rollback circuits are likely independent.
You'll likely have to use a generic sub.
I don't know.
The thermistors are in separate circuits, it appears. Since there is only one red LED, I'd assume that both circuits drive the LED. Since there are two supplies, teh front and rear rollback circuits are likely independent.
Amplifier is currently idling with +-28v and +-38v rail voltages and no thermal led light, just the green power led. I am ready to begin testing it out for reliable longevity.
Below is an explanation of how I ended up fixing the remaining problems just in case it may help someone else with a similar issue:
Removed the thermistors and they measured 115k and 122k ohms out of the board but they both measure 22.5k when in the circuit.. I replaced them with 150k resistors and there was no change. What I finally ended up doing is compartmentalising the circuits and comparing them to each other. Since there are two identical power supplies in this one chassis, I was able to use the properly working circuit to find the problem in the other. I was also able to determine that the MMBTA56 is functioning properly as a substitute for the 3b(BC856] transistor in the protection light circuit.When I removed the 4.7k ohm resistor that is connected to the base of this component in both circuits I was able to compare the voltage on both ends of it and then move forward in the circuit where there were different voltages. I actually had to take pictures of the two circuits and number and label the transistors and their connections in order to create a schematickof the protection circuitry. There are certain reference voltages of 2.8v, 4.9v, 1.6v, etc. that are present on the LM358Ms, transistors, and resistors in the vicinity of the logic ICs and the PS driver boards. I finally cornered the differing voltage on the other end of the 4.7k resistor pad when i removed two side by side 1k resistors in both supplies. After removing these two, the voltages matched on both power supplies. I tested all four resistors out of the board and three measured within tolerance. I reinstalled those and replaced the fourth one. Powered up and everything matched up on both supplies.
I surely created more problems for myself with the amount of digging and probing I had to do in this amplifier but I now understand much more about its operation than could be explained to me, since there is no schematic available. I hope it has been repaired to a reliable state of operation. I hope the information and voltage readings prove useful to site users in the future. Thanks to Perry for his assistance and patience as it has taken almost a year to restore operation to this amplifier. I will update the thread if there are any further issues as this item will be personally used by me. Thanks again.
Below is an explanation of how I ended up fixing the remaining problems just in case it may help someone else with a similar issue:
Removed the thermistors and they measured 115k and 122k ohms out of the board but they both measure 22.5k when in the circuit.. I replaced them with 150k resistors and there was no change. What I finally ended up doing is compartmentalising the circuits and comparing them to each other. Since there are two identical power supplies in this one chassis, I was able to use the properly working circuit to find the problem in the other. I was also able to determine that the MMBTA56 is functioning properly as a substitute for the 3b(BC856] transistor in the protection light circuit.When I removed the 4.7k ohm resistor that is connected to the base of this component in both circuits I was able to compare the voltage on both ends of it and then move forward in the circuit where there were different voltages. I actually had to take pictures of the two circuits and number and label the transistors and their connections in order to create a schematickof the protection circuitry. There are certain reference voltages of 2.8v, 4.9v, 1.6v, etc. that are present on the LM358Ms, transistors, and resistors in the vicinity of the logic ICs and the PS driver boards. I finally cornered the differing voltage on the other end of the 4.7k resistor pad when i removed two side by side 1k resistors in both supplies. After removing these two, the voltages matched on both power supplies. I tested all four resistors out of the board and three measured within tolerance. I reinstalled those and replaced the fourth one. Powered up and everything matched up on both supplies.
I surely created more problems for myself with the amount of digging and probing I had to do in this amplifier but I now understand much more about its operation than could be explained to me, since there is no schematic available. I hope it has been repaired to a reliable state of operation. I hope the information and voltage readings prove useful to site users in the future. Thanks to Perry for his assistance and patience as it has taken almost a year to restore operation to this amplifier. I will update the thread if there are any further issues as this item will be personally used by me. Thanks again.
Back again with some more questions:
1) Is the +-38v rail voltage that I have in the front channels correct or is it lower than it is supposed to be? There is approx 3.1v at the PS drivers on this channel while there is approx. 2.9-3.0v at the PS drivers for the rear channel. I read in other posts that it should be approx. 3.5v. I was unable to confirm from other threads exactly what the rail voltage should be for 450/4 front channels or a 300/2 which should be the same.
2) I am using a car battery for power supply and it is currently discharged down to 12.2v at rest. Is this in any way responsible for the rail voltages I am seeing, but isn't the regulation doing its job and keeping the rail stable despite the dropping input voltage? I am putting the battery to charge and will take some readings after its voltage has increased.
3) If there seems to be no noticeable problems but the front rail voltage still remains at +-38v, can I anticipate any future problems from using it as is?
1) Is the +-38v rail voltage that I have in the front channels correct or is it lower than it is supposed to be? There is approx 3.1v at the PS drivers on this channel while there is approx. 2.9-3.0v at the PS drivers for the rear channel. I read in other posts that it should be approx. 3.5v. I was unable to confirm from other threads exactly what the rail voltage should be for 450/4 front channels or a 300/2 which should be the same.
2) I am using a car battery for power supply and it is currently discharged down to 12.2v at rest. Is this in any way responsible for the rail voltages I am seeing, but isn't the regulation doing its job and keeping the rail stable despite the dropping input voltage? I am putting the battery to charge and will take some readings after its voltage has increased.
3) If there seems to be no noticeable problems but the front rail voltage still remains at +-38v, can I anticipate any future problems from using it as is?
The voltage read with a multimeter will vary, depending on the meter you use so it's not really reliable.
The gate voltage read by a meter will vary with duty cycle which will vary depending on the DC supply voltage and many other things.
I don't know what the rail voltage should be but the regulator will hold the correct voltage until the supply reaches 50% duty cycle.
In notes, I have just over 28v as the rail voltage for the front channels. This may not be correct but it would appear that that would allow it to produce rated power.
The gate voltage read by a meter will vary with duty cycle which will vary depending on the DC supply voltage and many other things.
I don't know what the rail voltage should be but the regulator will hold the correct voltage until the supply reaches 50% duty cycle.
In notes, I have just over 28v as the rail voltage for the front channels. This may not be correct but it would appear that that would allow it to produce rated power.