Hi,
I got this amp from a friend, and since I dabble with some electronics, I thought i'd have a go.
Its a Dragster DB 505 5 channel amp.
http://www.flickr.com/photos/55331647@N00/1571391294/
I replaced all obviously burned components, ie. all of the output devices and corresponding power resistors, the power supply transistors, input capacitors and one or two other misc components. The big resistor by the terminal block is replacing a smt resistor that was open circuit incase you were wondering.
I disconnected the rectifier diodes to aid troubleshooting. I'm only getting less than half the output from the toroid that I was expecting. I'm getting about 9vac per secondary!
I know the basic idea of SMPS, but I must be missing something obvious.
The four power supply transistors all get warm, but the two in the far left get hotter. The voltages at the legs of the devices all compare the same (ac and dc), and i'm getting about 4vac on each primary of the toroid.
Hope someone can give me some advice on what to check next.
Cheers.
I got this amp from a friend, and since I dabble with some electronics, I thought i'd have a go.
Its a Dragster DB 505 5 channel amp.
http://www.flickr.com/photos/55331647@N00/1571391294/
I replaced all obviously burned components, ie. all of the output devices and corresponding power resistors, the power supply transistors, input capacitors and one or two other misc components. The big resistor by the terminal block is replacing a smt resistor that was open circuit incase you were wondering.
I disconnected the rectifier diodes to aid troubleshooting. I'm only getting less than half the output from the toroid that I was expecting. I'm getting about 9vac per secondary!
I know the basic idea of SMPS, but I must be missing something obvious.
The four power supply transistors all get warm, but the two in the far left get hotter. The voltages at the legs of the devices all compare the same (ac and dc), and i'm getting about 4vac on each primary of the toroid.
Hope someone can give me some advice on what to check next.
Cheers.
Measure the DC voltage on the power supply transistors. There are two banks of transistors. Post the three voltages on each bank with the amp powered up. All of the transistors in each bank should have the same voltage. For this, touch the black lead to the ground terminal of the amplifier.
AC voltage with a multimeter can mean many things because they don't all respond the same way to high frequencies. DC is somewhat more reliable. Nothing beats a scope but I realize not everyone has one.
AC voltage with a multimeter can mean many things because they don't all respond the same way to high frequencies. DC is somewhat more reliable. Nothing beats a scope but I realize not everyone has one.
In most supplies, the FETs will remain cool for at least a minute. If they remain cool for that long, I wouldn't be concerned. If they begin to heat up as soon as you power it up and get too hot to touch within 1 minute, I'd be concerned.
If they get hot quickly, double-check the gate resistors. If they're OK, you could try reducing the value by ~20% to see if they run cooler. The new FETs could be slightly different than the originals even though they are the same part number.
If they get hot quickly, double-check the gate resistors. If they're OK, you could try reducing the value by ~20% to see if they run cooler. The new FETs could be slightly different than the originals even though they are the same part number.
The only real concern is shorted output transistors. If they're not shorted, I'd replace the rectifiers and power it up through a 10 amp fuse or with a 2 ohm power resistor (25w to 100w) in series with the B+ line. This will prevent damaging the power supply if there is a problem. Clamp the power semis to the sink before powering it up. This will help protect them in case there is another fault.
I think the power supply is OK if it takes 4-5 minutes for the FETs to heat up.
I think the power supply is OK if it takes 4-5 minutes for the FETs to heat up.
Hi, Ben12345,
If you have fixed the SMPS problem, please check whether your Dragster has zobel network(s) on its output(s).
This zobel is needed to always load the amp at high frequencies. Without these, the amp will be easily oscilating.
If your Dragster don't have zobel, just solder 10ohm+100nF across +and- of terminal blocks of each channel.
If you have fixed the SMPS problem, please check whether your Dragster has zobel network(s) on its output(s).
This zobel is needed to always load the amp at high frequencies. Without these, the amp will be easily oscilating.
If your Dragster don't have zobel, just solder 10ohm+100nF across +and- of terminal blocks of each channel.
I tried to fire it up with little success;
The status light was green, but the SMPS was screaming and the 50w resistor on the input got pretty warm.
I was using a 10a 12v PSU, so there should be enough current there just to fire it up.
All of the output FETS are new and not shorted.
The rectifier diodes are the only power devices not replaced. They measure infinity in one direction, and have resistance in the other.
Any ideas?
The status light was green, but the SMPS was screaming and the 50w resistor on the input got pretty warm.
I was using a 10a 12v PSU, so there should be enough current there just to fire it up.
All of the output FETS are new and not shorted.
The rectifier diodes are the only power devices not replaced. They measure infinity in one direction, and have resistance in the other.
Any ideas?
With the semis clamped to the sink and the resistor in series with the B+ line, measure the voltage across the emitter/source resistors. There should be little or no voltage across them. If one or more have significanly more than the others, you know that there is a problem in that channel. The voltage will be very low so a few millivolts difference can be significant.
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