I am working on a Memphis 16-MC1.2500. Guy states that he plugged a remote bass control knob from a different amplifier into it and it smoked.
I found the two 50N06 transistors for the secondary power supply were both blown, and the B+ feed to the secondary supply transformer was burned. I replaced the bad transistors and repaired the burned trace. Gates and drivers for those transistors were good.
Now the amplifier will power up and I get clean output, but the main power supply is not functioning correctly. The drive signal for the power supply FETs is not correct, and the output of the power supply transformers is not correct. (See first image for the drive signal). It also will sometimes protect upon power up, but not always. However, I get a little over 80V+/- rail voltage and the output with a 40HZ signal is clean on the scope as well as through a speaker. No abnormal DC offset.
Nothing in the amplifier seems to heat up even after running idle for a few minutes, except for D417 and D418 near the output driver board. These are connected to the +12/-12V supply. So not sure if it's normal for these to heat up or not. They are getting up to around 150*F in a minute or so. The power supply does make a buzzing noise as well from the transformers which I assume is from the horrible drive signal.
If I remove the rectifiers the signal from the power supply up to the rectifiers is a normal square wave again.
I'm not sure exactly where else to investigate.
I found the two 50N06 transistors for the secondary power supply were both blown, and the B+ feed to the secondary supply transformer was burned. I replaced the bad transistors and repaired the burned trace. Gates and drivers for those transistors were good.
Now the amplifier will power up and I get clean output, but the main power supply is not functioning correctly. The drive signal for the power supply FETs is not correct, and the output of the power supply transformers is not correct. (See first image for the drive signal). It also will sometimes protect upon power up, but not always. However, I get a little over 80V+/- rail voltage and the output with a 40HZ signal is clean on the scope as well as through a speaker. No abnormal DC offset.
Nothing in the amplifier seems to heat up even after running idle for a few minutes, except for D417 and D418 near the output driver board. These are connected to the +12/-12V supply. So not sure if it's normal for these to heat up or not. They are getting up to around 150*F in a minute or so. The power supply does make a buzzing noise as well from the transformers which I assume is from the horrible drive signal.
If I remove the rectifiers the signal from the power supply up to the rectifiers is a normal square wave again.
I'm not sure exactly where else to investigate.
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Lower the 12v supply voltage on the supply feeding the amp. Does that give you the expected drive signal?
No it does not. If I lower the voltage the power supply will start to have correct square wave drive signal. It will begin to build and then cut out completely, then try again, and eventually protect.
To clarify though I am not using a variable supply, this is through a current limiter, (head light) which does reduce the input voltage to the amplifier.
Then I don't understand the problem. I thought the problem was the drive signal. What did I miss?
The problem is drive signal. I checked again and it will very very briefly start out as a clean square wave from the 494, and then it will go away, but the amplifier remains functioning. With the limiter in series it also starts off as a square wave, but now its protecting after a few seconds through the limiter.
I have attached some photos from startup to when the signal starts to act up. These are taken right at the output of the driver IC. Keep in mind that the amplifier is on and functioning even during the last picture with clean output. Even though it appears there is no drive signal.
I have attached some photos from startup to when the signal starts to act up. These are taken right at the output of the driver IC. Keep in mind that the amplifier is on and functioning even during the last picture with clean output. Even though it appears there is no drive signal.
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The supply has regulation so the duty cycle is reduced as the rails meet their target voltage.
When viewing PS drive waveforms, it's more useful to set the trace to the center reference line and set the channel to DC coupling.
When viewing PS drive waveforms, it's more useful to set the trace to the center reference line and set the channel to DC coupling.
So are you saying what i'm seeing is normal? Why does it protect sometimes when powering up though? Even without the current limiter. Then if I power it off and back on it seems to be ok
It's normal for a regulated power supply. Look at this (sorry for the quality) video. Look at the relation of the output level vs the power supply drive pulse width.
http://www.bcae1.com/temp/audioandpowersupply01.avi
I'm not sure why it's going into protect. Is it possible that your power supply doesn't have enough surge current capacity and the voltage is dropping for an instant?
http://www.bcae1.com/temp/audioandpowersupply01.avi
I'm not sure why it's going into protect. Is it possible that your power supply doesn't have enough surge current capacity and the voltage is dropping for an instant?
I am not familiar with this particular amplifier, so I don't really know how the protection circuits are laid out but I will see if I can narrow it down.
The following diagram may help if you don't have anything better:
http://www.bcae1.com/temp/soundstreamxxx6500.zip
http://www.bcae1.com/temp/soundstreamxxx6500.zip
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