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Old 26th November 2010, 07:34 PM   #1
Mote is offline Mote  Estonia
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Default Magnat Crusader 720 failing to switch on

It is a 4 channel amp 720W max. When I try to switch it on, it stays in protection mode for 5 seconds. I guess it is normal because I have another Magnat amp which is in good working order, it stays also in protection mode for 5 seconds and then switches on. But with this one, as soon as the protection LED goes out and power LED on, it makes my power supply switch off as if there was a short circuit. My power supply can handle up to 16 A of current, I think my amp is just drawing more and thats why power supply switches off.
All the output transistors are OK, board looks good, no burn marks...
Only thing that was strange to me, was the biggest diode on the board, right next to B+, remote and ground terminals. I set my multimeter to diode test (2k ohm resistance), I checked the smaller diodes near power terminals, and these were OK as far as I understand checking diodes: + wire on anode, - wire on cathode - 1.1k ohm resistance, + wire on cathode, - wire on anode - more resistance than multimeter can display.
Checked the big diode the same way as well but that was more like measuring resistance on capacitor, slowly growing no matter which way the test wires were. I did not solder components out. Has the big diode failed or am I checking them incorrectly? If it has failed then why?
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Old 26th November 2010, 07:41 PM   #2
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If the meter value is changing, you're likely charging/discharging the rail caps. The diode is probably OK.

Remove both rectifiers to see if the amp will power up without tripping the supply.
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Old 27th November 2010, 09:15 AM   #3
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Without rectifiers it trips supply instantly...
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Old 27th November 2010, 02:27 PM   #4
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If it only trips the supply after remote voltage is applied (not with the B+ and ground connected) and there are no solder bridges on the rectifier pads, you may have a shorted transformer. It's also possible that the driver IC is defective. To eliminate it as the faulty component, remove the power supply FETs and measure the DC voltage on pins 9 and 10 of the IC. They should have ~5v DC.
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Old 27th November 2010, 07:57 PM   #5
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Supply trips even without remote connected. Rectifier pads I checked with multimeter, no continuity between pads. While removing the FETs I discovered that one of them is shorted, dont know how I missed that earlier. Removed all of them and now I am going to check voltage on IC pins.
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Old 27th November 2010, 08:07 PM   #6
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pins 9 and 10: 4,55 V
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Old 27th November 2010, 08:12 PM   #7
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You stated that you checked the output transistors. Did you also check the power supply FETs?

Did you check to see if any read 0 ohms to either of the other legs?
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Old 27th November 2010, 08:14 PM   #8
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What am I missing here? If it's shutting down the power supply, how are you able to measure the voltage on pins 9 and 10?
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Old 27th November 2010, 08:22 PM   #9
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Sorry, I did not express myself clearly. After removing FETs supply doe not trip anymore, thats how I was able to measure voltage on pins 9 and 10.

First time I did not check FETs :S
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Old 27th November 2010, 08:24 PM   #10
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Are they defective?
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