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Old 23rd May 2010, 12:35 AM   #1
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Default jl audio 500/1

this amp is blowing the mosfet transitors q600 to q607
i have 25 k htz at base of mosfets, and voltage looks ok, but every time
I solder 1 set of transistor {2 transistors at a time } they short out.
the one thin I notice is that when I apply power the scope signal
start out of shape, and within 3 seconds signal looks to take shape.
q 608 to q 611 measure ok, this are the driver for the output mosfet,
and r 600 to r607 are 47 ohms, hope to hear from you soon.
thanks in advance.
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Old 23rd May 2010, 01:46 AM   #2
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Are the 47 ohm resistors within tolerance?

If they've been replaced, are there any broken pads/traces on any of them.
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Old 23rd May 2010, 07:23 AM   #3
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2 of this resistors measure 49 omhs
and the rest 47 ohms,
seen Im taking measurement from the
driver transistor to the MOSFET gate
pads. I don't think there is any broken.
pads. Again like to mention, that when
I 1st apply power to this unit the scope
wave time start to increse untill the wave
is shape up. is this normal?
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Old 23rd May 2010, 07:30 AM   #4
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That's normal. It has a soft start.

Does it destroy the FETs or draw excessive current if you remove the rectifier?
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Old 25th May 2010, 12:26 AM   #5
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I test the amp without the rectifier, and mosfet are no longer geting
destroy, I then place back the rettifier, and remove the audio output
transistor, and no proble. without the audio output transistor in circuit,
I measure the voltage at the rectifier output, and got
28 volts, any sugestion to check at the audio output section?
THANKS IN ADVANCE.
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Old 25th May 2010, 03:57 AM   #6
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Are the outputs shorted or leaking?
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Old 25th May 2010, 11:38 AM   #7
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No, transistor measure no leakage, or short.
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Old 26th May 2010, 02:50 AM   #8
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back to this unit, now without any changes, no parts added.
this amp is blowing the power supply output mosfet in about 2 1\2 seconds.
once the rectifier is on the circuit, with or without the audio mosfet.
the power supply mosfet are blowing in about 2 1\2 seconds.
by the way, Im only using 2 power supply mosfet
at a time. let me know if that is ok. thanks in advance.
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Old 26th May 2010, 03:09 AM   #9
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Take the rectifiers out of the amp and set your multimeter to ohms.

test them from legs 1-2 then 1-3 then 2-3 post your results
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Old 26th May 2010, 04:55 AM   #10
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If you're using one FET in each bank of power supply FETs, it should be OK.

You may have an intermittently shorted transformer. To check it, install 8 FETs in the supply and clamp them tightly to the heatsink. Insert a 10 amp fuse in the B+ line feeding the amp and power the amp up. Twist/push/pull on the power transformer to see if it will blow the fuse.
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