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Old     (timmy)      Join Date: Jul 2001       06-01-2003, 3:18 PM Reply   
well it happened. I think I fried some FETs in my memphis ST-500D mono amp. they are FQP50N06. I need 6 of them. anyone know of a good source that will sell such a low quantity?
Old     (timmy)      Join Date: Jul 2001       06-01-2003, 4:47 PM Reply   
also, how does one check for a bad FET using a multi-meter? rather than replacing all 6, it would be nice to just replace the bad one(s)
Old    markb            06-01-2003, 6:03 PM Reply   
This should help you troubleshoot it with a multi-meter.......

go to the output transistor failure section on the following site:

Old     (aneal000)      Join Date: Feb 2002       06-02-2003, 6:49 AM Reply   
low quantity you should try if you are replacing the fets don't forget to check the bias resistors/transistors... as they have a tendency to go at the same time due to the high currents of a shorted fet.
Old     (timmy)      Join Date: Jul 2001       06-02-2003, 7:55 AM Reply   
Tony, thanks for the suggestion. digkey is the first place I tried, and they had similar FETs but not the same ones. I went to and ordered some.

i think the protection circuitry and the fuses helped stop further damage, there are no burn marks anywhere on the board and no components look toasted. however, when bench tested with lower current, the protection light kept coming on (as opposed to the internal fuses popping when in the boat) and the FETs were instantly hot. the FETs that got hot are the 6 that are connected to the torodial coil, so they are likely the power supply. the other 9 fets did not seem to have any problems.

if this does not provide a fix, it will be time to (sadly) look at other amps. I really liked the way this amp sounded and worked. simply could not get it to clip.
Old     (aneal000)      Join Date: Feb 2002       06-02-2003, 8:24 AM Reply   
That works...

Yeah you can usually tell if they are part of the power supply or output pretty eaisly based on where they are/what they are connected to. The fets are probably being driven by some sort of PWM circuit. The driver chip for that might be bad also... causing them to get hot fast. One other thought, you do have the heatsink back on when you are testing, even with low power. Otherwise you will run into thermal runaway real fast and could explain what you are seeing. But if the heatsink is connected and you are seeing the fets get real hot real fast with low power connected, then you need to start looking "upstream" driver transistors, IC, etc...

Or toss the thing and find a new used one one eBay! Good luck!
Old     (aneal000)      Join Date: Feb 2002       06-02-2003, 8:29 AM Reply   
Rereading the posts... have you changed them yet? The origional ones are getting hot? Or the new replaced ones are getting hot? If the origional ones... then yeah replace them and do the same test with low current to see if the protection circuit is comming on.

If it is the new ones then see my previous message.
Old     (timmy)      Join Date: Jul 2001       06-02-2003, 8:37 AM Reply   
i have not replaced them yet. i ordered them today so they'll be here in a couple of days. if it doesn't work I may try to replace the driver IC, as it is $0.92. if that does not work, then I am pretty sure I can sell it on Ebay. for some reason people like to pay for broken stuff.
Old     (timmy)      Join Date: Jul 2001       06-02-2003, 11:17 AM Reply   
ok here is the bad part, after desoldering the FETs I was able to check them and they check out OK according to the testing procedure that marb pointed out on that website. now I am at a loss to figure out what is wrong :-(
Old     (aneal000)      Join Date: Feb 2002       06-02-2003, 1:14 PM Reply   
If the PWM circuit driver is bad, it could be telling the fets to be on all the time, causing the high current draw and the blown fuses when installed in the boat. If you are bored and have the extra .92 cents to replace the driver chip it wouldn't be a bad place to start. Otherwise it sounds like it is time for a scope and schematic. Good luck.
Old     (cyclonecj)      Join Date: Jul 2001       06-02-2003, 8:31 PM Reply

Mouser has them for 1.36, Newark has a minimum order and I believe Digikey does too. Allied is 50$ minimum, unless they changed it. Check ALL the emitter resistors for damage, they usually fail with a dead short of a chip. You probably don't need a scope, just your nose! Check all output devices if it is blowing fuses. Power it up with no output devices, if things still get hot, replace the ones in the power supply. They are cheap. If the toroid is wasted, you are SOL. Smell it, if it smells burnt, it's burnt like toast.
Old     (timmy)      Join Date: Jul 2001       06-02-2003, 8:47 PM Reply   
I'll be checking the output FETs tomorrow.

and again, weirdly enough no components are toasted.

thanks again for the help, I will keep you all posted
Old     (timmy)      Join Date: Jul 2001       06-03-2003, 7:23 AM Reply   
looks like i have some bad output FETs
Old     (timmy)      Join Date: Jul 2001       06-13-2003, 7:01 AM Reply   
Quick update, amp is fixed and it works like a charm. Thanks for the help!
Old    markb            06-13-2003, 7:35 AM Reply   
Cool.....You just fixed a killer amp that would have been garbaged by most!


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