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LJM106 View Drop Down
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    Posted: 29 January 2022 at 5:45pm
Hello.
I'm new here and new to trying to fix amplifiers as a hobby I have so knowledge of electricity so safe in that way.

I have a C-Audio XR3801 which the "soft start" fuse kept blowing, I am currently checking the mosfets... 

Came across one j162 where the ohms reading keeps climbing up between source and drain where the other two I've tested so far are showing 2ohms as the service manual says...

Is this a faulty mosfet and if so is there a replacement available instead of replacing with j162?

Hope you can help 
Thanks. 
Luke. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 29 January 2022 at 7:02pm
Hi, 

You can isolate the power rails to each channel to 'Test' which channel has the fault (just lift off the wires from one side going from the main board to the transformer / rectifiers). Make sure the protection fuse is a 1.6A quick blow.

Take out the faulty J162 FET from the amplifier and see if the amplifier powers up.

I find the best way to do it is to find which side has the problem, i then measure the FET's with a meter and any that measure wrong i de-solder the three FET legs and then measure the FET again but put my probes on the FET legs only (so the FET is isolated from the PCB board when you are measuring it).

I would only replace the faulty FET / FET's with the correct / same type rather than a different FET as a matter of course / matching issues and reliabilty etc.

Big smile
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Post Options Post Options   Thanks (0) Thanks(0)   Quote LJM106 Quote  Post ReplyReply Direct Link To This Post Posted: 29 January 2022 at 8:01pm
Thanks for the reply!

I'm actually taking out every mosfet off the heatsink and testing each one in turn before screwing back in ready for soldering I have two j162 that at suspect at the moment as both seem to be making ohms reading to increase. I believe I'm testing them correctly. 

I'm worried about starting the amp at the moment just incase I blow something up. 

I see in the service manual using a 100w bulb inplace of the fuse when testing so I will try that when I have found which I believe to be faulty mosfets... am I doing it all wrong lol as I'm new to amplifiers lol... I mainly do disco lights refreshments and repair... I install and maintain traffic lights for a living but amplifiers are new ground.

All help is much appreciated!
Thank you. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 29 January 2022 at 8:21pm
Hi,

I have repaired a load of these amplifiers & taking all the FET's / Class H Diode off of all the heatsinks is a massive job & takes hours.

I would just de-solder (solder-wick will do this very quickly) ALL the Source, Gate & Drain leads from the PCB board (but keeping the FET's physical mounted to the heatsinks), i found this is the quick and easy way of doing it and this allows easy testing of each one via a meter / FET tester and if they test O.K., just re-solder it back onto the PCB board. I had a few of these where even getting the M3 nut & bolts off each FET is a pain in the neck due to them having locktite on the threads.

Isolating which side is faulty would also save you half the time rather than taking a load of FET's off a working side. Confused
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Post Options Post Options   Thanks (0) Thanks(0)   Quote LJM106 Quote  Post ReplyReply Direct Link To This Post Posted: 29 January 2022 at 8:34pm
Hi get want your saying! Is true takes a wile!! 

I can solder suck off the solder. So could do that and keep in place (shame the leads arnt long enough to put little clips on without touching pcb) 

As I brought this amp off ebay with no knowledge of its history other than the seller saying "believe this amp blows the soft start fuse" is as much as I know as when received and looked at 'soft start' pcb the fuse was blown and the 47ohm resistor had made a lovely black burn mark on pcb... checked the relay contacts and they arnt stuck.

So if the meter readings keeps increasing wile testing the FETs need replacing?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 30 January 2022 at 5:45pm
you can't just test those mosfets with a multimeter as a floating gate can cause it to acquire a charge, meaning the mosfet slowly turns on, (or ff, it can happen from either direction!) this is probably your meter reading creeping up and is normal. It can also happen in circuit across a whole bank of them.



Kevin

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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 30 January 2022 at 6:47pm
[QUOTE=kedwardsleisure]you can't just test those mosfets with a multimeter as a floating gate can cause it to acquire a charge, meaning the mosfet slowly turns on, (or ff, it can happen from either direction!) this is probably your meter reading creeping up and is normal. It can also happen in circuit across a whole bank of them.


This is true Kevin, i do use a MOSFET tester and just put the probes on all three terminals. So would he be better just de-soldering the Source & Drains, to allow the gates to be tied down ?.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 30 January 2022 at 9:37pm
I use an oscilloscope-based component analyser attached to S/D of the bank and then a croc clip on the gates to tie them to one or other depending on the n or p channel I'm testing. Sometimes the gate will respond to couple of fingers, but it takes experience to cut corners like this.

Providing you can get the fets to remain off tells you there can't be a shorted one in there and so that can't be the problem of the soft start fuse blowing (or conversely proves there is one or more short and so you found the cause).

Then when the amp is up and running I do a gate current test in the usual way to check for any lazy fets. If you dont test for this you may get a bouncer.
Kevin

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Post Options Post Options   Thanks (0) Thanks(0)   Quote LJM106 Quote  Post ReplyReply Direct Link To This Post Posted: 01 February 2022 at 12:23am
So can I just desolder Source and Drain pins but keep the Gate soldered in the pcb, set my DVM to ohms and check to see I'm getting 2ohms?

When you say floating Gate does it need to be grounded as such?

How do you do a Gate current test?

I'm limited to test equipment really, my manager gave me his little oscilloscope that plugs into USB on laptop but haven't used it as I'm worried I'd blow it up haha...

Appreciate all the help I can get.
Thank you. 
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