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Power Amp Repair - fuses

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Old Croc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 26 January 2024 at 11:55am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote spacebiscuit Quote  Post ReplyReply Direct Link To This Post Posted: 29 January 2024 at 2:39pm
Currently I am trying to establish if my fuses continue to blow because of the replacment MOSFET or another damaged component.

Given that the first batch of MOSFETs which blow the fuses are giving comparable readings in my tester to the 2nd batch of MOSFETs which have not yet been installed, can I assume that they did not die with "a lower voltage applied" and the issue is another component.

Or, could a poor quality MOSFET cause the fuses to blow yet reman undamaged?

I've checked the other components on the board and I'm sure the results are comparable/similar so I'm a little bit stuck for what to do.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 29 January 2024 at 5:58pm
When in any sort of doubt and have fuses blowing, etc., you want to stick an incandescent, or halogen bulb in series with the device and save yourself some aggro.

If there is a fault, the light will come on and tell you that all is not well. I use this method all the time when switching on stuff for repair - or stuff repaired but untested. It's simple and saves fuses as well as unexpected blow-outs, fireworks, etc.   Smile


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Post Options Post Options   Thanks (0) Thanks(0)   Quote spacebiscuit Quote  Post ReplyReply Direct Link To This Post Posted: 29 January 2024 at 7:52pm
how does this method with the bulb help - where am I adding them to my circuit. Trying to figure how they will save my fuses - won't they expose my board to the danger that the fuses are protecting?

Is this the same principle:

https://www.youtube.com/watch?v=wRFRwOnLsZI&ab_channel=UncleDoug


Edited by spacebiscuit - 29 January 2024 at 8:23pm
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Xoc1 Quote  Post ReplyReply Direct Link To This Post Posted: 29 January 2024 at 8:40pm
A mains voltage bulb wired in line with the live feed acts as a current limiter. If there is a dead short across the amp, the bulb sees the majority of the voltage & lights up. As the bulb gets hot it resistance increases protecting the amp under test. The amplifier only sees a few volts & does not blow the fuses. You can switch off again & try to find the fault.
If the amp is not drawing excessive current, once the PSU has charged the bulb will go dim.  Fuses are really a last resort to stop the amp from catching fire - The fuses may protect you, but they don't protect the amp from failure.


Edited by Xoc1 - 29 January 2024 at 9:28pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Xoc1 Quote  Post ReplyReply Direct Link To This Post Posted: 29 January 2024 at 9:21pm
I did not find any 2SJ352 fets to test in my spares but found a 2SJ162 Hitachi which is similar. Your test results seem way off. Did you use the self test before measuring your FET's?

Note the diode from Drain to Source which is missing from your test pic!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote spacebiscuit Quote  Post ReplyReply Direct Link To This Post Posted: 30 January 2024 at 12:08am
Yes I did do the self-test before test - just tested again but now i am getting "unknown of damaged part" on the 2SJ352s since I put them on the board, and one of the n-MOSFET is showing as a capacitor now.

I guess the replacements were not fit for purpose and have now only made matters worse :) 

I've tested the two original 2SJ351s on the working channel (still on the board though) and they don't show the diode either but I guess it is irrelevant since they're toast anyway....

Is it time for me to trash the review for these in the hope that someone else doesn't but this fake crap?

Have no idea how I am going to source a working pair.






Edited by spacebiscuit - 30 January 2024 at 12:09am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 30 January 2024 at 8:59am
Originally posted by spacebiscuit spacebiscuit wrote:

how does this method with the bulb help - where am I adding them to my circuit. Trying to figure how they will save my fuses - won't they expose my board to the danger that the fuses are protecting?

Is this the same principle:

https://www.youtube.com/watch?v=wRFRwOnLsZI&ab_channel=UncleDoug


Yes, that's it. Limit the current flowing through the DUT and so avoid unnecessary damage to the circuit &/or board. And avoid constantly having to replace fuses!!   LOL LOL LOL






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Post Options Post Options   Thanks (0) Thanks(0)   Quote spacebiscuit Quote  Post ReplyReply Direct Link To This Post Posted: 30 January 2024 at 11:41am
Ok I'm going to make myself a bulb limiter - makes sense.

are the following assumptions correct:

- Would these fake crappy MOSFETs produce the behaviour I am experiencing - tripping fuses

- Will the bulb limiter protect any new MOSFET that I fit in the event that another component is damaged and also causing the circuit to trip the fuses

I've sourced what I believe to be genuine MOSFET - the seller has re-assured me that they are genuine.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 30 January 2024 at 1:00pm
Yes, my incandescent lamp limiter is an essential tool for me and also something to do with all those (now) illegal bulbs! Use at least 2/3 x 100W or equivalent.    Smile

In theory, it should protect the circuit from any damaged/fake component causing fuses blowing and further damage on the board.

Fake MOSFETs failing will definitely cause fuses to pop.




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Post Options Post Options   Thanks (0) Thanks(0)   Quote spacebiscuit Quote  Post ReplyReply Direct Link To This Post Posted: 10 February 2024 at 12:54pm
Replacement 2SJ352 received - my tester gives this result - promising?








Edited by spacebiscuit - 10 February 2024 at 12:56pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote APW Quote  Post ReplyReply Direct Link To This Post Posted: 10 February 2024 at 7:14pm
Unless these are NOS (New old stock) from a reliable source, they are almost certainly fake… there are so many fakes out there that it’s unreal and NOS stock of these is like rocking horse poop.

Unless you know the source forget about these obsolete fets… the only real viable option for new fets is using Exicon ones from profusion that M4trix posted about on the first page of this thread. 

Profusion are the main dealer for Exicon so you are guaranteed that they won’t be fake.

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