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Power Amp Repair - fuses

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spacebiscuit View Drop Down
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    Posted: 28 December 2023 at 11:16pm
I have a Mosfet T-400 Power Amp developed an issue with the volume control so I switched the Potometer. during the change I damaged the tracks a little so soldered in some wires to address this. After reapir I tested on the bench and was ready to reassemble the channel into the main unit.

I then did something really stupid and reversed the polarity, when i switched the amp on there was a brief moment of smoke before I switched the power off but clearly damage has already been done. The amp has two replaceable fuses which have a red and green led. When I power the amp up the green fuse blow immediately and there is then a repeated pulsing sound coming from the speakers.


I've used a multimeter to check the diode (seems there is only one), the transistors and resistors and caps. There is no noticeable sign of burnt out components and the readings I am getting tally with the corresponding component which I am verifying against on the good/working channel.

the smoke looked like it was coming from the bottom right hand corner where the small additional board which is labelled as "cuttoff" sits (detached in photo with brown, orange and green wires). I was convinced this was the issue but switched it over to the good channel and it worked so seemingly this is not the issue.


The small board I swapped out:



This is the rear of the board I repaired after I damaged the tracks:



I'm kind of out of ideas, I have used my bag of spare fuses so my trial and error approach to repair is not working.

I have not removed any components from the board to test with the multi-meter - should this matter?

Thanks in advance.



Edited by spacebiscuit - 30 January 2024 at 12:26am
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kedwardsleisure View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 28 December 2023 at 11:47pm
It's not the most complicated board, why not remove and test all the semi's, store them safely then go over the resistors with the multimeter in situ, lifting any that dont test.

Do you have the facility to test the mosfets?
Kevin

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spacebiscuit View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote spacebiscuit Quote  Post ReplyReply Direct Link To This Post Posted: 29 December 2023 at 4:26pm
I've removed two of the MOSFET, I tested with a multimeter:

Red probe to Source (center pin), black probe to D (multimeter beeps) then to G and multimeter beeps again. If I am not mistaken this indicates a damage?

The piece is "J351" i guess this:

https://www.web-bcs.com/transistor/tmfet/j0/J351.php

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EmPro View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote EmPro Quote  Post ReplyReply Direct Link To This Post Posted: 02 January 2024 at 8:03am
mosfets are done,buy new ones,don't bother with those ones anymore.
EmPro Audio
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 03 January 2024 at 3:48pm
difficult to test mosfets properly with a multimeter, really you need a component analyser or tester, this is because on a good one the gate can float and turn the d-s junction on, making them look like they're short when they're not. If in doubt, just replace them
Kevin

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Post Options Post Options   Thanks (0) Thanks(0)   Quote spacebiscuit Quote  Post ReplyReply Direct Link To This Post Posted: 18 January 2024 at 6:43pm
I ordered some replacements P-MOSFET's - I fitted them but unfortunately now both fused are tripping when I power up the amp - before the repair just the fuse with green led was tripping.

Not sure if the MOSFETs were poor quality or faulty - they came from China as I can't find them stocked in the UK.

With MOSFETs removed the fuses do not trips when I power up the amp.

I've ordered more of the P-MOSFETs and a cheap component tester (LCR-TC7) to test the MOSFET before I try to fit them.

I also have ordered some N-MOSFETs (board has 2 of these) but I'm not sure if the current pair need replacing and I'd like to avoid removing them un-necessarily if they're ok but I'm not how else I can test them.

Any thoughts or ideas?




Edited by spacebiscuit - 18 January 2024 at 6:43pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote EmPro Quote  Post ReplyReply Direct Link To This Post Posted: 18 January 2024 at 8:51pm
Mosfets from china...fake as hell...i bought just once some V-FET's that supposed to be original from a dead power supply amplifier...but i received some odd like new mosfets...they blew up on start up right away,never going to do that mistake again,but in your case just wait for the tester to arrive and try to find genuine parts...

Edited by EmPro - 18 January 2024 at 8:52pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 18 January 2024 at 8:59pm
the mosfets are almost certainly going to be fakes, what sort are they, renesas? Are they laterals or v-fets? Laterals are often faked and v-fets you should be able to get from a franchised distributor.
Kevin

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Post Options Post Options   Thanks (0) Thanks(0)   Quote spacebiscuit Quote  Post ReplyReply Direct Link To This Post Posted: 18 January 2024 at 10:12pm
Absolutely no doubt that they are fake, but reassuring to know that others have had a bad experience with these because it hopefully indicates that there is not another problem with my amp other than crappy replace. Ent parts.

Not sure of the type but datasheet:

https://www.alldatasheet.com/view.jsp?Searchword=2sj352&gad_source=1&gclid=Cj0KCQiAtaOtBhCwARIsAN_x-3KQYfu6nbf2rsHy5yRkj0QEEbWaDo-CfMM8i3hH9k96EWfls6wi_5gaAmXLEALw_wcB

I have looked but can't find any of these from UK sellers, seems they are discontinued hence my only option to data has been the Chinese import. 


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Post Options Post Options   Thanks (0) Thanks(0)   Quote M4trix Quote  Post ReplyReply Direct Link To This Post Posted: 19 January 2024 at 12:21am
You might try Exicon's transistors such as ECX10N20/ECX10P20. Similar specs, a bit faster than 2SJ352/2SK2221. Either way, you don't have many options since they're way obsolete. Renesas still has the option to buy them but only if you order couple of thousands. LOL


Edit: Check the other components thoroughly or you'll have an expensive damage again. Ouch
Btw, what's with the mounting hole on the right side of the board ?


Edited by M4trix - 19 January 2024 at 1:34am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 19 January 2024 at 10:40am
yes those are renesas lateral fets, highly faked.

I remember my first fake one of these, well before buying from china/ebay was a thing, I only needed one and it came from a tv component supplier (remember them?). It went into a C-Audio amp, I forget the model, and when I switched it on the legs blew off (the amp kept working).

I think my last batch of these were supplied by Matrix.

Kevin

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Post Options Post Options   Thanks (0) Thanks(0)   Quote spacebiscuit Quote  Post ReplyReply Direct Link To This Post Posted: 19 January 2024 at 6:06pm
Why are they faked - in demand and out of production?
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