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mosfet replacements please help!!!

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jbl_man View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2017 at 1:48pm
Originally posted by young simba sounds young simba sounds wrote:

between the resistor is the two brown little ceramic capacitors thats the last thing, i have one that has gone, any info on what would be the best as there is no indication on them? also this is a pic of the board in repair 


Out if interest,what amplifier is this from? Those individual heatsinks look like they would be a dust-trap!
Be seeing you.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote young simba sounds Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2017 at 1:57pm
out of a please ka2401!!!
scoops...yes scoops the only SUB for me
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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2017 at 6:44pm
Originally posted by young simba sounds young simba sounds wrote:

AngryAngryAngry doing my head in trying to find these fets i dont think i am but is there an alternative i can use

numbers on fets are as goes sml buz908d

and buz 903d 

any help would be much appreciated thanks!!



Sorry i am confused, your asking for the above FET'S yet in the board that you posted a picture of they are BUZ901D & BUZ906D as well as BUZ903D & BUZ908D ?????ConfusedConfused i am sure these need to be ALL BUZ901D & BUZ906D throughout both channels (the last one i repaired had ALL 906D & 901D).


Edited by simonp1100 - 17 May 2017 at 6:47pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ceharden Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2017 at 6:47pm
IMO the cost of the replacement FET's will be much more than the value of the amp.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2017 at 6:48pm
Originally posted by ceharden ceharden wrote:

IMO the cost of the replacement FET's will be much more than the value of the amp.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote young simba sounds Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2017 at 6:49pm
thats what was in there, i got another two of these amps all fets are buz903d and 908d but i was just going to put back in what i took out
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Post Options Post Options   Thanks (0) Thanks(0)   Quote young simba sounds Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2017 at 6:50pm
in the long run this amp was a spare anyway just incase i needed parts but i thought id have a shot at it to learn on basically so money is not much of an object but rather more of a learning amp
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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2017 at 7:00pm
I would make sure that you replace all the BUZ906D & BUZ901D with BUZ908D & BUZ903D as the 906D & 901D are rated at 200V devices & the 908D & 903D are rated at 250v devices.

These amplifiers were well known for FET'S exploding hence why they may have changed / up-rated the output FET's due to the high supply rails.

Do not mix the two together (either all 200v devices or all 250v devices).


Edited by simonp1100 - 17 May 2017 at 7:02pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote young simba sounds Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2017 at 7:05pm
thanks simon, this must be an older model to the other two then as this one also has different capacitors also not as well built as my other two, i might just put this one to rest lol
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Post Options Post Options   Thanks (0) Thanks(0)   Quote _djk_ Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2017 at 10:25pm
"Do not mix the two together (either all 200v devices or all 250v devices)."

Does not matter.

If the ±V is 100V or less the 200V parts will be fine

The difference between the 200V and 250V parts is the 250V parts are obtained on a selection basis.

They start out with the same die .

The ones that test better than 250V become 250V parts.

Then they lower the test spec to 200V, and the ones that pass are sold as 200V parts.

Hitachi did the same with the 2SJ48/49/50 series rated at 120/140/160V.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 18 May 2017 at 5:25am
Originally posted by _djk_ _djk_ wrote:

"Do not mix the two together (either all 200v devices or all 250v devices)."

Does not matter.

If the ±V is 100V or less the 200V parts will be fine

The difference between the 200V and 250V parts is the 250V parts are obtained on a selection basis.

They start out with the same die .

The ones that test better than 250V become 250V parts.

Then they lower the test spec to 200V, and the ones that pass are sold as 200V parts.

Hitachi did the same with the 2SJ48/49/50 series rated at 120/140/160V.


Because of the past record with these amplifiers having FET's blowing with the 906D & 901D fitted and with the supply rails being OVER 100v, stick with the higher voltage FET's for longer long term reliability. Repaired quite a few of these in the past and have seen many PCB's damaged beyond repair due to FET's blowing and taking out the board as well. Clap

By the way these amplifiers were made in Bristol (Flowers hill). 


Edited by simonp1100 - 18 May 2017 at 5:25am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote odc04r Quote  Post ReplyReply Direct Link To This Post Posted: 18 May 2017 at 10:45am
Tbh an amplifier with +100V is not a great idea for a learning project anyway. As the FETs are valuable then you're probably better off stripping it for parts or selling intact as spares second hand, then investing in a more sensible amplifier for learning with.

The P3A on ESP audio is a great starter to build, and as long as you have a half decent dual DC power supply then you won't have to build any mains power supplies to be able to have a listen at low levels. If you simulate the design first you will learn a lot as you build it and test it out, mechanical as well as electronic with regards to construction and biasing. It's a good way to start learning vs jumping right in at the deep end with an unknown quantity.
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