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Fixing a MOSFET power amp
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v-un-v
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PostPosted: Sun Aug 06, 2006 4:48 am    Post subject: Fixing a MOSFET power amp Reply with quote  Mark this post and the followings unread

I've just been away for the weekend visiting friends and of the things that was sprung on me was a homemade mosfet kit amp that keeps blowing the bridge rectifier.

But I'm stumped because I've never come across this sort of problem before. Anyone here have any clues?


The large electrolytics are showing the right voltages across their tags- and appear to be okay (but I suppose you never know?). Any other pointers??

Tom Smile

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PostPosted: Sun Aug 06, 2006 5:01 am    Post subject: Reply with quote  Mark this post and the followings unread

Is this something out of Elektor magazine?

Bridge rectifier? You are talking PSU here. This one isn´t intergrated on the main PCB? Any chance you can isolate it and see if it works OK without the amp circuitry? You gotta start somewhere.

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PostPosted: Sun Aug 06, 2006 5:07 am    Post subject: Reply with quote  Mark this post and the followings unread

elektro80 wrote:
Is this something out of Elektor magazine?


No. Some awful Maplin job! Shocked Laughing



elektro80 wrote:

Bridge rectifier? You are talking PSU here. This one isn´t intergrated on the main PCB? Any chance you can isolate it and see if it works OK without the amp circuitry? You gotta start somewhere.


Again no. This time it was coddled together with the psu separate from the 2 (yes 2 individual) mono mosfet amp boards. At the back of it there is a sort of 'space wired' psu mess ( Laughing ) and a large tordial transformer.

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PostPosted: Sun Aug 06, 2006 5:11 am    Post subject: Reply with quote  Mark this post and the followings unread

Oh oh...

Shocked

Lemme think...

Do you have scans of the schematics?

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PostPosted: Sun Aug 06, 2006 5:12 am    Post subject: Reply with quote  Mark this post and the followings unread

Coddled together? Codpiece style? Shocked
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PostPosted: Sun Aug 06, 2006 5:16 am    Post subject: Reply with quote  Mark this post and the followings unread

Only thing I can think of is figuring out if the PSU can drive both mono blocks. Then isole the PSU and find out if it works as advertised ( or has been built correctly).

As for the amps: Check out the schematics and find out if there are any "idle current" suprises somewhere.

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PostPosted: Sun Aug 06, 2006 6:05 am    Post subject: Re: Fixing a MOSFET power amp Reply with quote  Mark this post and the followings unread

v-un-v wrote:
Ia homemade mosfet kit amp that keeps blowing the bridge rectifier.


When switched on or after a while ?

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PostPosted: Sun Aug 06, 2006 10:08 am    Post subject: Reply with quote  Mark this post and the followings unread

elektro80 wrote:
Only thing I can think of is figuring out if the PSU can drive both mono blocks. Then isole the PSU and find out if it works as advertised ( or has been built correctly).


Yes the psu drives both monoblocks and the actual amp has been functioning well for 10 years or more.

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PostPosted: Sun Aug 06, 2006 10:08 am    Post subject: Re: Fixing a MOSFET power amp Reply with quote  Mark this post and the followings unread

Blue Hell wrote:


When switched on or after a while ?[/quote]

when switched on.

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PostPosted: Sun Aug 06, 2006 10:12 am    Post subject: Reply with quote  Mark this post and the followings unread

As in:

Power on

...Zap!!!!!

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PostPosted: Sun Aug 06, 2006 10:13 am    Post subject: Reply with quote  Mark this post and the followings unread

?????

Shocked

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PostPosted: Sun Aug 06, 2006 10:41 am    Post subject: Reply with quote  Mark this post and the followings unread

elektro80 wrote:
?????

Shocked


Indeed

?????!

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PostPosted: Sun Aug 06, 2006 10:44 am    Post subject: Reply with quote  Mark this post and the followings unread

Like Maggie Thatcher in an electric chair?

Shocked

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PostPosted: Sun Aug 06, 2006 10:50 am    Post subject: Re: Fixing a MOSFET power amp Reply with quote  Mark this post and the followings unread

v-un-v wrote:


The large electrolytics


How large exactly ?

Could be the startup current for the caps is too high maybe (but only when switched on at the "right" moment then, so it shouldn't fry itself not always, just often). Especially as toroids tend to be good transformers (in the sense of being able to supply lots of current).

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PostPosted: Sun Aug 06, 2006 4:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

Well if it worked fine for 10 years, I'd put new rectifiers in which are rated at a higher current than the old ones. Then see what the quiescent current is, and where it's going. I know this sounds dodgy, but whatever the problem, it blew the rectifiers rather than itself, last time.
Does it have any type of output protection circuitry? The only thing I can think of that could carry enough current to blow the rectifiers without going open itself would be the mosfets. Or the caps?
I just had a thought. Even if they "seem" okay that doesn't mean the caps are okay. And after 10 years they are the thing which will definately be getting "past it".

I don't know how much help that is. I'm just thinking out loud. Smile

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PostPosted: Sun Aug 06, 2006 4:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

That is a reasonable suggestion.

Tom, does the amp stage have some sort of relay/soft kick in circuitry on the outout somewhere. Some of these designs can really act up.

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PostPosted: Mon Aug 07, 2006 1:44 am    Post subject: Reply with quote  Mark this post and the followings unread

elektro80 wrote:
does the amp stage have some sort of relay/soft kick in circuitry on the outout somewhere. .


Nope. This is why I asked the question in the 'DIY' section! Wink

Jan- I made a note of what the caps were, but I seem to have misplaced it. But I'll get back to you on that one.

The caps? Yes- I suspected the caps immediately. I also know the man who originally put it together. I would suspect that he'd taken the "Oh that'll do" route ! Laughing

I don't have the amp here (I'm back at home)- but I'll pass the suggestions on, and report back when something good is happening Very Happy

Thanks folks! Smile

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