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Old 25th August 2014, 03:42 PM   #1
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Default C-Audio RA2001 Repair

Hello. Newbie here - thanks for having me.

I have a studio amplifier on which one channel has failed. I was given the amp a couple of years ago and told that it was in fully functional order, but when I first turned it on this weekend I discovered that Channel B doesn't work at all. (I'm sure it is the channel which is not working as I have tried it with a known-good signal and speakers, and these both work on Channel A but not on Channel B).

The amp is a C-Audio RA2001, and I'm planning on using it as a home HiFi amp. I don't know a lot about amplifiers and repairing them but having taken it apart it looks like this;

Click the image to open in full size.

A bit of googling tells me that the most common failure on an amplifier is the output stage MOSFETS - presumably the MOSFETS bolted up to the heat sink in the middle of the amp. A close look at them doesn't reveal any as being obviously blown (i.e. charred, singed or with scorch marks next to them on the heat sink). However this one looks like it may have leaked something and is possibly cracked:

Click the image to open in full size.

Does this look blown to you? What are the chances of this being the source of the problem?

I have a reasonable grasp of electronics and know how to test a MOSFET with a voltmeter, but do the MOSFETS have to be removed from the board before testing? (i.e. Do they individually need to be unsoldered before being tested?) I also read that sometimes an amp will be restored to an operational state if a blown MOSFET is removed from it as it will work (in a less than optimal way) without it, allowing you to diagnose the problem. Is this the case, and is it as simple as this?

If it proves not to be the MOSFETS then does anyone have a good step-by-step guide to diagnosing problems with amplifiers?

Thanks, in advance, for any help.

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Old 25th August 2014, 04:27 PM   #2
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Join Date: Apr 2008
Location: Carlisle, England
IF you test the mosfets with an ohm meter with the legs closest to you the two right hand pins often go short circuit if faulty.
You can test them in circuit with speaker disconnected.

Fixing amps can be fraught with problems.
Just because a mosfet is blown doesn't mean simply replacing it will fix the amp.
Sometimes components further back in the circuit can have failed or have been blown up by the mosfet failing.
On the other hand you might be lucky and it is just a mosfet blown.
PCBCAD50 software. http://www.murtonpikesystems.co.uk
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