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Need to teach words to Carlsbro cpx1200!

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Young Croc
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    Posted: 20 March 2012 at 1:06pm
Am currently looking at above mentioned amp which belongs to a client. It has developed a hum on one of the channels (see what I did there in the title! ). Client does not have manual. So this is a plea, does anyone have any technical info/service manual/circuit diagram of this amp?

It's a british one, not Chinese. According to a friend, who is a former carlsbro employee, it was built in 1996. This he deciphered from the serial number. He said that when manufacturing shifted to china in early 2000, all the spare parts and documentation in the factory in England, got thrown in the skip, .

Although it has proved reliable over the course of the 18 months or so years my client has owned it, I have my suspicions that this amp has at some point been tampered with by the inexperienced. Under no load, the amp need only be turned on for about 15 mins or so, before the fan speeds up and pushes lukewarm air out the front of the amp.

Not long after the customer bought this amp, I replaced the captive mains lead, which was dangerous, with an IEC connector and gave it a general clean inside and out, taking great care to put everything back the way I found it. This is the first time I've had a proper look at the amp, I wonder if the fan has been turned through 180 degrees? Of course without documentation, I don't know for certain!

Sorry for the long epistle!, any help much appreciated.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jack_eng11 Quote  Post ReplyReply Direct Link To This Post Posted: 20 March 2012 at 6:02pm
Hum on the outputs whenever anything is played through an amp is normally a problem with the main (largest) filter capacitors. They don't filter the AC properly and leave all sorts of ripples on the supply lines which are then passed on to the rails of the output transistors.

Could also be the caps powering the lower voltage (probably +/-15V) rails for the op-amps in the pre-amp section.

These are the most likely problems, it might not be a bad idea to replace them, especially considering they have been in there for a while.
Jack
"Near enough is good enough, and good enough is best"
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Post Options Post Options   Thanks (0) Thanks(0)   Quote csg Quote  Post ReplyReply Direct Link To This Post Posted: 20 March 2012 at 7:31pm
regarding the fan - these push in the back and out the front. It is normal for them to idle a little warm.
“The fact is this is about identifying what we do best and finding more ways of doing less of it better”
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Post Options Post Options   Thanks (0) Thanks(0)   Quote TMH Music Quote  Post ReplyReply Direct Link To This Post Posted: 20 March 2012 at 11:27pm
Cheers for that info guys. Friend of friend has also told me it may be worth checking the ribbon cables, and internal earth connections. Well remembered about the fan CSG! If memory serves, from a previous post, you used to have a rack-full of these? Am loving what this amp does when hooked up to a pair of twin 18 BR! Sounds sooooooo much more than 600 per chan.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote TMH Music Quote  Post ReplyReply Direct Link To This Post Posted: 14 April 2012 at 8:07pm
Just a quick update for you guys, managed to get this amp sorted, turned out most of the noise was a couple of ribbon cable connector problems and a corroded internal earth connection, which I have now re-sighted in place where it makes a much more secure connection. By the look of the earth connection, if I didn't know better, I'd say this has had a drink spilt in it at some point and this is what has caused the corrosion, the earth bond was in a " not very easy to clean place".

Customer had it back a while ago, and called me the other night to say he used it at a gig and it was fine. There is still a barely audible, tiny bit of hum on the output, so Jack you may be right about the caps starting to break down, will have to keep an eye on this.

Once again, many thanks for the help guys
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