QED A240CD Blows Fuse on Powerup - diyAudio
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Old 2nd July 2008, 09:20 AM   #1
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Default QED A240CD Blows Fuse on Powerup

I have a problem with a QED A240CD amplifier. The problem is that when I try to switch it on it blows the internal fuse. My local hifi repair shop said that it is too old and that to repair it I need to find a circuit diagram and an engineer. This is exactly what I intend to do!

Does anyone have a circuit diagram for this amplifier?

Also where in the amp would be a good place to start looking at?
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Old 2nd July 2008, 09:38 AM   #2
Nanook is offline Nanook  Canada
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Default pop the lid....

and look around the power transformer and where it mounts to the circuit board (either directly or with wire jumpers from the transformer to the circuit board).

Also check the obvious spots where the speaker connectors attach to the circuit boards and where the bananas are inserted. One of the criticisms of the SA240 was that bananas had to be inserted into recessed jacks and the jacks were surrounded by the metal back panel. Use insulated banana plugs . Check to make sure there are no shorts from the jack and the rear panel and look very closely at where the jacks mount to the circuit board for cracks between the traces on the circuit board.

If the fuses are popping that quickly, the signs of shorting should be obvious.
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Old 2nd July 2008, 12:45 PM   #3
sreten is offline sreten  United Kingdom
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Default Re: pop the lid....

Quote:
Originally posted by Nanook

If the fuses are popping that quickly, the signs of shorting should be obvious.
Hi,

If an output transistor fails by going short it is not obvious at all.

However any competent technician should be able to fix it without
a circuit diagram assuming the cause is a a single failure point.

/sreten.
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Old 2nd July 2008, 03:04 PM   #4
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ok, I have opened it up, it all looks ok to me.

I have not managed to use this amp yet, I have not powered it up with speakers attached, my current amp does not use banana plugs, is there a possibility that when I try to power it up with banana connectors in it will just work?

I can post pics if needed.
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Old 2nd July 2008, 03:04 PM   #5
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ah, so an output transistor could be the problem, how can I tell if it has shorted if it is not obvious? What sort of equipment could be used to test the transistors?

Also how much do output tansistors cost? If it is just a case of soldering I know people who could do it for me.
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Old 2nd July 2008, 04:19 PM   #6
sreten is offline sreten  United Kingdom
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Hi,

If its a fairly standard amplifier then there are fairly standard things
you do if you know what you are doing. Disconnect the supply to
the power amplifier, still blows ? power supply problem, etc.

A power transistor should measure as two back to back diodes.
If it is blown it will not. Most output stages can be checked with
a multimeter in situ (power off). You just need to know what you
are doing to an extent. For FET outputs its not so easy to show
they have blown open, but blown short is easy to ascertain.

If the problem is more complex then a circuit diagram helps.

/sreten.
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Old 2nd July 2008, 06:51 PM   #7
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ok, how would the power amp be disconnected, it is all on one board.
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Old 2nd July 2008, 06:51 PM   #8
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I have taken a picture of the inside of the amp, it looks quite simplistic inside but it is all on one board so I don't know how I would disconnect the power amp.

Click the image to open in full size.
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Old 3rd July 2008, 03:07 PM   #9
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How much would a new transistor (or transistors) be likely to cost?
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Old 3rd July 2008, 06:08 PM   #10
Nanook is offline Nanook  Canada
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Default as far as transistors costs,

get the numbers off them (they're attached to the heat sink at the back of the amp. As sreten suggests, disconnect the output of the power supply, and try again.

stew
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