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QED A240CD Blows Fuse on Powerup
QED A240CD Blows Fuse on Powerup
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Old 4th July 2008, 12:53 AM   #11
brainf is offline brainf  Philippines
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I have this amplifier. Internally, mine looks different though (probably an earlier revision of the board), and uses push buttons for input selection.

The output transistors are BD911 and BD912. A kind gentleman from Canada sent me apartial/hand-drawn schematic. Let me know if a copy will be useful.

Armourhome (www.armourhome.co.uk) may be able service this if you're located in the UK.

best regards,
leslie
Manila, Philippines
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Old 4th July 2008, 11:10 AM   #12
Funky971 is offline Funky971  United Kingdom
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Thanks, if you could email me the schematic to flaming_fire_extinguisher AT hotmail DOT co DOT uk

Also where would I be able to buy the transistors and how much would they cost?
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Old 4th July 2008, 02:11 PM   #13
brainf is offline brainf  Philippines
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Sent you the partial schematic over email.

I see BD911 and BD912 listed on www.uk.farnell.com for approximately 60p each. These are not hard to find transistors and any good electronics supply shop should carry them.

~~~~~~~~~~~~
Any experts out there with ideas on how to tweak this little amplifier and bring it up to date? I notice it uses the NE5532AN in the pre-amp circuit. Any newer/better alternatives out there to try?

cheers!
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Old 4th July 2008, 02:15 PM   #14
jaycee is offline jaycee  United Kingdom
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Looks like it'll be a very simple circuit. It should be easy for any competent electronics engineer to service.

I suspect the shop you took it to don't have competent engineers, just people who can follow service manuals

If the transistors are BD911/912 then they are only about 50p each. The driver transistors may also have failed - from this distance they look like BD139/140 which are also cheap.
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Old 4th July 2008, 02:46 PM   #15
Funky971 is offline Funky971  United Kingdom
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Where are the driver transistors located?

Im new to understanding components in amps

When I went into acoustica hifi to try and get it repaired the guy there asked me what sort of amp it was and I told him that it was a QED and he told me that it was too old without even looking at it. Should I try again but tell him that I think it is the transistors that have shorted? Also I could take the schematic thanks to brainf.
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Old 4th July 2008, 05:54 PM   #16
Funky971 is offline Funky971  United Kingdom
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I have opened the amp up again and the transistors are BD911/912 and I could also see a BD140 so suspect the other is a BD139 but I cannot tell because the view is blocked by another component.

I have found BD911 and BD912 on the Maplin website and also BD140. However I cannot find BD139. Where would I be able to pick one of these up?

Also, step by step, how would I test the transistors using a multimeter? I have never done it before but have managed to track down a multimeter.

If it came to it how would I disconnect the power amp?
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Old 4th July 2008, 09:20 PM   #17
Nanook is offline Nanook  Canada
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Default your numberd transistors...

essentially are complimentary pairs, I think (usually consecutive numbered transistor are).

But before taking a huge plunge (not financially, but personally if you "ruin" the amp ---which is pretty unlikely), please follow sreten's instructions. They make the most sense to start, and as always, his approach is a logical one.

I recall the QED SA240 as being pretty strongly reviewed years ago. I still have a 6 input QED "PCC" passive preamp with an Alps "Blue" 50k attenuator. It is still my reference for "transparent sound", although I know there are better out there. The phono stage, although slightly noisy, was reportedly very good.

QED products have never been widely available in North America, so I think most of the opinions from folks from the UK make a lots of sense as well.

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Old 4th July 2008, 11:02 PM   #18
lohk is offline lohk  Europe
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Looking at my schematic of the QED A240 Ser. 2 (1988) shows a pretty standard amp with a complimatary "triple darlington" output stage. Sadly my schematic shows no types of the transistors used (one page went missing over the years, I suppose) and the copies are quite bad. The A240 doesn't seem to have any short circuit protection so it it is probably simply blown - that means one ore more output and driver transistors are blown. Do exactly as suggested by sreten and measure the transistors in situ just to check which one (switch the multimeter to the diode setting!).

The A240 wasn't my favorite amp in those days but is very good nevertheless - and of course not too old to repair. The BD911/912 are easy to find, and so are the BD139/140 - all pretty standard. Look at the first stage of the output (T8, T10), are these BC639/640? If anything else isn't burned this could be a solution.

There should at least be somebody in your vicinity to help you to repair these - it seems difficult if you have never done it yourself. Search for classical "british hifi".
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Old 5th July 2008, 11:00 AM   #19
Funky971 is offline Funky971  United Kingdom
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Sreten says to disconnect the power amp and try to power it up. How would I go about doing this?
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Old 5th July 2008, 12:02 PM   #20
lohk is offline lohk  Europe
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Finding faults in circuits always work in that way: You have to encircle the possible faulty part. That means following the signal path until you can isolate the part where it is broken or isolate the sections to identify the faulty one. Sreten meant that you should isolate the power section to get sure that the fault is there.

Looking at my schematic (copied from the original very amateurish looking original service sheets) I see that there are two 33V power rails, perhaps running from the rectifier/caps directly to the power amp. The preamp sections have their own +24V regulated section taken from the positive rail and another one -24V (?) to the CD board only. To isolate the power section does not seem to possible easily.

But it is very likely that the power amp has blown transistors. Try to identify them first.
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