| Funky971 |
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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| Nanook |
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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| sreten |
| 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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| Funky971 |
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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| Funky971 |
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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| sreten |
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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| Funky971 |
| ok, how would the power amp be disconnected, it is all on one board. |
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| Funky971 |
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.
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| Funky971 |
| How much would a new transistor (or transistors) be likely to cost? |
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| Nanook |
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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| brainf |
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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| Funky971 |
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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| brainf |
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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| jaycee |
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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| Funky971 |
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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| Funky971 |
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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| Nanook |
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.
stew |
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| lohk |
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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| Funky971 |
| Sreten says to disconnect the power amp and try to power it up. How would I go about doing this? |
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| lohk |
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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| Nanook |
a couple of thoughts;- if you are having this much trouble identifying basic parts, you should stop
- sreten made a suggestion to disconnect the transformer output from the amp, but you don't really understand things, so you are asking very basic questions, so you should stop and "re-group"
- because you don't understand the basics you could seriously injure yourself--please stop working on this NOW!
Now I know this is "diyAudio" and DIYing can be fun, but I suggest that you stop working on getting this QED working. Not because it can't be done. You need to educate yourself on basic electronics, so you have a(at least) a very basic understanding of electricity and electronics. The transformer can push enough current that you can be seriously injured or worse (':dead:').
If this was a "find" it will wait until you learn a little and maybe have tackled some basic projects on your own, or through evening "interest" classes at your local community centre or college or high school.
I am not trying to be overly critical, it is just that based on some of your questions, you simply have a non-existent understanding of this. You really need to take some time and learn some skills. It really is for your own safety I am making these comments. It is not personal, nor am I attacking your "intelligence". What I am suggesting is that it seems you are already "over your head". I would hate to find out that somehow you were injured working on this.
So put the QED away for now, and take a course, buy a good book on basic electronics (Radio Shack or Tandy used to have a really basic, but informative book for only a few $$), and gain a little confidence. If you need the amp fixed now, get it into a repair shop.
I'd still encourage you to take a course, regardless. Once you learn skills, nothing (except perhaps age...) can take that away form you.
stew |
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| Funky971 |
| I am not going to work on this amp on my own. I am very interested in amplification and am trying to learn about it. I do understand that electronics can be dangerous which is why I am working with a friend who knows what he is doing. I am just trying to learn as much about it as I can before I see him next week to test the transistors with a multimeter. If the transistors come up as fine I will try again to take it to a repair shop. |
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| Nanook |
I just would really hate to see you get injured ...
If your friend is experienced, then I'm sure in person he can help you , and you'll end up learning. The Tandy book, if still available, is almost as basic as it can get. A bonus was that it was really cheap to buy.
I hope I didn't come across as condescending, I really wanted to underline the safety issues.
stew |
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| Funky971 |
| thanks, i appreciate your concern. From experience with computers I have learned that it is important to know what I am doing before I try it and would not want to do anything too complicated until I am sure what I am about to do is right which is why I am getting in person help with this. I have also been researching into testing transistors for my own interest. |
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| brainf |
lohk... you are right, they are triple darlingtons. The first stage uses BC546 (BC556) , the Second stage BD140 (BD139) and finally, the Third Stage BD911 (BD912).
Funky971... Nanook raises some very good points about working with potentially lethal mains voltages present. I'm glad to note you are working with someone knowledgable and are using this opportunity to learn about amplifiers and transistors.
Good luck and let us know how it goes! |
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| brainf |
| Here's a picture of the version I have, btw: |
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| Funky971 |
| emailed you some more shots of mine internally. Mentioned in the email about the speaker connectors but I can see from here that you have the same options as me when plugging in speakers. |
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| brainf |
Thank you Funky.
Lot's of changes between version 1 and version 2 are evident in the layout and features but not in the speaker connections. You'll need to look for narrow profile 4mm banana plugs and just make sure they don't short each other out when plugged it. (Version 1 has output fuses visible in the back-right portion - just below the power cable, while version 2 lacks them).
But before you get to that stage you'll need to sort out the blowing fuses first. What fuses did you replace them with? (The markings on my board lists them as "1.6A Anti-surge" for a 220-240V mains voltage).
Assuming that the blowing fuse is of the correct rating, I'd try to rule out a shorted transformer by disconnecting the secondary windings from the board (the two yellow and two white wires), powering it up with a new fuse and see if it blows again. (If it does, you've got a shorted transformer which needs replacement). If it doesn't then the next place to look is at your output devices (as already mentioned by others here). |
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| Funky971 |
| Ok, thanks, the fuse in mine is the 1.6 amp antisurge. When I was trying to get the amp fixed I took it to two shops. The first one was more of a PA shop than a hifi shop. They opened up the amp and tried a new fuse however they did not have the exact one (I think it needed a T type fuse and they put an F type in instead). They put a different rating in because they said it would compensate for it being the different type. That one blew so he checked the wiring of the plug and tried another one which also blew. He then realized it was a hifi amp and told me to go to the second place which wouldn't even look at it. I am going to go to maplin this week and get some of the correct fuses. |
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| jaycee |
| Judging by the black mark on the fuse in the picture, it is blowing due to a fault - DONT simply put a new fuse in, you will be causing further damage! |
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| brainf |
Got this from a gentleman in Canada (Ivan) who's given his permission to share it here. It's a partial schematic of the amplifier he made while repairing them in the past.
Hope it helps! |
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| Funky971 |
| Me and my friend opened up the amp today and had a look, as suspected the BD911 on the left channel has shorted out. Because they are so cheap and it is a lot of effort to take the board off to take the transistor off we are going to replace all four output transistors. He also ran some multimeter checks on the transformer which came out ok. |
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| brainf |
| Thanks for the update and let us know how it turns out! :D |
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