QED A240CD Blows Fuse on Powerup

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To be specific, 50 Hz hum (not buzz) is derived from the mains supply - up to the transformer primary winding(s) in the audio system devices.

A Buzz usually identifies a 100 Hz or rectified mains source after the transformer/power supply and that often means the electrolytics are failing or worse. Just how loud is this noise relative to signal and does it vary with volume setting? As this could seem to give contradictory results to your assessment, try a different source - even an MP3 player with an isolated (battery) supply. If the buzz disappears you will be more certain of the problem, whether a hum loop or fault in the player (or other source) itself.
 
Ian

thanks for very prompt reply
no buzz or hum with only ipod attached, with rest of system attached buzz is constant and too loud to be ignored regardless of volume setting

I am beginning to think I have a loop - I am being a bit ambitious with my system
3 amplifiers, 3 sets of speakers all fed from single set of source components - using record outputs to feed 2 amplifiers from the primary amp attached to all the sources.

maybe daisy-chaining is going to be better than this "v" topology - time for more tests I think

thanks
 
I need help!
Got an A240CD Version 2 with left channel not working. Checked transistors and they're all good. Checked bias and right channel is at 120mv but nothing at left channel. I've checked the pre-amp by connecting the A240CD to a separate power amplifier and all was perfect. Just wondered if anyone could point me in the right direction where to test next?
Thanks.
 
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I wonder about your measurements or at least, how 120mV came into the picture. That suggests you also have about 120 mA bias current when measured across the series 0.47 Ohm emitter resistors of the output transistors - quite high for the tiny output stage heatsinks in the A240 amplifiers and would result in a very warm heatsink, just idling.

If you haven't already used it, there is a hand drawn schematic and service checks of voltages here for others to see: QED A240CD - Manual - Signal Path Optimised Stereo Amplifier - HiFi Engine
This gives us an idea of whether the amp is healthy or not by locating the appropriate tests points and confirming the voltages. Note that this a CFP design output stage and bias current is optimal at nice, low bias currents of only about 10-20mA. Indeed, the schematic indicates 10-20mV should be present when measured across both 0.47 Ohm resistors in series (about 1 Ohm) like the arrows show.

I know that many people see a single adjustment pot. and feel they must turn it to see if this makes things work better...but don't play! They were not intended for regular use and will soon fail. So, you may need to replace these if there have been previous owners who liked to play. Just set to 15 mV and proceed with testing the voltages shown on the schematic for both channels. Let us know of any other differences you find between them too, preferably on a copy of the same schematic, if you can edit it ;)
 
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Thanks for the response.
My skill is very limited so I'm restricted to basic tests etc. With the image on a previous page, I was able to locate and test the bias of each channel. The bias of 120mv (on the good channel) I'm pretty sure was down to the trim pot as I hardly touched it and easily achieved 15mv which is now stable. There is 0v on the faulty channel. I have tested the transistors which read the same from one channel to the other.
Where should I check next?
 
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The process is not difficult - these really are basic measurement tests. Just take care not to slip with probes and short anything while poking around a live amplifier. Well, I'm sure you realise the consequences of short-circuiting high current supplies through delicate components or even yourself.

Identify the other measurement points by the marked values of the components, if you can't simply read the their corresponding identification numbers on the PCB. Then measure the voltages expected as per the schematic and compare left to right channel readings - same as you already have done with the emitter resistors. With some more clear differences to consider, we may be able to offer some help by pinpointing the problem(s). If you have difficulty interpreting a schematic, there will surely be someone local who is familiar with them and can assist.

You can't really be sure about power transistors in-circuit, so how did you check the transistors as ok? You can get a fair indication of transistor health by checking Vbe, the diode voltage between base and emitter in operation and when combined with a voltage test for any short between collector and emitter, you can be more certain. For a definitive test of power transistors though, you would probably need to remove and test these at the currents they were intended for and that is not simply done with a multimeter alone. You need to check their driver transistors as well, because in a CFP design, these function as a part of the output transistor - sometimes called a "compound transistor" in this case.

Unfortunately, without more evidence, we can't say much about the missing bias current alone, as other associated components like the pot. or solder connections could have broken or failed in many different ways, confusing the search.
 
Thanks Ian,

So far I've just compared Right channel to Left channel, testing each component just using the continuity mode on my meter. Everything seems to be near enough exactly the same from one channel to the other. The only thing I haven't tested is the capacitors but all of the resistors, transistors, diodes all seem to test the same. I've examined the solder work because it's as if that is what's wrong but it all looks good. Like I mentioned before, I've tested the pre-amp section by connecting it to a power amp and it performs perfectly.
Could it be the caps? They all look fine with no bulges, in fact the inside of this amp all looks like new.
 
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The continuity mode on most meters is just a tone which sounds if the resistance between the probes on an unpowered circuit is less than a nominal value, around 70R. It does not properly measure or compare the voltage between two sets of points on a live circuit, which is what is called for.

You have already measured voltages to set the quiescent current so there shouldn't be a problem measuring the other specified voltages and comparing with those written - assuming you followed the link and downloaded the schematic for yourself (free after registering at Hi-fi Engine) and looked at the docs. If you are having problems understanding the steps, say so or get help.

Attached is a snip of the hand-drawn schematic. The corresponding parts values are also listed and you'll probably need them - as explained.

I note from comments at HFE that there have been problems with the balance control which is said to be wired to switch itself out and allow direct connection at its extreme left setting. It almost sounds like a candidate for where the left channel may be getting lost, and the control may be at fault. You could cobble up a signal tracer by just fitting an insulated wire from the preamp L out that is carrying a signal on it to prove this.
 

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My meter's not just a tone but gives a value also. I know it's not the best method but I guessed that if the values it shows are exactly the same from one channel to the other then it would at least point in the direction as to where the problem was when I came to a component with a value that doesn't match that of the other channel.
Looking at the schematic, what does lV(number) and 0v(number) mean? I take it these are the areas to test?
Thanks.
 
post75 sch.
The Vbe multiplier has the VR in the C to B side. This can cause damage if the VR wiper goes open circuit.
This should be changed so that the BE side has the VR and the CB side has the fixed resistor.
Even better if the VR on the BE side is a series pair of {fixed plus VR}, where the VR has the wiper shorted to one end of the track.
 
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Standards for electrical and most scientific notation change quite regularly with the times and the committees that govern them. That could just mean the current chief chair-polisher wanted to make their mark but to the layman, it looks like a passing novelty. Yet there is immense value in unambiguous, printer-friendly text since reading hand written figures and older printer type are notoriously error-prone. Ongoing change and old data though, leave us with a morphed confusion of systems that only exacerbates the problem, one we unfortunately have to live with.

So 10.5 volts for example, appears simply as 10V5. 0.6V would be 0V6 etc. (i.e. no decimal point used) Yes, these are the voltages I described earlier but use the meter's voltage ranges, not the continuity/resistance ranges and refer to the download schematic as this is clearer and includes the list of parts values which you can then check against the actual components if you need to be certain you are in the right location.

Regarding DMM measurements: If a meter rounds off a measurement by virtue of its display system, it may be missing significant figures which leads you to believe 2 measurements are the same but actually could be quite different. I guess if the meter was a model that allowed checking by manually changing ranges to show enough significant figures for valid comparisons, this could be checked. Ordinarily, it isn't a problem with recognized brands and models but I have seen low cost meters with fancy features that really cause more problems in use than they are worth, whether their owners realise it or not.
 
Right, I get you.
So, I've just tested as per the schematic voltages in the shown areas and the bad channel reads 0v all the way back from the output to the last measurements I took which were across R71 and across R66. The good channel matches the schematic perfectly.
So the problem is further back, where to check now?
 
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