I've recently completed some work on my QED A240CD amplifier, replacing some capacitors and transistors. This included the 2 filter caps. I now seem to have a strange issue where my speakers are buzzing even at zero volume. I cannot hear any sound coming from the amp itself when speakers disconnected. It seems to be an intermittent 'fault' because yesterday I played music all day with no problem. When it occurs it affects both speakers. The weird thing about this is that when I either hovered my hand over the internals or lightly touched the outer casing of some of the capacitors the buzzing would stop. The lid was not fitted at this point so I thought that maybe the lid needs fitting to maybe 'shield' or 'deflect' any external interference. It in fact made it worse. I only had to hover the lid over the top of the amp and the buzzing would get worse. My suspicions are perhaps some earth fault but I'm aware it could be caused by earth loops, cable placement or having the equipment close to external electrical interference. I've tried moving the amp well away from other equipment and ensured cable routes were good. The amp has always been plugged into the same wall outlet as other attached equipment. Anyone have any ideas what could be causing this strange (but true) phenomenon? Thanks, Andy
Welcome to the forum, Andy. I'll give your thread a bump!
Is your A240CD the original version?
Or is it the Series 2 version with the separate listen and record selection switches?
Is your A240CD the original version?
Or is it the Series 2 version with the separate listen and record selection switches?
For reference, the A240CD Series 2 Service Sheet is downloadable here: https://elektrotanya.com/qed_a240_series_2_service_sheet_sch.pdf/download.html
It may be helpful if you state which capacitors (other than the filter caps) and transistors you changed, because thereby may lie your problem.
It may be helpful if you state which capacitors (other than the filter caps) and transistors you changed, because thereby may lie your problem.
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Hi. In fact I changed all electrolytic caps and as far as possible for poly caps. I also matched paired values as close as possible when selecting which to fit. I also did the same for the transistors with regard to hFE values. I also replaced the relay and bridge rectifier. All replacements were bought from a reputable source (DigiKey). One thing I did notice when hovering or touching the ends of the capacitors was that the hum dissipated on all except for either C30 and C31 or C10 (I’ll check later when I get chance to confirm which it is). When I touched the end of those the buzzing got worse. My suspicions are around an earth 🌍 issue. And one other thing that might help is that when switching between the various listen/record switch positions some had no buzzing where others did. As you can probably gather I’m a fair newbie at the diagnostic part of all this but am very keen to learn.
What was the original fault that prompted you to do all this work? Changing transistors just for the sake of it can be a recipe for disaster and tbh recaps often 'go wrong' for various reasons.
'I now' reads as if you did not have these problems before.
'when hovering'. If there is absolutely no physical contact and just being near the circuitry causes unexpected behaviour then that is a classic sign of very high frequency instability. You need to look with a scope at what this buzz looks like and whether in fact there are stability issues.
That could be a problem in itself as electrolytic caps have increasing self inductance at higher frequencies and that can sometimes be a necessary part of a design to ensure stability... or alternatively it can be viewed as an aspect of a design that is not even considered and only becomes apparent when alternative parts are substituted.
Transistor types can also play a part and although generic replacements (if that is what you used) usually are fine it is still something that can not be guaranteed. Always make sure replacements are obtained from authorised distributors and not from eBay and the like.
Sorry if that all sounds a bit negative but they are all points to consider.
Hi Mooly and thanks for the comprehensive reply. I understand the point of view of 'don't fix it if it's not broken' but sometimes the OCD get's the better of me. And of course I'm a newbie too. As stated in my message all parts were sourced from DigiKey who are a reputable supplier. I've also just this minute tried another variable. I removed the MM Phono Board (standard board) and this eliminated all buzzing. When I replaced the board the buzzing came back. This was all with nothing connected to the inputs and outputs. Maybe a problem on the phono board?
It is very hard to say with certainty. If it did not do any of this before the work was done then you have to go back over all the work that was actually done and check, check check. If that is the scenario then its an inescapable fact that something done has caused this issue.
The phono stage itself is a very high gain amplifier and prone to stray noise pickup but that should in itself should not be altered by the parts swaps in theory. What is an unknown is whether it or some other area is now unstable. This effect of things altering as you move nearer or further from it is a classic symptom of a circuit oscillating at possibly many MHz and the effect of your hand etc effectively alters that condition or can even cause it to start and stop. Only a scope check would confirm all that.
With the phono stage being such high gain you should really have shorting links fitted to the inputs as if they are floating and open circuit they can pick up stray noise... but not to the point of causing it all to do strange things.
The phono stage itself is a very high gain amplifier and prone to stray noise pickup but that should in itself should not be altered by the parts swaps in theory. What is an unknown is whether it or some other area is now unstable. This effect of things altering as you move nearer or further from it is a classic symptom of a circuit oscillating at possibly many MHz and the effect of your hand etc effectively alters that condition or can even cause it to start and stop. Only a scope check would confirm all that.
With the phono stage being such high gain you should really have shorting links fitted to the inputs as if they are floating and open circuit they can pick up stray noise... but not to the point of causing it all to do strange things.
Thanks Mooly, I shall set to with the scope in a day or so and report back. I’ll also check through the work I have done so far in case I’ve missed something obvious.
