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Old 10th January 2009, 07:38 PM   #11
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Quote:
Originally posted by xenithon
It was a demo unit from an official distributor in the US (I am in South Africa). I have been in contact with the distributor and he is refusing to accept a return.
This shouldn't be too difficult for a competent tech to fix, it's just a bit of PSU ripple probably caused by defective components rather than a design flaw.

This would be an open-and-shut case under UK consumer protection law, demo does not mean unserviceable and the unit is obviously not of merchantable quality. They took on the modification request, and they should guarantee their work. The device was obviously not tested after modification, what kind of procedures do they have?

Best I can suggest is that you threaten the distributor with publicising their name and contact the manufacturer and get them to put pressure on the distributor, they're probably more concerned about the good name of their product than the distributor. Rant a bit, you've got reason to.

Worst case, you can probably get somebody local to fix it and the components won't cost much, in which case post the manufacturer and distributor names in this thread, ...it's not much but it's something.

w
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Old 10th January 2009, 07:46 PM   #12
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Quote:
Originally posted by wakibaki

This shouldn't be too difficult for a competent tech to fix, it's just a bit of PSU ripple probably caused by defective components rather than a design flaw.
Agreed.
Quote:
This would be an open-and-shut case under UK consumer protection law, demo does not mean unserviceable and the unit is obviously not of merchantable quality. They took on the modification request, and they should guarantee their work. The device was obviously not tested after modification, what kind of procedures do they have?
If I understand correctly, it was the US distributor that did the modification, not the SA distributor. If so, the US distributor would be responsible.
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Old 10th January 2009, 08:11 PM   #13
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Quote:
Let the technician measure the input voltages of the stabilisers with the scope and compare them to the R channel. I bet there's a significant difference.
In the power supply section there are just those three stabilizers, which I assume are common to the power supply. That is, there aren't L/R paths for these are there? Apologies for the newbie questions; I have very limited technical/electrical knowledge so want to make sure I ask the technician the right questions

Quote:
This shouldn't be too difficult for a competent tech to fix, it's just a bit of PSU ripple probably caused by defective components rather than a design flaw
I am hoping so too. Would you think that the problem is with the stabilizers themselves or the rectifiers/smoothing caps which come before them?

Quote:
The device was obviously not tested after modification, what kind of procedures do they have?
I have checked this with him in a number of emails and they maintain that they did test it. What was odd was that when I tested it briefly (20-30 seconds; did not want any longer due to safety concerns of an unearthed component) a few nights ago with an unearthed cable - it seemed not to exhibit the noise. Last night when I tested with both an earthed and unearthed power cable again, the noise exhibited with both. The only thing I can think of is that when they tested it, it was brief too and there was no issue? .

Quote:
Worst case, you can probably get somebody local to fix it and the components won't cost much
That is the hope, yes. I am waiting for the schematic as the technician (understandably) does not want to start tinkering or making any changes at all without knowing what is actually meant to be happening on the PCB, especially the power supply section. I have already been in touch with the manufacturer for assistance.

Quote:
If I understand correctly, it was the US distributor that did the modification, not the SA distributor. If so, the US distributor would be responsible.
Correct. In fact, there is no distributor here which is why I imported (I would always opt for a local channel if possible). The North Star site has a company listed but they stopped bringing in North Star almost 2 years ago.
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Old 10th January 2009, 09:31 PM   #14
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Quote:
Originally posted by xenithon
In the power supply section there are just those three stabilizers, which I assume are common to the power supply. That is, there aren't L/R paths for these are there? Apologies for the newbie questions; I have very limited technical/electrical knowledge so want to make sure I ask the technician the right questions
It's quite simple. If both channels share the same power supply, then both channels would suffer from ripple coming from that supply. Since the R channel is not affected, it can only mean that it has its own power supply. Two toroids instead of one IMO is a giveaway (at least, that's what it looks like). The section in the attachment may also be some power supply circuits.
My μΩ DAC1 has three transformers, one for the digital circuits and two for the analogue circuits. One is for the L and the other for the R (I measured slight differences in the voltages, so I'm pretty sure they're separate).

Quote:
I am hoping so too. Would you think that the problem is with the stabilizers themselves or the rectifiers/smoothing caps which come before them?
Stabilsers are pretty solid stuff. I'm not saying that they can't fail because they can, but they're not the usually cause of power supply problems. I have tested thousands of pcbs with stabilisers on them and I seem to remember only one failure of a stabilser!
But these were new products coming right off the assembly line.

Quote:
I have checked this with him in a number of emails and they maintain that they did test it. What was odd was that when I tested it briefly (20-30 seconds; did not want any longer due to safety concerns of an unearthed component) a few nights ago with an unearthed cable - it seemed not to exhibit the noise.
Not earthing will only be dangerous when there is a serious fault that causes the case to have the same voltage as the mains. When the device is not faulty, there is no immediate danger in not earthing it. But in normal use, yes, you should earth it.

Quote:
Last night when I tested with both an earthed and unearthed power cable again, the noise exhibited with both. The only thing I can think of is that when they tested it, it was brief too and there was no issue? .
That's the problem with intermittent failures. This kind of failure is the most difficult for a technician to find, if it happens at all on the test bench.

Quote:
That is the hope, yes. I am waiting for the schematic as the technician (understandably) does not want to start tinkering or making any changes at all without knowing what is actually meant to be happening on the PCB, especially the power supply section. I have already been in touch with the manufacturer for assistance.
It may be worth taking a closer look at the pcb (take it out to have a look at the solder side too). Maybe the US distributor left some debris that shorts intermittently. It happened to me a few times...
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Old 10th January 2009, 11:55 PM   #15
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Yeah, I found a picture on TNT audio, it seems to have completely symmetrical R & L supplies. It COULD be intermittent, a dry joint on one of those big caps on the 9V LH supply would probably do the trick. It could be as simple a fix as reheating the solder joints on it. Not a big risk even for a novice...

w

Oh, if we can speculate this far, sight unseen, your technician should have considered some of this. An LV PSU is something he should see every day. If he can't manage thought one, a schematic isn't going to help much.
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Old 11th January 2009, 05:32 AM   #16
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Hi guys. Thanks so much for the assistance thus far. I could not find any debris on the PCB. Would dry solder be visible at all?

I have some more photos which I uploaded to a hosting site (as they are larger than the size limit here).

The first one is an overview of what looks to be the power supply section. At the top left (very blurred in this photo) it says "ANALOG SUPPLY" and at the bottom right it says "DIGITAL SUPPLY". I am not sure if that "DIGITAL SUPPLY" section is the area which continues to the right in the photo. Also, you will notice that the three stabilizers are marked (top to bottom) IC2, IC3, and IC12.

Click the image to open in full size.

The second image is of what looks to be the "ANALOG SUPPLY".

Click the image to open in full size.

The third image is of what looks to be labeled the "DIGITAL SUPPLY".

Click the image to open in full size.

wakibaki - I understand you sentiment, and he does is very experienced in the field. He just wants to proceed with caution and rather have the schematic on hand than work based on assumption and speculation etc.
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Old 11th January 2009, 09:14 AM   #17
jitter is offline jitter  Netherlands
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Nice looking layout. The silk screen (= white printing) on the top also clearly defines the different functions. They even let the DAC straddle the line because it has both digital and analogue sections!
The only violation I've seen so far is the that the colours of the toroids' wiring don't match the description, which they probably did on purpose to avoid a twist in the wires (as these are symmetrical, that is no problem).

The idea I had of one toroid per channel was wrong. One toroid has 9-0-9 and the other one 18-0-18 written near the connection on the pcb. Also, only two wires from one toroid seem to be used for the digital power supply, all the others are located on the analogue part of the power supply.
My guess: the 18-0-18 toroid is used for a symmetrical power supply for the analogue output section. One part of the other toroid is used for a lower analogue voltage. The digital part probably needs +5V and +3.3V which are fed by the other winding.
I think there is more stabilising going on further on in the digital and analogue power supplies as I see some more TO-220 (name of that particular shape) ICs to the right.

Now that I've had a closer look at the layout, I'm not so sure anymore that L and R channels have completely separate (apart from the common GND) power supplies. So, waiting for the schematic is probably not a bad thing to do. Without it one can only get so far...

In the meantime it would be helpful if you could post what the types of all these TO-220 ICs are. I'm guessing there's LM317/337 (adjustable) and 78xx/79xx-series (fixed, e.g. a 7805 would be +5V fixed, a 7912 -12V, etc), but other types may also be used.
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Old 11th January 2009, 10:23 AM   #18
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Quote:
Nice looking layout. The silk screen (= white printing) on the top also clearly defines the different functions. They even let the DAC straddle the line because it has both digital and analogue sections!
Yes it is very neatly laid out and labelled. Below is a photo of the entire PCB which may give a better "big picture" view of what may be happening, where the power is going, and if/where it splits between L and R output. It shows the neat delineation between the "analog supply", "digital supply", "analog output", "digital input" sections etc.

Click the image to open in full size.

Quote:
I think there is more stabilising going on further on in the digital and analogue power supplies as I see some more TO-220 (name of that particular shape) ICs to the right.
Does the above "big picture" photo reveal anything perhaps?

Quote:
Without it one can only get so far...
Yes, the technician said he can start tinkering but would rather the schematic at hand for effective troubleshooting. By the way, I also have a close-up photo of just the analog output section, in case it helps.

Click the image to open in full size.

Quote:
In the meantime it would be helpful if you could post what the types of all these TO-220 ICs are. I'm guessing there's LM317/337 (adjustable) and 78xx/79xx-series (fixed, e.g. a 7805 would be +5V fixed, a 7912 -12V, etc), but other types may also be used.
This is where my limited knowledge really starts to show . Which components are you refering to? And would you need the white writing that is printed on those components?

Cheers
X
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Old 11th January 2009, 10:53 AM   #19
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Quote:
Originally posted by xenithon
[B]

Yes it is very neatly laid out and labelled. Below is a photo of the entire PCB which may give a better "big picture" view of what may be happening, where the power is going, and if/where it splits between L and R output. It shows the neat delineation between the "analog supply", "digital supply", "analog output", "digital input" sections etc.

Does the above "big picture" photo reveal anything perhaps?
The power supply traces seem to be hidden from view (they're probably on the solder side of the pcb). So I can't really tell from the picture if they are separate.

Quote:
Yes, the technician said he can start tinkering but would rather the schematic at hand for effective troubleshooting. By the way, I also have a close-up photo of just the analog output section, in case it helps.
Other than some flux residue from solder wire (the brownish stuff) between the pins of the two transistors in the lower left corner (L channel, BTW), I don't see anything out of the ordinary.
This may just be the result of touching up at the factory or a repair at a later stage...

Quote:
This is where my limited knowledge really starts to show . Which components are you refering to? And would you need the white writing that is printed on those components?
Let me help you. Usually three things are printed on the component: the brand, the type and a batch code. I'm only interested in the type (several brands produce the same type of very common components).

In the attachment you can see an LM337 and an LM317, the writing below the type is not of intrest to me. You will find similar printing on the case of other types.
It would be useful to have the reference designator (i.e. IC1, IC12, etc.) that is printed on the PCB next to the component too (e.g. IC53 = LM317).
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Old 11th January 2009, 10:59 AM   #20
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Another example of two different stabilisers:

On the left a KA7808 (can also be found as LM7808), on the right an L4805.

All examples are TO-220 shape (don't mind the rounded off corners of some).
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