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Old 13th January 2009, 06:38 PM   #31
jitter is offline jitter  Netherlands
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Agreed, it's strange, but don't forget we don't know what's the cause yet. Judging by your descriptions it has all the signs of PSU failure, but keep in mind something else might be wrong (too).
Besides, you only know the voltages of three of the regulators and there are eleven in total...
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Old 14th January 2009, 06:04 PM   #32
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Arrrrrrrrggggggggggggggg!!!!

Ok, now that's off my chest.

The technician spent the past couple of evenings doing a thorough analysis of the various components. Firstly as you may recall, one of the initial findings over the weekend was that the 4 transistors in the left analog output stage seem to have been replaced/changed. This is clearly evident as they had large, uneven solder blobs, whereas every other component on the PCB had very small, uniform, consistent solder. So this was NOT a mint, demo unit, but one on which repair work had taken place. Over and above the uneven solder blobs, there is significant solder flux residue around these components.

These components, labelled J3 and J4, are FETs in the left channel's analog output stage. These were obviously replaced at some stage - either when the voltage change was performed (though I suspect that is highly unlikely as the voltage change would not go anywhere near the analog output stage) or at some point earlier in this DAC's life.

The PCB is double-sided. A replacement of these FET's would require the entire PCB to be removed from the chassis so that they could be properly soldered on, but it is clear that this was not done. Instead, the replacement was only done from the top/exposed side of the PCB (the easy but incorrect and unreliable way). As a result, the contact is problematic and not properly being made, resulting in hum/noise problems in that left channel. The suspicion is that one of the 3 legs of the FET is making improper/inconsistent contact resulting in a ground issue which in turn results in the hum. The contact is inconsistent so it may occassionaly be made and grounded, hence the intermittent nature of the beast.

Irrespective of when the problem with the FETs occurred, the fact remains that an incorrect attempt was made at fixing it and this has resulted in the hum/noise. Properly fixing this requires the entire PCB to be taken out, which involves desoldering all the jacks, LEDs etc. - a time consuming, messy and costly effort (not to mention difficult when this product has never been worked on and without any schematic to assist).

EDIT: for additional info to this thread, those components (J3/J4) were isolated as the problematic ones as the signal was checked on an oscilloscope just prior to going through the FETs and it is clean, but directly after the FETs the signal shows significant ripple, distortion and clipping.

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Old 14th January 2009, 07:42 PM   #33
jitter is offline jitter  Netherlands
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Quote:
Originally posted by xenithon
Arrrrrrrrggggggggggggggg!!!!

Ok, now that's off my chest.

The technician spent the past couple of evenings doing a thorough analysis of the various components. Firstly as you may recall, one of the initial findings over the weekend was that the 4 transistors in the left analog output stage seem to have been replaced/changed. This is clearly evident as they had large, uneven solder blobs, whereas every other component on the PCB had very small, uniform, consistent solder. So this was NOT a mint, demo unit, but one on which repair work had taken place. Over and above the uneven solder blobs, there is significant solder flux residue around these components.

These components, labelled J3 and J4, are FETs in the left channel's analog output stage. These were obviously replaced at some stage - either when the voltage change was performed (though I suspect that is highly unlikely as the voltage change would not go anywhere near the analog output stage) or at some point earlier in this DAC's life.

The PCB is double-sided. A replacement of these FET's would require the entire PCB to be removed from the chassis so that they could be properly soldered on, but it is clear that this was not done. Instead, the replacement was only done from the top/exposed side of the PCB (the easy but incorrect and unreliable way).
Not necessarily. I have done many succesful repairs that way on boards that were mounted on big heatsinks with many screws. Double-sided (or multilayer) pcbs have metal plated holes. The only way repair 'from above' will fail is when the plating has been damaged during removal of the component(s). If the damage is not too bad, soldering from both sides will get an acceptable joint.

Quote:
As a result, the contact is problematic and not properly being made, resulting in hum/noise problems in that left channel. The suspicion is that one of the 3 legs of the FET is making improper/inconsistent contact resulting in a ground issue which in turn results in the hum. The contact is inconsistent so it may occassionaly be made and grounded, hence the intermittent nature of the beast.
If the plating of the holes has been damaged, this is possible. But the repair may also have damaged the FETs themselves. IMHO only one way to find out: removal of the pcb and replacement of the FETs.

Quote:
Irrespective of when the problem with the FETs occurred, the fact remains that an incorrect attempt was made at fixing it and this has resulted in the hum/noise. Properly fixing this requires the entire PCB to be taken out, which involves desoldering all the jacks, LEDs etc. - a time consuming, messy and costly effort (not to mention difficult when this product has never been worked on and without any schematic to assist).
It may be worth checking if the components on the front side are acually attached to it. Looking at a picture that shows the LEDs and push buttons it seems to me that the pcb can be pulled away without unsoldering them.

Quote:
EDIT: for additional info to this thread, those components (J3/J4) were isolated as the problematic ones as the signal was checked on an oscilloscope just prior to going through the FETs and it is clean, but directly after the FETs the signal shows significant ripple, distortion and clipping.
And what about the ripple on two of the voltage regulators?
The tech could remove the offending FETs the quick'n'dirty way (from the top) and see if that has any effect on the PSU.
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Old 21st January 2009, 03:22 PM   #34
jitter is offline jitter  Netherlands
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And? Any progress?
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Old 21st January 2009, 03:59 PM   #35
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Hi there. I sent a stern email to the distributor and the manufacturer stating my dissapointment in how a faulty unit was sold as a "mint demo" unit. I requested a refund which I got and have shipped it back.
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Old 22nd January 2009, 10:05 PM   #36
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Cool.

...well, not cool.

I guess you're still out the tech's time and a load of aggro, but at least they've come up to the minimum in terms of customer support.

Better luck next time.

w
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Old 24th January 2009, 06:20 PM   #37
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Xenithon, this may be nothing but looking at the pictures of the Analog Supply you can see that R27 and R30 are different resistors, the two caps just below the resistors are different also, I assume that we are looking at L and R channels on the board. Strange that these parts are different in a new piece of gear.

Phill
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Old 24th January 2009, 06:39 PM   #38
jitter is offline jitter  Netherlands
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Different, yes, but same value.

This may have happened as early as in the manufacturing stage.
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