I would appreciate your help in troubleshooting my dac. - diyAudio
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Old 3rd November 2004, 10:47 AM   #1
protos is offline protos  Greece
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Default I would appreciate your help in troubleshooting my dac.

My Perpetual Technologies dac P3A decided to take a turn for the worst.One channel is distorting.I put ascope on the + input to the I/V stage and I got the following image.Now I am not a digital expert and I was wondering what to do next in terms of diagnosing the problem.Do you think it is possible that the cs4397 or one of the other main chips is responsible and can this be replaced-how can you desolder smd?
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File Type: zip 102_0294.zip (62.2 KB, 69 views)
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Old 3rd November 2004, 11:07 AM   #2
protos is offline protos  Greece
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This is the dac naked.
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File Type: jpg 102_0290_2.jpg (48.7 KB, 429 views)
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Old 4th November 2004, 01:23 AM   #3
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Default DAC fault finding

Hi Protos,

Your photo of the positive input to the I/V converter is pretty meaningless and will not help with fault finding.
This is because this circuit is a) a differential to single-ended filter, NOT a current to voltage conversion stage, and b) differential so measuring half of the signal is useless.

I suggest you (if possible) measure both simultaneously and subtract the negative signal from the positive... This will then show you the actual signal which would then be filtered by the circuit. Compare this to the output. if it is the same the DAC is at fault.

Check that all signals are present and accounted for. Check that they make sense. i.e. that fBCK is 64xfLRCK. Find, or source a single tone to the DAC (test cd etc) and listen to the distortion. Is it ticking (indicates a clocking error) or is it a distortion that looks vaguely sinusoidal (indicates a problem with the analog output stage).
Is anything getting hot? As this indicates several things (HF oscillation, clocking, an internal fault etc).

Good luck with it!!

apollyon25
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Old 4th November 2004, 07:50 AM   #4
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Hi Protos,

I modified one of these Perpetual Technologies dac P3A couple of years ago. They run HOT, so you may have some cold solder points on the PCB.

Use a test CD (Technics for example) and start tracing the problem from the "back" (from the RCA connectors), OR from the RF digital input. It would probably be easier for you to start from analogue out RCAs.

Itís a bit crowded inside with a lot of SM components, so if you are not sure what to do, take it to service!

Nick
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Old 4th November 2004, 10:50 AM   #5
protos is offline protos  Greece
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Thanks for the replies.
I further checked and measured and this is what I have:
1.cs4397 outs measure identical and clean on 1khz sine
2.There is no apparent distortion on the scope at -20db with the opamps
3.If I take out the output opamps (unfortunately I had to destroy these opa 627 to take them out but I have some more) and measure the output of the cs4397 AFTER the first series resistor 1K I find that the +R output is double that of any of the other outputs.This is the first point where you can measure the distorted sine wave when the opamps are in.
4.Might this lead to clipping in the summing amp?Would such a result on the scope indicate partial clipping?But why is this distortion/clipping obvious on the + input of the first opamp and not on its output only?
5.I resoldered most of the network but the problem persists.The problem appeared suddenly during normal play so there was no physical stress on the PCB/components and none that I can see now.
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Old 5th November 2004, 04:59 AM   #6
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If the output is anything like this http://www.diyaudio.com/forums/attac...amp=1071911103
where is the distorted voltage?
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Old 5th November 2004, 12:05 PM   #7
protos is offline protos  Greece
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The first point at which it can be seen is right after the 1k resistor at the output of the cs 4397.As I said before the output of the cs4397 before the resistor is clean and absolutely symmetrical on all four diff outs..If I remove the opamps the sine wave after the 1k resistor has no distortion but it is double in size compared to the other diff signal or even to the symmetrical signal in the other correctly functioning channel.
I am completely stumped because I measured and compared readings with the multimeter from one channel to the other between most of the nodes connections etc and they match so I cant see a bad connection or a failed resistor or cap (which would be surprising in an opamp stage with low VA).
I have substituted the opamp but the problem is the same .It has something to do with why the +R output just after the dac is swinging double what it should be.However I still don't understand why the distortion is appearing all the way back up to just before the output of the DAC since I dont see feedback there according to your circuit.
Since the the swing is double the others I would assume (resistive divider theory) that the impedance to ground following the 1k resistor would be off spec and lower than it should.However that is not what my multimeter is telling me because it is reading similar impedances to ground.
Slightly off topic but why does the above schematic sum up the two diff outputs in one opamp then take its output plus one of the two diff outs and sum them again in the second opamp?
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Old 5th November 2004, 12:21 PM   #8
protos is offline protos  Greece
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Sorry now that I am looking at the schem closely I see the second opamp goes to ground.Are you sure?That doesn;t make any sense to me.
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Old 5th November 2004, 06:12 PM   #9
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Quote:
Originally posted by protos
Sorry now that I am looking at the schem closely I see the second opamp goes to ground.Are you sure?That doesn;t make any sense to me.
No, it's only drawed a little bit complicated. The Signal runns thru the 10 Ohm Resistor to pin 2 and to the output.
It's also an active feedback for the negative input of the first op.
Don't know why they put the output from the (2nd) op to half negative supply via 5k (10k || 9k9).

Regards

Jobstens
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Old 5th November 2004, 06:46 PM   #10
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Errr,protos, its not my circuit. It from a thread started by someone called protos.
How to convert P3A DAC to balanced output?
When you measure the + and - outputs are they completely isolated bar the 1k resistors? It could well be a duff chip.
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