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MOSFET-MAX tube biasing issues (Solved, now about GrubDAC issues) - Page 4

post #46 of 52
11/13/11 at 6:10pm
Thread Starter 

Sorry for not making myself clear. I tend to get kinda lost when describing my problems. The LED is on (always has been), and the test point readings are spot on (the only ones I found were 3.3V and 5V). There is no solder on the crystal chip, and I'm measuring out the pins' connections right now, I've already just seen some pins that look like they're a bit short on solder though, so I might just try to add some more solder to all the pins before I measure the more tricky ones. Again, thanks for your patience.

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post #47 of 52
11/13/11 at 7:54pm
Quote:
Originally Posted by hitman47 View Post

Sorry for not making myself clear. I tend to get kinda lost when describing my problems. The LED is on (always has been), and the test point readings are spot on (the only ones I found were 3.3V and 5V). There is no solder on the crystal chip, and I'm measuring out the pins' connections right now, I've already just seen some pins that look like they're a bit short on solder though, so I might just try to add some more solder to all the pins before I measure the more tricky ones. Again, thanks for your patience.


No problem!  Let us know how it goes, though.  Again if you can post some pics, that will help.
 

 

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post #48 of 52
12/22/11 at 6:31am
Thread Starter 

Well, it seems it's time to bump this thread again. I completed the second grubDAC and it worked in the beginning (this is starting to sound familiar..), however after a week or so of not using it I tried to fire it up again and the sound had a lot of cracks and pops in it. I reflowed U1, it worked again for a few minutes until I unplugged it and plugged it in again, now it's stopped working completely (isn't recognized by windows). There's no voltage on the 3.3V point and the VRM gets insanely hot. The LED is lit, but it has a flicker to it. I've already replaced the VRM twice, both times to no avail. The symptoms stayed the same. Any clues as to what might be blown?

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post #49 of 52
12/22/11 at 7:22am
Quote:
Originally Posted by hitman47 View Post

Well, it seems it's time to bump this thread again. I completed the second grubDAC and it worked in the beginning (this is starting to sound familiar..), however after a week or so of not using it I tried to fire it up again and the sound had a lot of cracks and pops in it. I reflowed U1, it worked again for a few minutes until I unplugged it and plugged it in again, now it's stopped working completely (isn't recognized by windows). There's no voltage on the 3.3V point and the VRM gets insanely hot. The LED is lit, but it has a flicker to it. I've already replaced the VRM twice, both times to no avail. The symptoms stayed the same. Any clues as to what might be blown?

Sounds like something is shorting after the voltage regulator. Also check if the USB connector shell is shorting against something.

No voltage means the regulator is shutting down. The LED is driven straight off the USB. The reason it is flickering is that the host (PC) is actually tripping its current limiting, due to the grubDAC pulling too much power.
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post #50 of 52
12/22/11 at 8:39am
Thread Starter 

Is it normal that I get a fluctuating resistance rating across 2 of the 3 electrolytic caps (C1 and C15) (I assume that's not supposed to happen with a cap, and C5 doesn't behave that way) - in both "directions" so to speak, so it doesn't matter which DMM lead I put on which cap lead - with the DAC not plugged in? The reading floats around 20-70 ohms. I could try salvaging the electrolytics from my first build to replace them if that's the problem.


Edited by hitman47 - 12/22/11 at 9:35am
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post #51 of 52
12/22/11 at 9:52am
Quote:
Originally Posted by hitman47 View Post

Is it normal that I get a fluctuating resistance rating across 2 of the 3 electrolytic caps (C1 and C15) (I assume that's not supposed to happen with a cap, and C5 doesn't behave that way) - in both "directions" so to speak, so it doesn't matter which DMM lead I put on which cap lead - with the DAC not plugged in? The reading floats around 20-70 ohms. I could try salvaging the electrolytics from my first build to replace them if that's the problem.


Yes, it's normal.  You can't read resistance across a capacitor.  The DMM injects a small trickle voltage that ends up charging the cap.  The resistance will vary as the cap charges.  I can't tell you why your C5 doesn't act that way - it could be something as simple as you haven't placed the probes on that cap correctly.  In any event, it's a waste of time to measure resistance with capacitors.

 

P.S. You should start posting your GrubDAC questions in the GrubDAC thread.  I doubt that cobaltmute even has this thread subscribed.

 


Edited by tomb - 12/22/11 at 9:54am
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post #52 of 52
12/22/11 at 10:02am
Thread Starter 

OK, yeah I thought that because of the way caps work, there should only be a resistance showing up at the beginning as it's being charged (whichis what happens with C5). I'm just not sure how to check what's causing the short, as the VRM is turned off by overcurrent protection (I'm assuming) so measuring currents/voltages won't do the trick I think and checking for resistance across caps (they're usually what's connected to ground) doesn't work as you told me... I'm also gonna leave a link to this thread in the grubDAC thread, thanks for the tip.


Edited by hitman47 - 12/22/11 at 10:12am
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