[hitman47]

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.

 

[tomb]

Quote:

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.
 
 

 

[hitman47]

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?

 

[cobaltmute]

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.

 

[hitman47]

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.

 

[tomb]

Quote:

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.
 

 

[hitman47]

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.

 

[hitman47]

Alright, time to revive a dead thread!

I've been having some issues with scratching noises on the right channel of my Mosfet-MAX that tube swapping did not solve, but that seemed to be alleviated by jiggling the right tube around in its socked. The tube socket contacts appear to be pristine on both sides so I don't believe that's the problem. I figured I might want to check the tube bias and it was completely off from what I'd set it years ago. I corrected it and checked the MOSFET bias while I was at it and to my surprise, the right MOSFET showed some odd behavior: The bias voltage measured differently depending on which reference points I used. I proceeded to reflow the MOSFET joints as one of the transistors seemed to have some pretty bad cold joints, and I also cleaned the PCB again. Now, the cracking/popping noise is even worse and it doesn't change no matter how I move the tube around. Is this a shot MOSFET, or did I somehow make the cold joints worse? They are looking decent now, nice and shiny, although there's way too much solder on some of them - my soldering iron is currently semi-broken (snapped in two a few days ago) so I can't use solder wick properly.

Edit:
Alright, that was pretty unnecessary - I had another look at the leads of the MOSFET I suspected was causing the noise and on the component side of the PCB, they were encrusted with some gross-looking flux residue. I scraped it away and now the amp's dead quiet again.

 

[tomb]

  Alright, time to revive a dead thread!

I've been having some issues with scratching noises on the right channel of my Mosfet-MAX that tube swapping did not solve, but that seemed to be alleviated by jiggling the right tube around in its socked. The tube socket contacts appear to be pristine on both sides so I don't believe that's the problem. I figured I might want to check the tube bias and it was completely off from what I'd set it years ago. I corrected it and checked the MOSFET bias while I was at it and to my surprise, the right MOSFET showed some odd behavior: The bias voltage measured differently depending on which reference points I used. I proceeded to reflow the MOSFET joints as one of the transistors seemed to have some pretty bad cold joints, and I also cleaned the PCB again. Now, the cracking/popping noise is even worse and it doesn't change no matter how I move the tube around. Is this a shot MOSFET, or did I somehow make the cold joints worse? They are looking decent now, nice and shiny, although there's way too much solder on some of them - my soldering iron is currently semi-broken (snapped in two a few days ago) so I can't use solder wick properly.

Edit:
Alright, that was pretty unnecessary - I had another look at the leads of the MOSFET I suspected was causing the noise and on the component side of the PCB, they were encrusted with some gross-looking flux residue. I scraped it away and now the amp's dead quiet again.

Way to go in staying with the issue and getting it fixed!

  Yes, flux will eventually become conductive over time.  It's always a good idea to rinse the PCB as much as you can when finished populating it.

 

[hitman47]

Yeah, I go through lots of iso rinsing my PCBs but the transistor legs on the MAX are very hard to reach with a toothbrush (on the component side, that is) and seem to require extra attention