Electronics > Projects, Designs, and Technical Stuff
Audio HP Amp TI LM4811 - am I getting crazy?
Saimoun:
Thanks tooki for the summary.
I tested with my last chip (number 4) and the 4th PCB. I can now get one of the two channels to work. The other channel is fried I think - I get a 42Ohm resistance between input and output.
I think I basically fried all 3 other chips, which explains the very strange behavior on all of them.
My guess about what happened: the chip must be really sensitive to changes on the CLOCK input, since each rising edge changes the volume, so I think leaving it floating just for a second makes too many changes and the chip fries. And as for the second channel of the 4th chip - I had the input tied to GND while I was testing the other channel, I guess that killed it?
I also noticed how sensitive the voltage on the bypass pin is - just probing it with my oscilloscope is enough to make it drop by a few 100mV!
Saimoun:
Anyways - thanks all for the help, I guess the issue is sorted. I'll buy a few more and be even more careful this time.
OM222O:
the main problem that I suspect is the very odd shaped pad which you use to solder. Adding large amounts of solder, overheats the chip for a long period of time, burning the circuitry. Most reliable method for hand soldering SSOP packages:
1)Add solder using a normal iron to all the pads excessively
2)Add flux and go over the pads with an iron, removing excess solder, leaving just enough on the pads
3)Heat the pads at the same time using hot air, hot plate etc, enough to melt to solder on the pads
4)Grab the part with the tweezers, place it on the pads, keep pressing down and remove the heat source
This way there is minimal heating of the chip, practically no chance of shorts (if you don't mess up the placing step and move the chips around) and reliable joints on the first try.
Saimoun:
Thanks for the tip about amount of solder - but then I guess I can still use my technique just be very careful with the amount of solder? I am also really quick with the iron, leaving it less than a second on the pad usually.
I do not have a hot plate or hot air gun so for now this might be my only option. And the large pads *really* help.
tooki:
Honestly, I doubt heat is the problem. It could be, but I don't think it's likely. (And honestly, you don't need those giant pads. SMD is about technique, and one of the things to unlearn is that the soldering iron needs to be as small as the pad. It absolutely doesn't.)
Anyhow, I listed a whole bunch of things for you to look at, but you didn't respond to any of them. In particular, we still haven't isolated whether the issue is in the IC or in the PCB...
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