| Electronics > Projects, Designs, and Technical Stuff |
| I am at a loss as to figuring out how to track down this short. |
| (1/5) > >> |
| Hextejas:
I have built 2 of these amplifiers(Right and Left) and R works and L doesn't. For the faulty one, I have built it twice so the next time will make 3. I would like some guidance to try and figure out what I am doing wrong. I built it (I thought) according to the schematic but both times when I put it in the chassis and power it up, I get a .22 shot sound, smoke, and in one instance, a fire. Both times it seemed to be on resistor 4, 20K. The resistor looks like it has 2 chips taken out of it but it still tests 20K. Resistor 4 is soldered to the back side of the PCB. The PCB is mounted on 1/2 inch plastic risers. I completely desoldered board L1 and tested all the parts and they all tested ok, including resistor 4. The only thing that I couldn't or didn't know how to test was the LM3886 IC. So, I bought some new ones and put it in. So, L2 was with a new LM3886 and the salvaged parts from board L1. Before I put it into the chassis, I hooked it up to my bench power supply and ran it up to +30DC, and -30DC and it seemed to work ok. I put L2 into the chassis along with the R1. applied power and L2 .22 shot sound, and smoked again. I took L2 out and have been trying to test it outside of the chassis and it seems to work ok. However when I put it back into the chassis, at least I dont get any shots nor smoke but I think that it is not performing correctly. I took some voltage readings and tried to compare both boards. Something strange that I noticed was this. V- was measuring -5 Vdc with no connection to the power supply. With it normally connected, it should read -26Vdc I dont know where to start. I was going to build another amplifier board using the scavenged parts from L2 but I am reluctant to plug it into the chassis and apply power to it. I think that I will build it and swap it with the working amplifier, R1. If R1 shorts then that will tell me something,,,,, that I will have to think about. Could it be the wiring into the chassis or the power supply ? I dunno |
| Andy Watson:
--- Quote from: Hextejas on January 23, 2019, 10:36:08 pm ---. Before I put it into the chassis, ... --- End quote --- Have you checked that the tab of the LM3886 is isolated from the chassis? The datasheet suggests that the tab is connected to the "V-" pin. |
| Hextejas:
--- Quote from: Andy Watson on January 23, 2019, 10:55:00 pm --- --- Quote from: Hextejas on January 23, 2019, 10:36:08 pm ---. Before I put it into the chassis, ... --- End quote --- Have you checked that the tab of the LM3886 is isolated from the chassis? The datasheet suggests that the tab is connected to the "V-" pin. --- End quote --- Why no I haven't ! I actually have not been paying any attention to it. It is connected to the heat sink with some compound in between and I am pretty sure that the heat sink may be connected to the chassis. Do I need to pay attention to that ? |
| james_s:
It depends on the design, but usually yes. The tab will almost always be connected to some element of the semiconductor die. |
| cdev:
Here's what I do, I have some LCD containing material that changes color depending on temperature. So, this is a cheap form of thermography. For example, one changes color over the range 20 to 25 degrees C. Thats usually the most appropriate one. I would connect up the board to a current limited supply at a fraction of the voltage and current it normally draws and I would use something like my temperature sensitive sheeting to see where it gets warmest. I'd start there. That lets you see tiny variations in temperature visually. You can get this material at science museums, etc. Its not at all expensive. Since you also have a working unit, I would start looking wherever they differ. The working unit at the same (lower) voltage + current wont work either but any major difference in the two boards warming pattern should lead you right to the problem. Remember to start at a very low voltage, and limited current, just high enough to overcome any voltage drops and get a small current flowing, enough to warm something up just a degree or two, not to damage anything. |
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