7805 Extreme Overheating

[Strange Tales]

I've been working on this issue for a couple of days.

https://github.com/MagpieModular/microB ... 0Build.pdf
https://github.com/MagpieModular/microB ... r/Hardware (need DipTrace Freeware for the .dch file)

So, my 7805 gets ridiculously hot in like 20-30 seconds. Like, I can't get my finger off it quick enough hot. So far I've gotten in my brain that it's a short... somewhere. (probably) So far what I've gotten from multimetering the shit out of this board:

Voltage test on input is .3V
Voltage test on output is 0V (??????)

Impedence from 5V to GND is 15.4k.
I get continuity from the 5V output to Pin 2 of the 74HC4067. I don't know which cap is C32 (board isn't labelled, it isn't the cap below it) but if I get continuity to pin 2 I think it's safe to say the short isn't there.
I get continuity from the 10 pin header -> Diode -> 12V input.

If I'm getting continuity in and out of the 7805, that should indicate it's not a short here right? Could the problem be that I need an input capacitor for some reason (although others don't). This makes no fucking sense either, but I'm picking at clouds here and I'm about to start running in circles. I've taken a hot air gun over the board for reflowing purposes to make sure I didn't have a shitty solder job somewhere.

[multi_s]

is it a typo? .3v will not regulate to 5 v with a linear regulator. if not i a typo i would remove the 7805 and see if teh input voltage becomes correct. if ou cant find a short i would replace the 7805 or take the one you have off and test it off the circuit board.

[Strange Tales]

Yea that's what I'm starting decide. I did a quick test on it this morning before leaving for work this morning, it's down to .15V on input and 0V on output.

[eatyourguitar]

0.3v in 0v out is correct for a shorted output. the ground terminal is always grounded and the output terminal can be shorted to ground internally sometimes when the device is completely blown. instead of lifting the output, try lifting the input to see if the power rail comes up and your pedal power is OK. then disconnect the power and check the impedance from the reg out to reg ground pin. if it is shorted then remove it and check the part, PCB separately. you could disconnect the output pin first but that only tells you good info if the PCB does have a short.

so there are only 3 ways to go

bad regulator (shorted)
pcb power rails 5v<->0v connected
power supply went wonky

there is also option number 4 if you have two regulators with outputs fighting each other but I don't have the schematic.

[mattoverse]

Have you tried flipping it around? I have occasionally gotten things where the pins are flipped from what the datasheet says...

[Strange Tales]

0.3v in 0v out is correct for a shorted output. the ground terminal is always grounded and the output terminal can be shorted to ground internally sometimes when the device is completely blown. instead of lifting the output, try lifting the input to see if the power rail comes up and your pedal power is OK. then disconnect the power and check the impedance from the reg out to reg ground pin. if it is shorted then remove it and check the part, PCB separately. you could disconnect the output pin first but that only tells you good info if the PCB does have a short.

so there are only 3 ways to go

bad regulator (shorted)
pcb power rails 5v<->0v connected
power supply went wonky

there is also option number 4 if you have two regulators with outputs fighting each other but I don't have the schematic.

If I remember correctly, the 15.4k impedence I got was the 5V output to ground. The schematic is in the .dch file that I linked, I had to download the diptrace freeware to use it though. This board does have a LM1117-3.3 on it as well, but I have 0 problems with that part.

I can rule out the power supply. This is for my modular, and the entire rest of the case is working 100% fine (I even tested with power supplies on my case to make sure). I'll try lifting the input to see what happens.

Have you tried flipping it around? I have occasionally gotten things where the pins are flipped from what the datasheet says...

I can't flip it around, it's an SMD part and it has pin 1 labelled on the board.

[eatyourguitar]

if the input of the LM7805 is 0.3v, where is the voltage drop 0f 11.7v? is there a 47 Ohm resistor on your +12v rail where it enters the module? you can use this to calculate total power through the 47R and therefor check if the 7805 is pulling more than 100ma (datasheet) (shorted, failed part). there are different power filter resistors and different versions of 7805 so it would depend on that. I can only assume 100ma (7805 max output) 100ma (7805 max shunt) and 250ma (calculated 11.7v @ 47R Ohm's law) through the input pin of the 7805 (Kirschoff's current law)

[Strange Tales]

As far as I know from reading and following the schematic from the Diptrace file, the power enters through the 10-pin header (pins 9 and 10? It's whichever side has the red stripe), hits the Schottky diode D1, and then goes straight to the input of the 7805. There should be nothing in the path before hand.

Here is a screenshot from the PCB Layout file showing how the power goes in.

http://imgur.com/a/Guu7G

[eatyourguitar]

if there is no short from the reg out to ground, no short from reg in to reg out, and there is no short from 3.3v out to 5v out, and you definitely have the diode hooked up right or it would not even use any power at all. then that means the regulator has failed plain and simple. cut it out to make desoldering easier.

[Strange Tales]

Replace regulator -> all systems go.

Now to figure out the Mutable Dev environment, more fun!