Hoping for some help here guys, this one has me stumped.
My next door neighbor asked me to try and fix
this thing. It's a drag racing tool, basically a few met sensors (temp, bar, hum) and a little micro that calculates your optimum tire pressure based on those readings and a few other input parameters, pretty simple. He said it wouldn't power up after being dropped.
I put fresh batteries in it, it runs off 4@AA in series and sure enough, nothing, so I did the basic things; cleaning up and reseating the ribbon cable to the keypad and the pin header for the display...still nothing.
OK, time to get serious. To get it out of the case to start tracing things I had to desolder the battery tabs. They poke through the back of the case, hefty tabs that solder into big slots on the board, so I got it out and tacked a couple scrap component leads on to those power pads to hook some jumpers to a bench PSU and it boots right up. Mechanical right? Cold joint or something? No.
I have it back in the lower case with the battery tabs soldered back on and it will boot off a power supply clipped to those tabs, or off a separate battery 4 pack clipped to those tabs, but absolutely refuses to boot with its own battery case populated. I can boot it off the external pack and then add the internal batteries, then disconnect the external pack and it will stay on and operate normally, but turn it off and it will not turn back on.
I have traced the problem to a TPS7250Q 5V regulator (datasheet
here). This VR will not start with the batteries in the integrated holder. It starts every time with batteries clipped on.
I have tried varying the PSU voltage to see if it is over senstitve to that, it is not. I have tried fresh batteries and somewhat used batteries, both work in the external pack but not in the onboard pack. I have pushed and tugged on the + and - tabs while it is on trying to find a mechanical issue and there is none. I have verified voltage on the input pins of the VR when it will not start.
I am left with the puzzling possibility that something about having the battery connected by 20cm of wire vs direct to the board somehow makes a difference.
Please help, I am starting to entertain supernatural explanations here...tearing my hair out.