Troubleshooting a pcb board

[thekyguy11]

Hi guys, see if you can help me with this. If you would like to read the history, read below... If you want to get straight to my question(s), jump to the last paragraph... Thanks!!!

I've got an Innovate Motorsports LM-1 wideband oxygen sensor for my car and I'm having a problem with it. I bought it new and it has always worked in the past. I decided to make a custom display for it in my car, which required me to do some modifications to the electronics. First I removed the board and lcd screen from the plastic case. I de-soldered the ribbon that connects the lcd screen to the board, and re-soldered it on the opposite side of the pcb board so it would fit in the new enclosure (i made sure the ribbon was oriented correctly so the pins were in the same holes as before). Then I had to de-solder two momentary push buttons from the board.

At this point I connected a 9V battery and tested it to find that everything still functioned normally. I finished building the new faceplate for the screen to sit in and tested it again, and everything was still fine. I then de-soldered the on/off switch and soldered in two wires in its place (there are only 2 traces going to the on/off switch). I connected the two wires to a new on/off switch located in the faceplate next to the lcd. When I tested it again, the lcd screen was no longer working. I checked voltage at the 9v battery to find that when the switch is turned off, voltage is roughly 7.7v. When I turn it on, voltage at the battery drops to 2.4v. After de-soldering the two wires I had just added for the new on/off switch, I noticed the end of the ribbon near the lcd screen had gotten bent and was touching the ground pin and one other pin to a metal surface. I re-bent them away from the surface and suddenly everything worked perfectly again. So, I re-soldered the two wires for on/off and tested again, it worked. The last thing I did was add two more wires connected to a momentary switch to take the place of the calibration button which i had removed earlier. Then, the screen stopped working again and voltage was dropping to 2.5 again when on.


Am I correct to assume that when the voltage is dropping that low when measured at the battery (2.5v, down from 7.5v when "on"), that there is a short on the board or lcd somewhere? I tried to feel for a "hot spot" to find a short, but the only thing getting hot was the LM1086 and the battery itself. What is the proper way to find a short, or trouble-shoot the board/lcd screen?

Thanks again guys!!!

Kyle

 

[3v0]

It sounds like you created another short. The first thing I would do it to insulate the ribbon cable connection to keep it from shorting out again. Maybe a bit of epoxy. Also it would be my first suspect for the current short.

Look first to the stuff you modified for the short.

3v0

I noticed the end of the ribbon near the lcd screen had gotten bent and was touching the ground pin and one other pin to a metal surface. I re-bent them away from the surface and suddenly everything worked perfectly again.

 

[thekyguy11]

I have ensured the ribbon is no longer touching anything, and I completely de-soldered everything I installed in the board. I used de-soldering braid to get any and all solder off any of the pads I messed with. I can't "see" anything causing the short, which is why I want to try to pinpoint the problem through testing. The epoxy is a good idea, but I need to get it working first.

Also, the screen uses a 16 pin ribbon. Is there a way to check the voltage at each pin to maybe narrow down which path has a fault? This is the screen Innovate used, the **broken link removed**

thanks

 

[Chippie]

Try disconencting the lcd display altogether and see if the voltage returns, then add each lcd connection one by one, while monitoring the battery voltage....That should help isolate where the problem are is..

 

[thekyguy11]

that is a great idea. I was kinda thinking the same thing. Ill try that and post results!