Zagadka's Site, Tim's Ramblings - Tuesday, 04th October 2005

Tuesday.

Crappy day at work for no particular reason...

Finished off (read made working) the video distribution amp. The chip was working but the image was just about white. I tested it on the DVD, you could only just see the screensaver logo bouncing around the white screen. No contrast at all, in addition only the first amp and last amp from four were working (this chip has 4 amps). I started checking for more shorts and found one! There was a short between the PCB pin and a ground plane. I fixed that up with a scribe and tried it again. No cure.

I read the datasheet about the chip (MAX497CPE) hopeing to find some troubleshooting info. Looking at the example circuit diagram it clicked. I decided to check if the chip was getting power - or at least power to the right places. I started by fitting a new battery to my multimeter. Something I should have got ages ago. After seeing which pins should be getting power I started checking traces starting at the regulators. The chip requires both +5V (pins 9 and 15) and -5V (pins 11 and 13). This is achieved using a 7805 (positive regulator) and a 7905 (negative regulator). BAM! no -5V on 11 and 13! I started checking for un-soldered pins. On this board there are no vias so connecting the top and bottom layers is depended on soldering both top and bottom pads of some components.

I ran into a dead end. I started manually checking the traces again. Whats this! A mistake on the board; NO! Immediately I was looking at ways of jumpering the -5V where I needed it. Looking closer at the middle of the board the -5V jumped straight off the regulator, through a filter cap and to an empty pad. Hrmmmmmm... There was nothing in this hole because I was making this as a Standalone unit. It has pads here to daisy chain +/-5V to other slave boards. Gee! Where do those traces lead on the other side of the board? Bloody pins 11 and 13! BUGGER! I stuck a PCB pin in there immediately and soldered it both sides. I didn't bother with the +5V because there are no traces on the top of the board taking +5V anywhere, just a pad surrounded by the ground plane.