TDS644A Aquisition Board problem.


r221b@juno.com
 

Hello all. I'm in the process of trying to restore a TDS644A. Ive already stripped off the surface mount caps, cleaned all the solder pads, washed and dried the board as the group info here suggests. I started to replace the caps with new ones but decided to make some ohmmeter checks while the board was not fully populated. I've discovered something strange that maybe someone here can comment on. I've shecked all of the pins on the Aq. Bd. J700 and J101 to ground. I have a schematic for the TDS544A power supply and it seems the same as the 644A. J700 (40 pin ribbon connector socket) is the analog power supply connection and goes J5 on the PS. J5 connections are the same as the 544A schematic. I still have the aquisioion board out of the scope with nothing external connected. The ground pins on J700 are pins 5,6,13,14,15,21,22,23,24,31,32,33,34. The -5.1V pins are 16,17,18,18,20. The +5V pins are 35,36,37,38,39,40. My ohmmeter reads 209 ohms from the -5.1V pims to ground. I read 24.5 ohms from the +5V pins to ground. From that I think there's a problem with something connected to that +5V line. I'm guessing the resistance should be much higher. Possibly the same with the -5.1V line. Well, all of that isn't too strange, BUT when I was measuring the ground pins, pin 15 (which should be ground) seems to be connected to the -5.1V line and pin 16 (which should go the -5.1V line) measures zero! Effectively it looks like pin 15 and 16 are swapped! Now the power and ground pins all disperse inside of the PC board. I can't find any external traces or connections that connect the ground or the power pins together. My theory is that somehow the -5.1V line got shorted to the pin 15 ground and opened the connection. Possibly the pin 16 connection got shorted to ground too. My first instinct is that there's an IC that has both the + and - 5V supplies connected and is defective. Since this could be a major undertaking to find and fix, has anyone here had a similar problem with the TDS series boards? Is there anyone has a 644A or similar that might measure the J700 pins for resistance to ground and help me determine where to start? Please measure with the two power cables removed from the board. BTW, I have meticuously inspected the board for any visible shorts and couldn't find any. Of course it's possible that there is a problem under one of the chips, but I haven't removed any yet. I'd really like to find this problem so any suggestions (and measurements!) would be greatly appreciated.

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