Lodi, WI
Quoted from Catch86:I learned that replacing the LM339 differential amp on the Opto board of a Doctor Who is not always the fix. So about a year ago I was getting the "GND. SHORT ROW 1 IS STUCK CLOSED." So I tracked it down to U3 of the Opto board. So I cut the IC out and desoldered the pins out of the holes. I noticed that there was a lot of flux residue under the IC. I thought nothing of it and cleaned it. I then put a socket in and put the new part in. Piece of cake. Pin worked. Done. Right? Well, fast-forward to this past summer and I was getting weird problems of the VUK randomly firing while playing and the ESCAPE sequence randomly being completed without even hitting them and the "GND. SHORT" problem showing up again. I seemed to have begun around July 4th weekend when we had a party and the house was open and the humid air was coming into the house. The pins are located right next to the back door. So back to the problem. I just replaced U3 since it was now socketed and that would be it. Nope. So troubleshooting showed that it looked like maybe U2 was the problem. So I figured why not just replace U1 and U2 with sockets. Did that and the problem was still there but it came and went. Meaning it was intermittent and seemed to be better if I played it more. Ultimately I removed the IC's and just started ohming out the pins around U3 and U2. While doing this I found that I could measure a resistance between pins that should be wide open with no resistance. I then removed the socket from U3. Then the measurements were open with no resistance the way it should be. After that it was pretty obvious that the flux residue from the soldering was the cause. I then cleaned the board again and used a low flux solder and tried cleaning with flux off as best I could while the socket was installed. I measured again and now the measurements were reading open as they should. Now it works perfectly. So I learned that differential amps are very sensitive to flux contamination. So the flux was acting as sponge to the humid air.
Was this on the opto board underneath the pf?
