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Message ID: 1290     Entry time: Fri Feb 4 23:45:40 2011
Author: Zach 
Type: Electronics 
Category: GYRO 
Subject: RFPD board problem solved 

I was going to continue stuffing board #2 this evening, but I got the urge to take another stab at the problem with the first board. I removed some other components and was able to make the short go away. 

It went something like this:

  • I realized that even though I had removed all capacitors and diodes touching the -15V supply, I hadn't removed the AD620 itself, which was powered by it. Once I did this (and replaced it with a new one), the resistance between -15 V and GND was Mohms.
  • I plugged everything back in and turned on the power, but the lab supply still current limited the negative voltage.
  • Knowing that the AD620 was at least part of the problem opened the playing field to a much wider part of the board. I found that the 4.7uF capacitor in the bias path had failed, so I removed it. This was a polar tantalum cap that I had made very sure to put in facing the right way (which I did). I'm not sure what caused it to fail.
  • Again I plugged everything back in and turned on the power. This time, the current was below the limit, but still way too high (~250 mA). I found that the LMH6624 was getting pretty toasty, so I replaced this one too.
  • Finally, the board was acting normally. I replaced the +/-15 V regulators, as well as the -5 V one (during the tests I was just supplying +/-15 V to the test points) with new ones. I also replaced the bypass capacitors. I did not yet replace the tantalum cap, which I don't think is crucial at the moment.

So, we have a working board that is ready for (real) testing.

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