Quote:
Originally Posted by turrican9
First of all I found out that resistor R301 that was supposed to be 10 Ohm was infact showing thousands of Kilo Ohms. And that is why I had no +12V on the op amp. Resistor R302 which is set on the side that feeds the op amp with -12v was also supposed to be 10 omh. It showed like 50 ohm.
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Are you measuring those resistors in-circuit? Because you can only trust the measurement if they are unconnected (or at least one side). And it seems odd that resistors would go bad as they don't usually do that. If they do, it is usually from overheating and then it is normally easy to spot the discoloration.
If the opamp is shorted somehow, it could drive its supply low (because you have resistors in the path which increases the impedance), making it look like the supply is the culprit when in fact it is caused by something in the opamp circuit. - Thinking about it, perhaps a shorted opamp might kill the supply resistors if it had been left running like that for a long time.
Quote:
Originally Posted by turrican9
So reduced from two 1K to two 47 ohm resistors and the op amp is working perfectly. Resistance between pin 10 and 12 to ground went from 0.4K ohm in the original config, down to about 37 ohm. And again, the 2D board is working fine with the original 1K ohm resistors at R303 and R304 and around 0.5K ohms to ground. So maybe this is some kind of flaw with the old rev 1.1 board or maybe I'm missing something else causing the voltage drop, and "cheated" ghetto style to get get it working again? Could it be a ceramic cap or something that failed between the op Amp and these resistors?
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AVref should be ~2.5V even if you used 100kOhm resistors for R303+R304 as AVref is not loaded at all DC-wise. It is only loaded by C305+C306 and two opamp inputs (which is >megaohm). So if the voltage is driven low then either one of the two caps are bad (high leakage current), the opamp burned somehow or you have a short on the PCB. Making those resistors smaller may seem to be working, but it is just 'feeding' the leak and allowing it to continue and perhaps something will overheat and go pop at some point. Actually with 47+47 ohm on 5VDC, you are pushing 1/8W in each resistor and that is probably just at the limit of what they were designed for (the small leaded resistors are rated for 1/8W).
I would go back to 1kOhm and then hunt for the current leak by removing components until the leak disappears. My guess would be C305 as I have seen this one go bad before and have heard from others with the same experience. Also a bit weird though since ceramic caps don't go bad very often unless they were not soldered correctly or experience physical stress or vibrations.