Why cut the power to the attenuator during memory test? Makes no sense to me really.
Well I thought it was odd to have a 74 series chip controlling the -12 V rail so I shorted the transistor. It failed the self check. I unshorted the transistor (the device has -12V applied to that terminal while its running) and it passed the self check, meaning that it must cut power temporarily during the check...
Any other ideas? I went after a LM380 that I thought might be bad but its not the problem. Something on the ADC side has a 20 ohm resistance to ground, but I can't find it. I did a resistance map and thought the LM380 by the input from the RF shit was the problem but the "short" is still there after the IC was removed.
When I do a resistance test across a capacitor then one way it gives me 60ohm resistance and the other way gives me a 0 resistance that eventually rises to 20 ohms.... I Checked everything in the area and it seems to be the point of lowest resistance.
The thing is its a bitch to trouble shook because the short only apears when I plug the unit into the extension board, but I unplugged every thing that I could unplug from the unit and it seems that they decided to route some kind of ground loop in a insane way, I don't know if its some star ground or what.
I also noticed that the board has a screw terminal connector that touches the ground plane by about half a milimeter. When the board is removed the screw terminal does not make a connection to ground, but when the board is inserted and the tension from the housing causes it to buckle a bit the screw terminal grounds that section. I don't know if the engineers wanted to route the ground through the housing at that point... because the ground plane was 95% removed from under the screw terminals, just the corners overlap.... but only make electrical contact while the board is under stress.
FUCK
I tried putting a insulator between the screw terminal and the chasis ground incase its not supposed to make contact there but it did not matter.