I posted a while back about excessive LF noise in the A channel of my AM700 Audio Test Set.
Although I haven’t cured it yet, I have found an intermittent noise in the front end of Channel A and I am on the way to hopefully curing the problem. I believe that the noise is coming from the clamp circuitry. As a side issue, the switching power supply is apparently very close to not being able to turn on the +15 and – 15V rails due to startup inrush current tripping the Overcurrent detection circuitry of the Power supply. I am hot on the trail of this problem as well, comparing the old CRT supply schematics (mine) and the newer Color LCD version power supply, which shows quite a few changes.
Close examination of my power supply, showed a TEK factory mod that disabled the current sensing comparators on the floating +/-22V. The 0.1Ohm series current sensing resistors in that supply have been removed and replaced with 5 Amp solder-in pico Fuses. The stickers covering up the resistor nomenclature convince me that this was a factory mod, not one done in the field. Also, my unit is a factory refurbished unit. The pdf service manual that I have has no mention of this mod, in which TEK decided to replace the onboard current sensing/shutdown with fuses to better float this supply which feeds the analog generator outputs.
Does anyone have any documentation on this modification to the AM700 supply?