So a few days ago I received an "as is" 8845A for a decent price, and was surprised when it fired up without issues:
It didn't however take too long for it to start exhibiting one. You probably know but 8845A/8846A has I think kind of a nifty feature where even when you turn the meter off using the front soft power off button the meter doesn't completely shut off. The reference (LM399) and most of the system remains in standby for a quick boot up and warm up time.
Well, I randomly noticed the meter come alive from this standby mode into a continuous Initializing (boot up) loop. The meter is on a UPS and there was no power brownout at the time as none of my other gear was affected. Powering off the meter using the hard switch located in the back of the instrument and waiting couple of minutes resolves the issue and the meter comes back and boots normally but after awhile the issue randomly pops up again.
I've only done a cursory look at any obvious issues and I will document my process here.. I am open to any suggestions, as I don't have a service manual for this unit.
I've checked the unit with a thermal camera after it's been warmed up for any obvious signs of thermal issues. There are rows of linear regulators in the power regulator section of the board and one particular LM317 gets warm, but nothing is running particularly hot.
The unit is nice enough to offer well labeled test points for rail voltages:
I waited till the problem reappeared and started measuring. All low voltage rails appear fine.. +60V test point however showed an issue. The rail dips from ~65V to 32V every time the Initializing boot loop starts.
Located the DFO6S bridge rectifier in the vicinity:
And it measures fine.. the rectified output does not dip as the test point shows which is a bit odd. The AC input also looks fine without dips.
Anyone knows what the 16 pin IC marked "PE-65728 0733-J CHINA US. PAT 5015981" is? I looked up the patent and it pertains to IC inductor packaging:
Next to it there is this TN4CW MOSFET, one of the pins looks a bit suspect, maybe a cold joint so I took a pic for a closer inspection: