Raise your hand if you bet on U28 on board A4
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And it's calibrated (at least for low volt DC, 100 mV - 10 V)! Now, what do I do with the other nine 74ls377 chips?
Virtual beers to all...
Update: Just checked full DC calibration and it's friggin' perfect (I'm talking <5 ppm for all ranges save the 100 millivolts range that's off by around 10 uV and jumps quite a bit at 6 1/2 digits - it's a hard range to test). This was with the voltage standard I got last week from voltagestandard.com (within 1 week of calibration via a NIST certified Agilent 3458A). The 3456A matched all settings but the 100 millivolt one to within 5 ppm, the 5 and 10 V ones to within 2 ppm. The 5-digit Data Precision voltage standard in the pic is a tiny bit off. I can use the 3456A to calibrate it - the irony!
So, best $87 ($80 for the HP 3456A and $7 for the ten replacement 74ls377 chips of which I used one) I ever spent! OK, I guess the 8-10 hours I spent on the troubleshooting and repair have to be mentioned, but it was a good learning experience for me.
Update 2: Just measured a 480 ohm 1% metal film resistor to get a feel for the state of the resistance section and the reading came within 4 ppm of my Fluke 8506A. (476.404 ohms on one and 476.408 ohms on the other).
I am beginning to get the feeling that this unit was calibrated in the not too distant past and that that 74ls377 blew on the main board and it wasn't used since. I just wish it was in better cosmetic condition, but hey beggars can't be choosers
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Update 3: Found the "calibration on the unit's" Achilles Heel, the AC. It's showing about 1.5 V too high at 120 V US line voltage, compared to three other meters I have, which are all within a quarter volt of each other. I guess that can use some calibration in the future, but I don't measure AC all that much and have no way to really calibrate it either.