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HP3478A 5.5 Digit Bench DMM, First look and functional test

Just scored this meter from eBay, managed to get a good deal and it just arrived a couple of days ago so time to open up the box and see what I have The add implied it may have a fault and only specified it would power on and that there may be an issue with overload. Therefor got it for a good price So as you will see, I ended up with a winner, not only was it fully functional but on first look, pretty accurate too (Compared to my 4.5 Digit Agilent 1272A anyway). I took it for a spin through all the ranges using various supplies and my Vishay Precision Resistance standard, and while I have no way right now to confirm the accuracy or calibrate it. It seems to be very accurate and on resistance, surprised me, especially in 4 terminal measurement mode. Not bad for a meter over 30 years old and with who knows how long since the last calibration Anyway, I also took a quick look inside and identified a few of the key components although I guess wrong for some earlier in the video but figured it out by the end. It’s a shame if worked so well in some respects because now I will have to look for another one, I was intending to make a repair video out of it as the add implied a fault. I guess I can’t complain. I will follow up with a detailed look through the operation of it, probing various circuit test points to show how it works. HP (Now Keysight via Agilent) use some interesting dual slope or multi slope measurement techniques to perform ADC functionality so it is all based on timing, not absolute resister chain values etc. as is the case with traditional ADCs so we will examine the schematics and look at the waveforms to see how it all works