Hi George,
Very nice explanations. I will follow your advice and calibrate the 3478 as well on 4W. Its just that I was a bit less trusting about its calibration than the 3468, so my reasoning went to use 4W on what I perceived as being the better of the two. But as you point out the '78 having the the lower ohms range, it definitively would make sense to use it for low value 4W measurements. Although I have to say that with the zero cal on the 3468 and the chinese kelvin cable I was for the first time able to read and verify the value of some 0.01 resistors I had purchased some time ago to use as shunts in a power supply project but never used them. They are more like a U shaped piece of wire, and so far all of them. And same goes for basically any other low value resistor in my parts stock that I threw at the 68, it measured up quite well within the expected results.
I really need to compare resistor readings between the 68 and 78, which I did once, but then also this time write down the values for comparison and post them here just for the record. I will recalibrate the 78 in 4W and do a comparison of readings.
My reasoning to have one DVM on 2W and the other on 4W stems from the fact that the service manual seems to be very critical about which mode to select for calibration and recommends to to do so in the ohms mode that the instrument will be used (...the most). So it sounds like one would always have to make a small compromise, and having one of each basically sounded like being able to eliminate that compromise. Or not really.
For the time being and following your suggestion, I will not attempt to calibrate the 30k/300k ohms ranges by using the Red Devils. No need for absolute accuracy.
In regards to your suggestion about R201 did some research at Mouser, my preferred supplier:
To substitute R201 outright there are no 40.0K series S102 resistors available for that exact value. Closest I could find is Vishay PTF series 40K 1/8W 0.05% 10PPM/C at $3.38 each, 58 currently in stock. Would that be still an improvement over the existing resistor?
https://www.mouser.com/ProductDetail/Vishay-Dale/PTF5640K000AYEK?qs=sGAEpiMZZMsPqMdJzcrNwo3vE%252BHgilrQScfmxiejoUM%3D
Second best options would be another Vishay PTF series 40K 1/2W 0.1% 10ppm/C. One does cost $4.99 and the other only $0.89. Frankly, with a side by side comparison, can't seem to find any differences between these? two other than the selling price (both prices are for qty one unit purchase).
71-PTF5640K000BYEK
https://www.mouser.com/ProductDetail/?qs=41svs21nsHBzvOZxn1wkhQ%3D%3D
71-PTF5640K000BYEB
And there is actually a Chinese seller on ebay offering used Vishay S102K at $5, 40K 0.01% which as pictured are obvious pulls. But given ebay's history with remarked fake parts mainly from China I doubt this could be trusted to be the real deal, and even if it where given aging and accumulated drift, etc would probably preclude this as an option in this case. If this where the only option, figure probably better off staying with my current R201.
https://www.ebay.com/itm/122674485353
And on the other hand there are no 190M ohm resistors available at all, the closest standard value seems to be 187M ohm. Closest possible exact match for a series configuration would be to use four 47.5M ohm, but they are 100ppm rated, so not sure it would be useful long term beyond just finding out if the ranges would fall in compliance after connecting this resistor combination in parallel with R201.
As many others, looking forward to your video, and when the time comes hoping for some more of those Red Devil resistors that you might want to sell for use as standards. BTW did a G search on them and closest results is for Ohmite "Brown Devil" resistors, a whole different beast. Basically zip on the Red ones.
Thanks for all the Hi-Q information.
Alex
