It's been a week or two, but I've been busy with other tasks.
The latest work regards the RTC battery. The circuit is designed for a 3.6 V lithium thionyl chloride (LiSOCl2) cell, such as a Tadiran TL-4902. With the 3.6V, the circuit (National MM58174AN) draws about 18 uA, quite high by today's standards. The IC's operating current increases quite substantially with Vdd, so the designers added a series resistor to reduce the voltage to about 3.0 V which reduces the power consumption (though does waste energy in the resistor). With the reduced Vdd, it typically draws about 10 uA.
In my case, I replaced the existing TL-4902 with a Xeno XL-050FAX, which is of similar chemistry. Xeno suggests a depassivation technique for using cells which have been stored for >6 months by draining it with 30 mA for 10 to 30 seconds. I skipped this procedure, as I assume that it is mostly for high-pulse applications and that the passivation layer will slowly be remove through its lifetime.
I had contemplated using a LiMnO2 cell (3.0 V nominal) and get rid of the dropping resistor, but the LiMnO2's energy density is low enough that it would decrease the lifetime vs the LiSOCl2 cell.
(I did a quick run of a 5V reference, and noticed sub-ppm level steps during the auto-zero cycles. These steps do not happen when the input is zero volts. This will require further debugging, and inspection to see if some "charge-injection reduction" resistors were added to my auto-zero circuit or not. I pulled the integration capacitor from the board and tested it for leakage. The LC102 tester showed zero leakage at 100 V. I'll reinstall it (I have some replacements in stock in case I melt the capacitor or something. I may order some of the teflon standoffs to reduce PCB leakage, too, as Mickle has demonstrated on the input). Grounding the guard PCB did not significantly change the measured noise.)
(I ordered a replacement display that I was going to bodge in, but it turned out to be too deep, and doesn't fit between the blue filter and the PCB. I think I'll just order the correct part from eBay.)