HP3457A SRAM battery change

[dboy]

First post...
I have some general experience on repairing stuff, but I have never done any complicated stuff like changing a battery when a mistake will fry or ruin the device.

I have HP3457A DMM with a very old calibration and very old battery. It still holds its calibration but I would like to change the battery

I have connected it to mains power via an uninterrupted power supply, UPS.
Questions:
1. Will this prevent it from draining the internal SRAM battery?
2. Will the connection to mains prevent it from loosing its calibration if the SRAM battery dies or is desoldered and removed?
3. Does the instrument need to be turned on for the mains power to prevent it from dying, or is it enough to just have it connected to the UPS?
4. Can the old internal battery be desoldered and disconnected without loosing the calibration, as long as the 3457a is connected to the UPS?
5. Should the 3457a be turned on during the desoldering?
6. I understand that the soldering need an isolated soldering tip, which I don't have. I have a butane soldering tool, but it is not very powerful, would it be possible to just cut the old battery away and solder the new using the old battery pins? Kind of, mechanically remove the old battery, but solder a new in place.

If I desolder the old battery completely, and solder a batteryholder to the board as in this thread,

Is it safe to desolder the old the battery and replace it with a battery holder with a battery holder as long as the 3457a is connected to the UPS? This way it will be much easier to change the battery next time, it is just to connect the instrument to mains power, and then replace the battery.

There are several threads on the subject of changing the battery, but I'm not sure if my questions are answered in them, at least not so I understand the answers well enough... :-)

https://www.eevblog.com/forum/repair/hp-3457a-nvram-battery-change/msg3551646/#msg3551646

A way too complicated solution (for me) of backing up the cal-data.
https://www.eevblog.com/forum/metrology/dumping-cal-ram-of-a-hp3457a/

And regarding a possible problem
https://www.eevblog.com/forum/testgear/just-got-3457a-and-then-boom/

Regarding which battery to use
https://www.eevblog.com/forum/projects/panasonic-br-23a-chemistry-change/msg2737974/#msg2737974

[pqass]

I've done a battery replacement on my 3478A which, from your meters schematic, is similarly attached in the 3457A.
See a snip of the 3457A schematic below from page 236 from here.

The technique that I used involved creating an alternate 3-cell (4.5V) battery holder with series resistor and diode (just like CR635 and R647) and attaching it to a nearby cap on the NVVCC line and digital ground.  In your case, C642 is a good candidate judging by the attached picture (from the xdev.com page).

See my writeup here (bottom half) and here.

As for your questions...
The CR636 diode (and after reset, U637D; a PNP) will override the power coming from the 3V battery but only if +5V power is on.
So just being on a UPS (with meter off) is not enough.

Others have elected to have the meter on while they cut out the old battery and used an isolated/portable iron to solder a new one.  I did the opposite; isolated and off meter while I soldered in an alternate battery holder. Then confirmed that I saw 4.5V on the NVVCC line (with alternate battery) before cutting out the old battery and soldering in a new one.

Why didn't I just use a battery holder in the old battery location?   
A: I wasn't sure that the holder contacts wouldn't oxidize over time.  Although, since I have the cal data, I can afford to take that chance.

Also, my alternate holder is only for temporary use since the original battery also has a connection to U636A; a comparator.