Swapping buffer batteries in old test equipment without loosing calibration data

[Traceless]

Hey everyone,

recently a 1980s Prema 5000 6.5 digit bench DMM found a new home on my workbench. I was expecting the meter to need some cleaning and repair but it turned out that it was in pretty good condition and only requires some calibration in some modes/ranges. To my surprise even the backup buffer battery for the NVRAM that stores the calibration data, which must be >30 years old by now has not leaked and still measures 3.4 V (bottom left on the picture attached). However due to its age I think it is a good idea to replace said battery anyways. Ideally I want to perform the swap in two steps:

1. Make a backup copy of the NVRAM contents prior to removing the old battery.
2. Bodge in a 2nd backup battery so the NVRAM supply is secure during the swapping process.

My idea was to use a double AA holder with non rechargable lithium AA batteries (when new those measure usually something like 1.6 - 1.7V each). Since some here in the forum have probably done such a swap more than once I thought I'd ask for some advice and common pitfalls before anything goes sideways.

I also haven not yet been able to find a 1-to-1 replacement for the battery I guess those throughhole lithium batteries are out of fashion meanwhile. Does anyone know what that form factor was called or if there is a modern replacement that will fit as substitute? Modern system usually use coin cells - while those can deliver 3.3V I think they'll drain way to quick in the Prema.

Is there some adapter/programmer that I can use to read the NVRAM while still on board buffered by the old battery similar what the CH341A can do for modern BIOS chips when used with the clamp like this? Or in other words is there a programmer that can read 1980s NVRAM and comes with a suitable clamp?

[alm]

The battery looks a bit bigger, but 1/2 AA and 2/3 AA 3V and 3.6V Lithium cells are commonly available with solder tabs or wires. Search for instructions on the HP 3478A and 3468A, which use such a battery to store their calibration values.

My technique has been to both solder a backup battery holder with 2xAA (I don't remember if I used a series diode) and also use a floating soldering iron so I can keep the unit powered. Obviously only do this if the battery contacts are well away from any mains wiring. Now I have a TS-100 soldering iron (similar to this) that I can power from a bench power supply or other floating supply. What I used to do before that with my Hakko 936-clone was to heat up the iron, then unplug it from the station, quickly solder using the heat capacity of the tip, and then plug it back in to heat up again. This works best with big tips. Again, only do this if you feel comfortable working on live equipment.

I don't think it's possible to read parallel RAM in-circuit, though I can't say I tried. For the HP 3478A and 3457A people figured out commands to send via GPIB to copy the contents, although I'm not sure if they also commands to restore the contents. What might work if you're motivated is record all address and data lines with a logic analyzer, and reconstruct the contents from that. If you're lucky, it will read the entire RAM on start up to calculate the checksum.

[Traceless]

Hi alm,

thanks for the tipps. Well I can unplug the unit to avoid any dangers from mains. I also own a Pinecil iron which I could power from a USB power bank. Diode may be a good idea I'll also need to think about how to deal with the resitor that is tapped into the battery positive. I haven't had the board out yet so I'm not sure it the solder pad on the bord leads somwhere separate or if the battery positive solely goes though the attached resitor.

[David Hess]

Tadiran makes batteries in that form factor.  I like this one because of its good capacity/price:

https://www.mouser.com/ProductDetail/667-TL5155P

[BrokenYugo]

I put a 1k current limiting resistor in series with a temporary 3V battery, tack solder temp battery in circuit away from old battery solder joints, desolder old battery, solder in new battery, desolder temp battery. I haven't lost one yet, though admittedly I've only done this for really simple single chip video games save file type stuff. The device must be floating from earth when you do this to avoid the earthed soldering iron from shorting anything and inadvertantly clearing the contents. You must also obviously avoid shorting anything with solder bridges or careless placements of the tip. Work slowly and always know what you're doing before you do it.

I personally don't like the idea of non soldered connections in this sort of application.

[Traceless]

@David Hess: Nice, thank you! That looks like an exact replacement. For some reason I've stubled upon Tadiran but only managed to find this form factor. Replacing it with the one you linked will for sure look much nicer and be easier.

Also thanks to BrokenYugo for sharing your experiences.

[indeterminate]

Most of the kit from that era used eeprom for long term storage of variables, but if you have none of that then it looks like the 1k x 4 bit static ram chips are used.
You could build/modify sumthing like this to copy the ram
https://myretrostore.co.uk/product/2114-tester/

[Wollvieh]

I have the same task to do, with the Prema 5000 and a HP3478A. For the HP there are even comfortable Save/Restore-Programs available here, but you need a GPIP-Adapter (which can also be built cheaply with an Arduino). For the Prema I think a logic analyzer is the only way to save the configuration.

Maybe a Programmer (or a "RAM-Reader"-Clamp of any kind) is not a good idea, as there will be voltages on the bus with a lot of unpowered chips including the CPU. They might be in tristate, but they also could get damaged or at least disturb the readings. Someone with knowledge on old Processors will know for sure.

So in any case it's "for want of a nail" (in German it would be like the song "Ein Loch ist im Eimer") with a lot of additional tasks if you want to do it the safe way, instead of just boldly solder in a new battery.

In the Prema schematics there is a 10µ electrolytic cap across the uPD444 RAM, behind the diode and the resistor. Meaning if the capacitor gets leaky one day, it will drain the battery quite quickly.

I think I will not remove the board, be it in the Prema or in the HP, but instead cut the wires or foil from the old battery and solder the new one to it. You don't risk to damage the double sided boards while soldering and you avoid mechanical stress to the board as a whole. Old solder joints or socket-connections don't get better by bending them. Not to speak of having to unplug or even unscrew lines of the measuring circuitry.

[alm]

Prema meters I'm familiar with will store the initial factory calibration constants in EEPROM together with the firmware. So worst case you can fall back to those. They will automatically use those constants if the NVRAM is empty. But this is for more modern meters that used the Dallas monolithic NVRAM parts instead of an external battery like the 3478A.

As I wrote above, my preferred technique is while the unit is powered with a floating soldering iron. Obviously you need to be careful to stay away from the power supply and not short anything with tools (use insulated tweezers).