I did it ! I replaced the memory battery in my 3457A.
?
My used 3457A was built in the late '80s and still had its original lithium battery, with a 1989 date code on it. This battery obviously needed replacement.
Unfortunately, the battery is soldered to the processor PC board and provides permanent Vcc for a low-power CMOS SRAM chip which holds the multimeter's calibration constants. In other words, interrupting the SRAM supply would result in the loss of calibration data.
Thus, the battery must be replaced while the 3457A is powered up. This is a bit of a nail-biting experience, but it can be done. One useful trick I used is to prepare a cardboard shield which covers the CPU board and protects it from any solder drops and shorts, while allowing access to the battery. And of course the soldering iron must be floating.
?
Replacement batteries in the original size (lithium BR 2/3AA with solder pins) are still available for a few dollars, but I chose to install a slightly different battery, a Tadiran SL750 which has a 1/2 AA size, for which sockets are available. Using a socket means that in 10 years, the next battery replacement will be easier : with the multimeter powered up, just pop out the old battery and insert a new one. Another advantage is that 1/2 AA lithium batteries are very easy to find, there's a good chance that your local supermarket has them.
?
Now I'd like to share some thoughts and questions about this battery. As we all know, lithium batteries have a long, but finite life. Even though the current drain on the battery is less than 0.2 uA at room temperature, at some point in time the battery will die.
I read the 3457A user and service manuals, but I couldn't find a single mention of the battery. Nothing about battery life, no instructions on how to replace it, even nothing that says "contact your nearest HP sales office". To me, this doesn't make sense, especially when considering that this multimeter was introduced in 1986, at the time of the "old HP" which produced high-quality instruments, not expendable stuff. I could only think of a few possibilities :
?
- Maybe the engineers at that time believed that lithium batteries could last forever ;
- Or they thought the multimeter would be scrapped when the battery runs out, or before it does ;
- Or they decided that if a multimeter loses its calibration data because of a dead battery, it should be returned to HP for battery or processor board replacement followed by re-calibration. That's an expensive proposition.
?
None of these options make any sense to me in the context of the "old HP".
Further thoughts and comments will be most welcome !
?
Joel?
