Short: Hacking an external battery onto the Dallas DS1235 nvram

I was repairing a Tektronix 2430 digital oscilloscope. It had power supply issues, which I won't describe here, it just needed new electrolytic capacitors, as usual.

However, it was also forgetting calibration*, which pointed to the DS1235 nvram's internal battery being discharged. It was not hard to diagnose, especially that the scope was failing the nvram battery test.

The problem with this and the similar Dallas nvrams is that they have a built in battery, meaning that the lithium cell is attached to the ram circuit and the whole thing is potted into one sealed case. Sometimes these can work for decades, depending on for how much time the instrument is powered up, because then the ram is using external power. But, sooner or later all of them will become discharged. Unfortunately many of these types of components are obsolete, and there are a lot of fake ones on eBay, so it can be hard to find a replacement. There are workarounds by replacing them with other types of nvram devices, such as frams, but what I want to show here is how to attach an external battery. This is not new, there are several tutorials for doing it for various devices, but AFAIK this method has not been shown for the specifically the DS1235.

This nvram has two batteries and a DS1210 nvram controller chip, which is actually visible on the bottom. Also visible are the outlines of the battery, The goal is to remove at least one of the batteries and connect an external battery there. The DS1210 will automatically select the battery with the higher voltage, so it is sufficient to use only one battery.

The way I achieved it was by heating the potting compaoud with a heat gun which made it rubbery and it was easy to dig out the batter. Do not heat so much that the baterry explodes! Make a hole into the button cell, if you are afraid of explosion.

Once the battery is removed, it is simple from here. Solder some wires and a battery holder. I used a CR2032 cell. I used a dab of epoxy to fasten the wires.

I should have used a shorter piece of wire, but originally I wnated to put the battery holders to the side of the PCB. However, I realized that there is room on the top of the nvrams, so again epoxy came to help. Some Kapton tape was used to provent the case accidentally shorting.Not a winner of a beauty contest, but works.

 *In many cases people go great lengths to save calibration data from these nvrams. In the case of the Tektronix 24xx series Digital Scopes (2430, 2432, 2444) it is completely unneccesary. The scope is cabaple of nearly automatic self-calibration. Some external signals need to be provided in the form of DC levels and square waves, but these are easily obtainable. So don't save the nvram data, simply redo the calibration. That should be done periodically anyway. This is not the case with the 24xx series analog scopes like the 2445, 2465 etc. Their calibration is a tedious process and requires some quite spocialized equipment. Still it doesn't hurt to do it peridically, especially after repairs.