I finally decide that I had pushed my luck far enough with my battery backed NVRAM in my Tektronix TDS 380 oscilloscope. The specifications for the Dallas Semiconductor DS1644-120 Timekeeping RAM state that it is good for at least 10 years at 25°C, and mine had just turned 28 years old.

The DS1644 NVRAM stores all the calibration data for the oscilloscope, along with a Real Time Clock (RTC) for display on the screen and timestamp for saved files.
I did not want to replace it with one of the cheep Chinese knockoff IC’s that also have non-replaceable internal batteries, or one of the quality functional replacement’s with external battery for over $73 USD delivered.
Since I was not interested in the timekeeping capabilities of the DS1644 or replacements, I decided to try out a Ferroelectric RAM (FeRAM) that should be a drop in replacement except for lacking the time function.
I ordered a Infineon Technologies FM18W08-SG which is a 130 ns 256K device organized as 32 x 8 parallel memory, along with a SOIC28-DIP adapter board for a total of $18.80 USD.

There are quite a few screws to remove before the main acquisition board containing the DS1644 IC can be removed, including four screws under the front bezel. The floppy drive interface board and many cables also need to be disconnected.
After carefully unsoldering the DS1664 from the board, I placed it in my TL866II Plus programmer and read out the memory data and saved the file. I then placed the new FeRAM into the programmer and programmed it with the data. So far so good.
Important note: Be careful unsoldering the DS1644 IC as there have been reports of data loss attributed to overheating the IC. I unsoldered 4 to 5 pins at a time, then waited five minutes before working on the next group of pins until all pins were de-soldered.

I added a 28 pin socket to the circuit board so that in the future the next owner could easily replace my FeRAM modification if required.
Did I mention that the really nice thing about FeRAM is that it requires no batteries.
After putting the TDS 380 back together it was time to power the oscilloscope up and see if the replacement works.


Well everything worked fine, all the test’s passed, and the only indication that something is different is that the time and date on the display remains set to 16 Jun 2024 12:31.