I did quite a bit of research on replacing the NVRAM in Tek 2465B and some of the 24xx digital models (2430A, 2440) a few years back. I recently had the good fortune to also acquire a very nice shape 3458A. Its serial number and internal device datecodes suggest it was produced around 1999~2000. My unit contained two Dallas DS1230Y and one DS1220Y devices. One of the first things I did to my 3458A was simply pop in 4 standard RAM chips for a sub-$10 option 001. Note that this cannot be done with newer 3458As. I also de-soldered the 3 Dallas NVRAMs and installed MILspec sockets. I copied the contents of the old devices and programmed new parts. It's working well. On the subject of using FRAMs. In my previous assessment, FRAMs cannot be used in the TEK applications because all the Tek models I evaluated used the NVRAM as standard RAM when powered on. By that I mean that the scope's are continuously writing to the NVRAM while it is powered on. I measured the frequency of the write cycle and calculated that the FRAM would last 1~3 months of powered up service, whereupon it would become a volatile device. I suspect the 3458A may also continuously write to the Dallas devices, if so FRAMs would not be a good replacement. I also evaluated Symtek devices and concluded they WOULD. Symtek as since been absorbed by Cypress and the pricing for the STK16C88 is much higher than Dallas parts, they also don't make a DS1220Y equivalent, only a DS1230Y equivalent. For now the best solution for me is to have sockets installed and replace when necessary. --Victor

At 12:45 AM 8/27/2010, Victor Silva wrote... >1999~2000. My unit contained two Dallas DS1230Y and one DS1220Y >devices. >...On the subject of using FRAMs. In my previous assessment, FRAMs >cannot be used ... calculated that the FRAM would last 1~3 months of >powered up >service, whereupon it would become a volatile device. I've fixed a couple of IOtech EX488 boxes, which use a DS1216E with similar old-age problems. The DS1216E is a DIP socket which makes a 8x32K CMOS RAM chip nonvolatile, and also adds a real time clock. Internally, it's a couple of small lithium cells and a Dallas DS1315 chip. I suspect the DS1220Y and DS1230Y use a similar design, probably using a DS1312. I was able to use a Dremel tool to remove enough of the encapsulation so I could pull the batteries on the failed DS1216s. I then ran some small wire to external CR2032 battery holders (much greater capacity than the smaller cells which fit in the DS1216s). Finally, I replace the 1990s era CMOS ram chip with a modern one (.1 uA typ vs. 100 uA max standby draw). Because the 1315 works by selecting the best of the two batteries, battery replacement is easy - do one, then the other. Self discharge and parasitic discharge are the main factors to life now, so these should be good for 20 years without new batteries. One could pretty easily do something similar with the 1220/30 devices. If not direct surgery, then get a couple of DS1312s and hack up some sockets to provide non-volatile support using replaceable batteries.