[volt-nuts] HP 3458A CMOS RAM

[Victor Silva]

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