[time-nuts] PM6681 and Timelab (was: PM6680 or 53131A for TimePod)

[Mark Sims]

I once got in around 20 Tek DM5?? GPIB DVM modules for the TM5000 series mainframes.  They have a rechargeable nicad pack soldered to the circuit board.  All the meters still had good cal data but the cells were on their way out.  The meters are a pain to re-calibrate...   

I used a DIP clip to provide voltage to the SRAM chip while changing the battery.  Only lost cal dataon one meter.    I replaced the nicads with a CR123 lithium cell with a series diode to block the charging current.   On each meter I checked the standby current to the SRAM first.  A couple had rather high values that would drain the CR123 after a year or so, so I kept nicads in those.  After 5+ years the meters are still working on the CR123 cells.