[volt-nuts] volt-nuts Digest, Vol 12, Issue 11

[Poul-Henning Kamp]

In message <949887D3866942A3857625622A0587AF at LapTop>, "Roy Phillips" writes:

I found a HP3458A (on ebay for EUR1390) which had appearantly spent
its entire life with a german manufacturer of electromechanical
relays.

The only problem were a dead bit in the GPIB port, and a fresh
driver-chip solved that.

To say the CMOS RAMS were old would qualify for understatement of
the year award:  They were 5 year models (Suffix Y) datecoded in '89.

I used the "MREAD" GPIB command to make a backup copy of the CALRAM
chip to my computer, before unsoldering all three CMOSRams and
mounting new ones (in sockets).

Afterwards I checked the old CMOSRAM's battery state, by measuring
the voltage over the GND and VCC pins in  10GOhm input mode and
found a healty 2.9something volts.

This is a point I think many of us overlook:

If the meter has been turned on 24/7, the lithium cell does not do
anything but sit there, and the reverse current of the diode that
separates it from the chip is often enough to keep the battery in
fine shape, despite the fact that it is not in any way shape or
form intended to be rechargeable.

And I think it is pretty safe bet that most HP3458A's are left powered
24/7, because that avoids the entire issue of warm-up etc.

But yes, eventually you will run out of battery, but it was probably
the easiest chip I have ever unsoldered: the holes were suprisingly
large.

So I would recommend you make a backup of the CALRAM with the MREAD
command (I described how to in a post some time back) and let your
batteries run out.

If like me, you keep the meter powered, it may never happen.

Poul-Henning

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