HP 3468A multimeter - CMOS battery question

8/22/2011 9:44:27 PMPeter G Balazsy

(I'm hoping EDD might chime in on this one....)

I bought an ostensibly "good working" HP 3468A Bench multimeter for only $84.

It hasn't arrived but the seller did a few tests with it and says it is working.. plus it passes the "self test" mode too.

The meter has no internal adjustments and gets calibrated via software changes via front panel or remotely via RS232 port.

The original factory calibration offsets are stored in CMOS using a 2.9v .75mah Lithium battery. This battery is supposed to maintain values up to 10 years.

This meter was made about 1994 or so I think.. so that battery may be near end of life.

I don't want to pull it to replace it and possibly loose my settings ( then I'd have to send it out to be calibrated maybe $$)
I bought another 3volt lithium .8maH battery to replace it with.

How do you suggest I replace it live with out screwing anything up.

There is a diode -->|-- leading to the CMOS from the plus end and a .1uf with one end to ground...
.. that may hold up CMOS during the change but not sure that is really what it is there for and how long it will work while I make the swap.
Do you think that .1uf would remain charged long enough?

My plan is to try to somehow parallel in a new one then remove the old... but just how?

Any ideas?

8/22/2011 9:52:45 PMMac

I share your concern that the 0.1 would not have enough capacitance to maintain the settings. I have done this type of swap by soldering two leads to the current + and - and just connecting these to the new source, as you said, in parallel. Then just de-solder or snip out the old battery. The old one just received a bit of a recharge, prior to its exit!

8/22/2011 10:39:24 PMPeter G Balazsy

Here's the area in question Battery = BT551
The steering diodes seem to allow the CMOS line to be fed from the +5v line while unit is powered up or from the 3-Volt lithium battery when powered off.
Not sure what roll the .1uf cap serves.

8/22/2011 11:43:10 PMMac

Nice upload of the schematic. I see the mystery of the 0.1 cap, but as I'm sure you've suspected it likely suppresses the Vcc. I would solder in the + and - leads and connect up your new battery.

8/23/2011 1:54:54 PMEdd

Sir Peter . . . . .

Just replicate the temporary FUCHSIA circuitry and you are home free . . . flawlessly.

I usually carry a meter with me to check a stores available lithiums stock with a current short across them with a DVM and expect 100 of ma's of current . . . and pick out my best option.

(My meters pin sharp probe tips pierce thru any plastic packaging.)

73's de Edd

Nice upload of the schematic. I see the mystery of the 0.1 cap, but as I'm sure you've suspected it likely suppresses the Vcc. I would solder in the + and - leads and connect up your new battery.
:

8/23/2011 4:48:46 PMPeter G Balazsy

Thanks a bunch Edd:
... very reassuring.
nice pict help too!!

8/30/2011 6:38:57 PMPeter G Balazsy

Well... the deed is done. Thanks all.

I put two AA cells in series with a switching diode and first hooked that as a temp source to the circuit to supply 3v.

Then I clipped out the old 3v battery and soldered in the new 3v lithium.

It all seems to be working fine.

I guess I got that old battery out in the nick of time... as it has practically no juice left at all.
I connected a 2.5v 500ma lamp across it and it would not light at all.... however the two AA's in series lit it up just fine.

So I guess that old lithium was really weak.

It has a name:
Eternacell sdx
3volts No.9511
hp part no. 1420-0278

I replaced it with a 3 volt 800mah lithium.