Re: Tektronix DM5010 NiCa 2.4V battery replacement


Harvey White
 

On Sat, 21 Jul 2018 11:40:14 -0700, you wrote:

Hi everyone,

I need to replace the backup battery of a DM5010. It has leaked but I cleaned up the mess.
The service manual specifies a Nickel Cadnium 2.4V 150mAH at 14mA.

So far, I was able to find (on Mouser) a 2.4V 110mAH (https://www.mouser.fr/datasheet/2/331/PCBM-2.4_R_complete_11_Jan_25-21018.pdf).
Does it meet specification? In particular, we have 0.11AH vs 0.15AH.

I've absolutely no experience with rechargeable batteries. So I welcome your help.
I'd suggest a different approach. The backup battery will leak again,
sooner or later. In addition, even with 150 mah, if it's in storage,
it will go below the point where the cal constants are valid. You
will have to provide various (highly accurate) voltages, including 250
or so and 700 VAC to calibrate this.

Calibration, unless you have the equipment, is a pain. (there are
ways of working around the accuracy requirement, but you still need
the 700 volts AC and 1000 volts DC).

I'd put in a lithium battery, I'd suggest a Tadiran TL-2155, which
will fit in the slot. You MUST put a diode in series to prevent the
battery from being charged, however.

Before you go to the trouble, I'd suggest temporarily wiring in two
small NiCd batteries, capacity not super important. Let them charge
up. Then go through and attempt to calibrate the unit as best you
can. What you're looking for is bad calibration constants, indicating
that the battery corrosion has damaged part of the backup circuitry.

At that point, if you can fix it, good, if not, then you'll have to
figure out a workaround.

One workaround is to rebuild/remake the CPU board with a more modern
processor, an AVR mega (a big one) would work. However, you'll want
to know how the insides interact with the processor, and I do not have
anywhere near complete information on that.

Given that information, a more modern board can be made, the functions
may be expanded, and you can restore the DMM to full service.

I put the battery in, and the memory storage was still not working, so
rebuilding is going to be what I do, for now, unless I find one that
is still working.

Harvey



Best regards


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