Re: Tektronix DM5010 NiCa 2.4V battery replacement


Ke-Fong Lin
 

Hi everyone,

My plan is only to fix this unit back to working condition, and hopefully
have fun in the process.
So there's no plan to re-make the CPU board, that's just to much work,
especially for a 4.5 digit only meter.

The DM5010 uses a "simple" trickle charger for the battery. However, the
charge current may seem too high.
Original battery specifcation is 150mA so trickle charge current should be
0.1 x capacity = 15mA.
However, the replacement battery specifies a trickle charge at 4mA, so I
may have to tweek the current limiting resistor.

I've noticed there is already a subject on eevblog regarding the battery
replacement.
May be I should move there (would be easier to post photos).
Or would guys prefer to continue here?


Best regards,


On Sat, Jul 21, 2018 at 9:13 PM, Harvey White <madyn@...>
wrote:

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






--
/* Lin Ke-Fong
* anotherlin@...
* I always do what I say, especially if it's something stupid.
*/

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