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Old November 7th 18, 05:39 PM posted to
Jeff Liebermann[_2_] Jeff Liebermann[_2_] is offline
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First recorded activity by RadioBanter: Jun 2007
Posts: 1,332
Default MFJ-259B touchup paint?

On Tue, 6 Nov 2018 22:48:42 +0000, wicklowham

On 06/11/2018 07:29, Jeff Liebermann wrote:
Incidentally, the owner of the MJF-259B has graciously allowed me to
use it for an experiment. I plan to replace the 10 AA cell battery
pack (15.0V) abomination with a 4 cell LiPo or LiIon battery packet
(14.8V) and an external charger. I'm also looking into a replacement
LCD display that has backlighting. I'll post the results when it's
done and hopefully working.

Li-ion and LiPo cells are nominally 3.7 V but when fully charged 4.2 V,
hence 4 cells in series would initially give 16.8 V
Question : Would 16.8 V not be too high for the MFJ 259B ?

Yep. See Pg 4 of the manual:
Voltage must be more than 11 volts, and preferably less
than 16 volts, when the unit is on and operating. Maximum
"sleep mode" and "OFF" voltage (when the power supply is
lightly loaded by this unit) is 18 volts. The supply
must be reasonably well filtered. The supply must not
have a grounded positive lead!

The MFJ-259B can be used with an external low voltage dc
supply’s (MFJ-1315 AC adapter recommended). The ideal
supply voltage is 14.5 volts dc, but the unit will function
with voltages between 11 and 18 volts. The current demand
is 150 mA maximum. (Be sure you read the battery instructions
if you use also install batteries!)

So, basically the MFJ-259B will run on 11 - 16V. To double check, the
typical AA alkaline cell is at 100% at 1.55V and mostly dead at about
1.2V giving a range of 12 - 15V for running off the 10 cell stock
battery pack.

Meanwhile, if I use 4ea 18650 LiIon cells, I get a voltage range of
4.2V for fully charged, and about 3.5V where I want to stop. For 4
cells, that's 14 - 16.8V. Not a perfect fit for 11 - 16V but a diode
or two in series with the battery pack should work. Or, I could take
the easy way out and use fewer cells and a boost regulator to deliver
exactly 14VDC. Something like this:
Lots of other options including some boards that integrate a USB 5V

The above cells should NOT be discharged below 3.0 Volt in order not to
loose capacity .
It means the voltage of the pack should NOT drop below 12.0V .

Actually, I would quit at about 3.5V just to be safe. If I get
lavish, I'll add a 4S (4 cell) BMS (battery management system) like
one of these:

To make things really complexicated, I want to include a balance
charger connector, so I can equalize the voltage on all 4 cells using
one of the balance chargers commonly used by the RC (radio control)
Or, I could just use the boost converter idea, run it off of one LiIon
18650 cell, eliminate all the complexicated BMS and equalize stuff.
The MFJ259B only draws about 150ma giving a 2 A-hr 18650 cell:
2 A-hr / 0.15A = 13.3 hr
runtime. Of course, with junk Chinese cells, about half that, or
less. Or, with a single LiPo cell, I could use a cell phone charger.
Decisions, decisions, decisions.

I assume you intend charging the above cells outside the analyser
,otherwise you would have to include a max voltage circuit for each cell
when charging the cells in series. These circuit boards can be cheaply
bought from Chinese online retailers , but highly like there is NOT
enough space for the 4 cells + the circuit board inside the analyser

See above on equalizing charging. That usually fails badly with a
mixed assortment of Chinese cells.

There is PLENTY of space inside. Remember, the 10 cell AA pack is
going to disappear. At this point, I have no idea if I'll use an
internal or external charger. Much depends on how much butchery,
err... modifications need to be made to the power switching and wiring

Good Luck with the repairs

I may need some luck. I cleaned up the mess on the PCB as best I
could last night and applied power. Nothing on the display. A quick
continuity check showed that the battery wires were open. Replacing
those showed that both the on/off switch and the external PS connector
were not working. To be continued in a day or two.

Frank , EI7KS

Jeff Liebermann
150 Felker St #D
Santa Cruz CA 95060
Skype: JeffLiebermann AE6KS 831-336-2558