Tuesday, March 18, 2014

How to repair a Weighmax scale model xjusa that doesn't power on

    The Weighmax model xjusa scale doesn’t power up. It has been in use for about a year and was used heavily in a weight control program.  New batteries didn't help. Sure this thing was only fifteen bucks from Amazon but let’s take it apart and see if it can be fixed.

It's dead


Dis-assembly and troubleshooting

1) Pry off the four rubber feet from the bottom of the scale. 
2) Remove the four Philips screws that are now visible.
3) Remove the plastic weigh platform on top by turning it counter-clockwise. It will rotate off the load cell assembly.
4) Inspect the circuit board for defect or damage. 
5) Remove the two micro Philips screws holding down the circuit board. 
5) Remove the batteries and then reinsert. This resets the scale and it should do an auto power up.
If it auto powers up and weighs now as this one did, the weigh cell and the rest of the circuitry are good.

It looks like the power switch on the right is the culprit

6) With a volt meter test for voltage at the power on switch. Three volts is present and the scale actually powered up when the meter lead  touched the switch Contact. The switch is the prime suspect.

Removal of the switch. Needed: Soldering iron, solder, solder wick.

1) Remove the batteries
2) Using the solder wick and the heated and tinned iron, remove the solder from the solder pads and the  four legs of the switch. Remove the switch. It may be necessary to pry up on the switch with a small tool while applying heat to the pads.

De-soldering the power-on switch

3) After the switch is out, mount the replacement switch nice and level. It may be necessary to clean the remaining solder out of the pads with solder wick. 
4) Solder in the replacement. Radio shack or an online electronic parts house like Newark or Digi-key stocks these.  I re-purposed a used switch from a retired VCR. Remember those? 
5) Reinsert the batteries and test. It works!
6) Remount the circuit board with the two tiny Philips screws.
7) Reassemble the bottom.
8) Put the scale platform back on by twisting clockwise once the cover is seated on the slots for the load cell pan. 
9) Test with a known weight.

Back in use, weighing solder (in grams)


4 comments:

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