Scale don't work

[Givat]

Hi all,

I got a "E4000D OHAUS scale". I have had it for over 10 years.

Today the scale decided not to work for the first time.

I tried to find out what was wrong, I have connected the power supply to the power and used a voltmeter to see if the wires are cut or the transformer is broken - I couldn't find 2 wires that have 17volts between them. (actually I couldn't find 2 wires with any voltage between them)

 

And now for my questions:

1) Is there any component in scales that is prone to "die" first? How can I check if it is broken?

2) Does any one know why the scale power supply has 3 electric wires coming out of it and not only 2? (1 positive and 1 negative)

3) If I can't find 2 wires with some voltage between them - does it mean the wires are broken\the transformer is broken like I guessed?

 

thanks.

 

http://i1111.photobucket.com/albums/h479/questl/P1010687.jpg

http://i1111.photobucket.com/albums/h479/questl/P1010689.jpg

http://i1111.photobucket.com/albums/h479/questl/P1010691.jpg

[50AE]

Looking at the scale, its body is metallic, so I would suspect the third wire as a grounding one, though it could not be.

If you are sure you don't measure any voltage accross the wires, I do suspect the power supply. If it is a transformer based one, it's easy to fix, but harder if it's a switching mode.

Carefully open to check it out.

[Peret]

If you look at the label on the back and the power supply, it says +/- 17V (ok, the plus is sitting on the minus) - that means it wants 17 volts above ground, another 17 volts below ground, and ground. Three wires. 34 volts between two of them and 17 volts between all the other combinations.

 

If you're not measuring any volts anywhere, either the wires are broken somewhere or there is a fuse blown inside the PSU. The wires can break inside the plastic sheath at points of stress, like where it's bent sharply in your picture right where it comes out of the power supply, but it's improbable that all of them are broken, so you should still get some voltage. More likely there is a thermal fuse inside the unit that has blown and broken the 110V input. If you put an ohmmeter across the 110V plug (with it unplugged, naturally) it should give you a low reading. If it's open circuit, open it up and look for that fuse. It will be in the 110V circuit, right on the transformer somewhere, possibly wrapped in the winding tape.

 

If you can't fix it, I'll sketch up a power supply circuit you can build yourself as a replacement using easily available parts.

[50AE]

It will be easy to build a new one, you will need the proper transformer though, it has to have two windings with a 13V output if you want 17V+-.

[Givat]

If you look at the label on the back and the power supply, it says +/- 17V (ok, the plus is sitting on the minus) - that means it wants 17 volts above ground, another 17 volts below ground, and ground. Three wires. 34 volts between two of them and 17 volts between all the other combinations.

 

If you're not measuring any volts anywhere, either the wires are broken somewhere or there is a fuse blown inside the PSU. The wires can break inside the plastic sheath at points of stress, like where it's bent sharply in your picture right where it comes out of the power supply, but it's improbable that all of them are broken, so you should still get some voltage. More likely there is a thermal fuse inside the unit that has blown and broken the 110V input. If you put an ohmmeter across the 110V plug (with it unplugged, naturally) it should give you a low reading. If it's open circuit, open it up and look for that fuse. It will be in the 110V circuit, right on the transformer somewhere, possibly wrapped in the winding tape.

 

If you can't fix it, I'll sketch up a power supply circuit you can build yourself as a replacement using easily available parts.

 

I opened the transformer and it is filed with epoxy.

My dad had a guess that the transformer is a very accurate one and can compensate if it gets warm or cold. but we don't know that for sure.

A new transformer will cost me 130$(!!!).

 

The scale worth about 1800$ so I don't want to try and build a new transformer and to take a risk to harm the scale.

[mabuse00]

My dad had a guess that the transformer is a very accurate one and can compensate if it gets warm or cold. but we don't know that for sure.

What's inside the power supply besides the transformer?

Can you post a photo or is the whole circuit covered in Epoxi?

 

The transformer itself is never that accurate, the regulators behind are. Maybe it's not even a stabilised one and the regulators are inside the scale.