Need help with wiring

Thread Starter

Djard

Joined Oct 20, 2017
5
In my Viz model WD-747 digital multimeter, the hot wire to the 9V battery broke off. Since the wire had oxidized and was impossible to properly solder back onto the terminal (even after filing the braided strands), I replaced the wires.

But upon opening the device, one wire popped off (see in attached pic the wire resting on the cover at right), which, looking at the connector at the end, is not soldered. I conducted a continuity test to see if the multimeter works with the disconnected wire: it does, but only for a short time. And it fails when attempting to read voltage.

I looked for a terminal on the circuit board inside the multimeter to which the wire might have been connected but see nothing that even resembles the lug at the end of the disconnected wire. I'm hoping someone might be able to help me find where the wire should be reconnected.
 

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wayneh

Joined Sep 9, 2010
17,152
In my Viz model WD-747 digital multimeter, the hot wire to the 9V battery broke off. Since the wire had oxidized and was impossible to properly solder back onto the terminal (even after filing the braided strands), I replaced the wires.

But upon opening the device, one wire popped off (see in attached pic the wire resting on the cover at right), which, looking at the connector at the end, is not soldered. I conducted a continuity test to see if the multimeter works with the disconnected wire: it does, but only for a short time. And it fails when attempting to read voltage.

I looked for a terminal on the circuit board inside the multimeter to which the wire might have been connected but see nothing that even resembles the lug at the end of the disconnected wire. I'm hoping someone might be able to help me find where the wire should be reconnected.
Could it have pulled off the back cover, underneath the paper there. Maybe a piece of metal shield there?
 
The 9V battery clip might be tough to solder because of the plating. One would normally replace the entire clip. It comes with wires.

If it will fit Digikey has a nice riged clip for the battery.

I'd actually try holding the wire to the disconnected side of the battery clip or temporarily sandwhich it between the connectors.

As for the other wire, it looks like its missing from the shield material. The hole grommet should fit like a glove there.

Again, this could be very tough to solder.

You might want something like https://www.digikey.com/en/products/detail/panduit-corp/P22-4R-C/317721 with a #2 or #4 stud.

Attach it with a nylon screw and nut and possibly a metal washer and metal external tooth lockwasher being very careful with the placement. Solder the wire and use heat shrink.

I'd use something like https://www.mcmaster.com/washers/lo...ock-washers-9/?SrchEntryWebPart_InpBox=washer which is available in #2 and #4.

A Bellvillle https://www.mcmaster.com/belleville...isc-springs-7/?SrchEntryWebPart_InpBox=washer disc spring is constant force.

So, nylon screw, case, soft metal, flat washer. toothed washer, ring terminal, bellville washer, nut.

Searching for Thread chart ther should be 2 numbers of interest. Close fit and free fit. Close fit is tighter.

These https://www.digikey.com/en/products/detail/mpd-memory-protection-devices/BS6I-MC/252231 are a rugged battery clip.

The wire size should be close, but you can double it, solder and heat shrink.

The board should have provided a hole to strain relief the battery wire with an underwriters knot.
 

dl324

Joined Mar 30, 2015
12,871
one wire popped off
Carefully remove the paper on the back cover and see if you can match up the debris on the connector to something under the paper. The length of the wire is going to limit the places where it could have been.
 

LesJones

Joined Jan 8, 2017
3,169
If you have an old PP3 (9 volt battery.) you can dismantle it and use the connector part of it. It is normally possible to solder to the metal strip on the connector.

Les.
 

Thread Starter

Djard

Joined Oct 20, 2017
5
Thanks to everyone for the kind response, especially for the idea of replacing the battery clip. And I had a spare one in my box of scraps, too. Duh!

I looked under the paper on the cover, where there is a partly torn symmetrical hole. There is no metal there...only the plastic cover. Yet the tear on the hole looks fresh. The terminal on the disconnected wire looks like it might have been clamped to the paper. Why would the wire be attached to the paper, could it be some kind of antistatic technology?
 
Why would the wire be attached to the paper, could it be some kind of antistatic technology?
Think like a chewing gum wrapper or a potatoe chip bag. Their metalized. ran equipment that "could" to that on a small scale. See https://en.wikipedia.org/wiki/Sputter_deposition

We had a tiny machine at work that basically allows on to put an insulator in a Scanning Electron Microscope. You would need it if you were scanning bugs which we were not.

it's a shield, probably from your hand. Your body acts as an antenna for power line frequencies. Grab onto a scope probe.

That why I suggested using a nylon screw. It might be aluminum which won't solder well, You can solder to the brass hole. My confidence level is really high, that's where your wire goes.
 

Lo_volt

Joined Apr 3, 2014
189
I concur with KeepItSimpleStupid's answer, it's very likely a shield, especially since it's right over the big IC. That IC is the A-to-D converter that measures your voltages and it drives your display as well. It probably has an oscillator that may interfere with other equipment if the shield is not there. The paper serves to insulate it from the components below, but it's likely to be copper or aluminum foil on the other side. It looks like the little ring terminal was riveted or staked to the foil. If you haven't destroyed the foil in removing it, you may be able to solder the wire directly to it.
 

RIKRIK

Joined Oct 11, 2019
145
To me it looks like it came off the hole in the paper. Depending on how it was fixed over time it just failed. If you cut the old 9v battery connector off. New ones are dirt cheap. Also depending on what your using it for. The shielding wire you can get away with not using it. I don't know how old the thing is but "would I try peeling back the sticker" mmm.....
 

Thread Starter

Djard

Joined Oct 20, 2017
5
I carefully peeled back a little of a corner of the paper and used an jeweler's loop to look at the underside. Yes, it looks metallic. I placed a strong magnet on the paper and found the foil-like backing to be non-magnetic.

So I will try my hand at surgically riveting the terminal back onto the foil. I dare not use my soldering gun as the foil is sooooo thin.
 
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