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 Post subject: Fixing a Meter with a Mangled Needle - A Step By Step Guide
PostPosted: Feb Thu 17, 2011 6:11 am 
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Various meter problems come up frequently and I have not seen a good guide on dealing with them. Having fixed camera light meters and various electronic gear for years I have gotten pretty good putting these delicate device back into working order. I have been wanting to share that experience with the rest of you, so when a Kris got a Sencore CR-143 with a missing meter needle (see viewtopic.php?t=156224) I figured it would be a good opportunity to put something like this together. Now, enough chit-chat lets get to work.

First I survey the damage. (See the pictures below.) The meter needle has come off of the meter and has gotten mangled. At least we still have it, so the meter is fixable. (I don't know where to get raw needle stock.) If the meter needle were broken in half, it might still be fixable by gluing a tiny piece of wire inside the meter needle to support it from the inside. However, if the break is too close to the tip, the added weight will make the meter impossible to balance (this will make more sense later.)

Image
Image

The arrow in the first image above points out the brass pin which fits inside the meter needle. Yes, I said inside! Meter needles are hollow to aluminum tubes make them light enough to make a balanced movement. Unfortunately, that also makes them very delicate. What happened here was that the glue (Pliobond or something similar) which attached the meter needle to the brass pin gave way, allowing it to fall off and get mangled. The first order of business is to straighten it out again.

I start by VERY GENTLY straighting the bends with a pair of tweezers. I barely apply any force to the tweezers for fear of crushing the needle, but rather use them to push in the high spots on the sides of a crease and support the back side of the bend which I use my fingers to slowly and carefully work it out. Basally, it is just like straightening anything else - just go really slowly and carefully.

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Now comes the cool part. Since the needle is hollow, we can put a piece of wire into it to push out dents from the inside. Here I am using a resistor because its leads are the correct diameter. Again, I use the tweezers to push in any high spots, but I can do it more forcefully this time because the resistor lead prevents the needle from crushing.

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With the needle as straight as I can get it, I carefully slip it over the brass pin and use the tweezers to get it aligned so that the flat end is parallel with the meter face. I then apply a tiny drop of super glue to the junction between the meter needle and the brass pin. To apply just a tiny drop, I wet the end of a piece of wire with super glue and then touch the wire to the junction. Capillary action causes the super glue to be sucked into the meter needle for a strong bond. Be careful not to overdo it here - the meter needle needs to be light so it can be balanced properly. It is also important to use a good quality super glue. Good super glue is dramatically better than what you get at your local hardware store and actually makes a good permanent bond which I find is stronger than epoxy. The best super glue which I have found is called Zap-A-Gap and can be bought at http://www.amazon.com/gp/product/B000EY ... Z9771GR1RH

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(The picture above makes the needle look better than it is - it really has several dents/dings in it.)

Alright. With the meter needle back on the meter, I carefully use the tweezers to bend the brass pin up or down to get the whole needle parallel with the meter scale. I do this before installing the scale because the scale will get in the way once it is installed.

Now it is time to paint the meter needle. It is dinged up from being bent and straightened and we want it to be a consistent mat black. First, I use acetone on a cotton swap to remove all of the old paint (sorry but I don't have a picture of this step) and then I put new paint on using a paint-pen. I buy gloss and mat black paint pens from Micro-Tools http://www.micro-tools.com/store/P-1510 ... p-Pen.aspx . Just shake it VERY well, pump the tip a couple of times to get it nice and wet with paint, and then draw it slowly down the needle from the base to the tip. Allow the paint to dry for a few minutes and its done.

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With the meter needle repainted, I reinstall the scale. Usually they are attached with screws but this one was attached with a couple of pieces of foam tape, both of which disintegrated. A couple of drops of super glue on the old foam will hold it forever.

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Now on the fun part. We want to meter to read the same value no matter what position it is held in. This means that the meter needle must be balanced - it must weigh the same amount on either side of the pivot. To achieve this, tiny counterbalance weights are located opposite to the needle and can be slid back and forth along their shafts. Usually there is one which is in parallel to the needle, to adjust the horizontal balance (my language - I don't know the proper terminology) and another installed perpendicular to the needle to adjust the vertical balance (also my language.) Some meters, however, use one weight which can be bent in both directions, while others, like this one, use one sliding weight to adjust the horizontal balance while the vertical balance is adjusted by bending the shaft which this weight slides on. The red arrow in the picture below points out the sliding weight in this meter.

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The procedure to adjust these weights is simple. First we set the meter on a horizontal surface so that the face is parallel with the ground and then zero it. This is accomplished by rotating the zero-adjust tab (red arrow in the picture below) or, if it is very far out of adjustment, centering the zero-adjust tab, and rotating the pivot wire support (green arrow in the picture below) relative to it. (This meter using a tiny wire which suspends the moving coil of the meter as a spring to return it to zero. The piece supporting this wire can be rotated relative to the zero-adjust tab to act as a course zero-adjustment. Most meters use coiled hair-springs and jeweled bearings instead and don't have this kind of course zeroing adjustment.) Basically, just zero the meter with it sitting so that the meter face is parallel with the ground

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Next, adjust the horizontal balance. As shown in the picture below, hold the meter up with the needle exactly horizontal and the meter face perpendicular to the ground. Note if the meter needle is too heavy (moves down) or too light (moved up.) If the meter needle is too light (as is the case in the picture below) slide the adjustment weight toward the pivot with a tiny screwdriver or pair of tweezers. Otherwise, move it away from the pivot. Rezero the meter with its face parallel to the ground and repeat the procedure until the meter needle does not move more than the width of the zero-line between the two orientations.

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Finally, we set the vertical balance. Rezero the meter with the face parallel with the ground, then hold it up so that the face is perpendicular to the ground and the needle points straight up (as shown in the picture below.) If the meter needle points to the right of zero, bend the shaft with the horizontal balance weight to the left and visa versa. (Of course, if the meter has a separate vertical balance weight, just adjust it to the left instead of bending anything.) Again, zero the meter with the face horizontal to the ground and repeat the procedure until the meter stays within the width of the zero line between these two orientations.

Image

In a perfect would the meter would be balanced now, but in reality the horizontal and vertical balance adjustments interact somewhat, so you probably have to repeat the entire horizontal and vertical balance adjustments a couple of times until the meter needle stays within the width of the zero-line independent of the orientation of the meter.

Finally, if the cover for the meter is plastic rub it down with an unscented 'Bounce' fabric softener dryer sheet. This sounds crazy but it will dissipate static electricity on the face which can cause the needle to stick. Wipe off excess with a clean cloth.

Here is the completed meter. Note that the needle is actually pointing exactly at zero, but because the picture was taken at an angle it does not look like it is.

Image

To measure the full-scale current of a meter, connect it in series with a 1.00K precision resistor and with an adjustable DC power supply. Adjust the power supply such that the meter reads full-scale then using a DVM measure the voltage across the resistor. The value of the voltage is the full-scale current of the meter measured in milliamps.

Thats all folks! I Hope someone find this useful and feel free to add your own advice/corrections

-Matthew

P.S. If anyone has a Sencore CR-143 with a bad or missing meter, I would be happy to buy it off of you, or, conversely, sell you a working meter for it.

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Last edited by 7jp4-guy on Feb Sun 20, 2011 4:46 am, edited 5 times in total.

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 Post subject:
PostPosted: Feb Thu 17, 2011 7:20 am 
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Very cool. Thanks for the info!

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 Post subject:
PostPosted: Feb Thu 17, 2011 12:53 pm 
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Impressive documentation. Thanks for the effort. Another topic for the education file.


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 Post subject:
PostPosted: Feb Thu 17, 2011 3:08 pm 
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Location: Mpls, Minnesota
Matthew, Needle tubing used to be available a few years ago.I used to have a selection of different sizes for meter repair. I think Small Parts used to carry it.

Dave


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 Post subject:
PostPosted: Feb Thu 17, 2011 4:07 pm 
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No pictures here, are the rest of you seeing anything??? And Dave I had one witht the needle broken off about a 1/2" from the pivot and the needle was missing. Looked into the hypadermic needle thing and couldn't find anything that was that small or that thin and they are made from Stainless Steel, much to heavy for meter needles. The hollow meter needles, made aluminum and I tried winding my own using aluminum foil but that is dead soft 3003 alloy and the meter needles are made from a tempered aluminum. I found anything that was stiff enough to support it's weight in a 3" or so length was simply to heavy to balance.

Denny G


Last edited by Denny Graham on Feb Thu 17, 2011 4:19 pm, edited 1 time in total.

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 Post subject:
PostPosted: Feb Thu 17, 2011 4:10 pm 
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Pictures were here but they are gone now.

Dave


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 Post subject:
PostPosted: Feb Thu 17, 2011 5:24 pm 
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I don't know what is wrong with the pictures. :? I use the server space provided free with Comcast's Internet service to host my web page and all of the pictures in my posts and it has been acting very 'funky' for the last week or so - I have been unable to log in with my FTP client and it has been generally very slow. Perhaps they are doing maintenance or something.

-Matthew

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 Post subject:
PostPosted: Feb Thu 17, 2011 5:43 pm 
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I saw them earlier today, but not now.


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 Post subject:
PostPosted: Feb Thu 17, 2011 7:19 pm 
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I see 'em, no problem.


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 Post subject:
PostPosted: Feb Thu 17, 2011 8:39 pm 
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Good. It can see them too - it looks like the problem is fixed.

-Matthew

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 Post subject:
PostPosted: Feb Fri 18, 2011 5:14 am 
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That is the best meter repair post I have ever seen! Great job.


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 Post subject:
PostPosted: Feb Fri 18, 2011 5:36 am 
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Hi Matthew,

Great information, I learned a lot from your tutorial. Thank you very much for you hard work. P.S. Don't stop writing them :D

KT6G Paul


Last edited by KT6G on Feb Fri 18, 2011 6:08 am, edited 1 time in total.

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 Post subject:
PostPosted: Feb Fri 18, 2011 5:39 am 
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Thanks for all of the compliments. :D Do you guys think that I should put a link to this in the Hints & Kinks forum?

-Matthew

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 Post subject:
PostPosted: Feb Fri 18, 2011 6:11 am 
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That is an awesome tutorial! I am glad the needle was savable. I would have never even known it was hollow. Thank you very much for the information.

Kris


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PostPosted: Feb Fri 18, 2011 1:12 pm 
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Mathew,

Great post, and EXCELLENT pics. Well done.

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 Post subject:
PostPosted: Feb Sat 19, 2011 2:08 pm 
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Great post, I had no idea the meter needles were hollow. I've got a Sun dwell meter with a broken needle and I've got a NOS Sun milliamp meter that has the same type of needle. Is there a way to loosen the glue to remove a needle so I can do a swap?
Keith


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 Post subject:
PostPosted: Feb Sat 19, 2011 6:57 pm 
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Acetone will dissolve it, but you don't want to get it on anything but the meter needle joint. Try soaking a swab in acetone and holding it right on the joint. After a few minutes VERY GENTLY try to push the meter needle off.

-Matthew

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PostPosted: Mar Tue 01, 2011 8:59 am 
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Hey there Matthew --

WOW !! :shock:

Now THAT is a well-written restoration tutorial !!

Thanks to you, all of those meter movements in the junk box may have new life once again !!

I have never seen such a thorough article geared towards the repair of such a vital piece of older technology, the analog meter. And in today's world, some old meter movements are difficult to find, all the more reason to save them !!

Thanks so very much again ! Have you considered authoring an e-book with such amazing info ?? It would be an amazing hit here !!! :wink:

All the best --

Tom

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PostPosted: Mar Tue 01, 2011 1:20 pm 
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Very informative on meter repair and balancing!
Could you drop over to my 539C meter post? 8)
At the very least I'll try the balancing effort.
Thanks again, and look forward to any other of your movement repair/ adjustment posts.


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PostPosted: Mar Tue 01, 2011 9:10 pm 
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7jp4-guy wrote:
Thanks for all of the compliments. :D Do you guys think that I should put a link to this in the Hints & Kinks forum?

-Matthew



YES!

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