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 Post subject: Meter movements
PostPosted: Apr Tue 27, 2010 1:27 am 
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I'm curious about what the common problems are and their solutions.
I just fixed a Sprague tel-ohmike TO-6As' meter that was sticking a bit on the end of the scale. With some luck and being as careful as possible with a can of computer type blast-o, it's smooth throughout the scale, and accurate too. I suppose this is a general question, but in reference to the meter movements in both ME-26 D/Us I have. They seem nice and fluid, but when trying to calibrate (switching between ac and dc ranges to check for 0) it will jump a bit and stay there. On another one (it's a bit worse) the needle is quite moveable, but will stay where it is (sometimes even quarter ways up the scale) when it's turned off.
These movements are old. Anyone know of a standard movement "spruce up"? Any chance in applying a lubricant of some sort? If so whats the proper type?


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PostPosted: Apr Tue 27, 2010 3:12 am 
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That's a really interesting post. I have a couple meters that could use some sprucing up.

Hopefully someone knows something.

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 Post subject: Re: Meter movements
PostPosted: Apr Tue 27, 2010 3:20 am 
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quadlibet2001 wrote:
I'm curious about what the common problems are and their solutions.
I just fixed a Sprague tel-ohmike TO-6As' meter that was sticking a bit on the end of the scale. With some luck and being as careful as possible with a can of computer type blast-o, it's smooth throughout the scale, and accurate too. I suppose this is a general question, but in reference to the meter movements in both ME-26 D/Us I have. They seem nice and fluid, but when trying to calibrate (switching between ac and dc ranges to check for 0) it will jump a bit and stay there. On another one (it's a bit worse) the needle is quite movable, but will stay where it is (sometimes even quarter ways up the scale) when it's turned off.
These movements are old. Anyone know of a standard movement "spruce up"? Any chance in applying a lubricant of some sort? If so whats the proper type?

Usually there is a jewel in the pivot.... no lubrication needed or wanted. If the jewel is bad you can get odd meter movement. There is backlash adjustment, but I would not move it unless you know what you are doing. If there is a lot of play it might mean the jewel is gone. They can crack and just disintegrate, which makes the movement loose.

Spraying cleaner or even air into it a movement can un-coil the hair spring or springs.... that is a bad things. I would not use any cleaner because it can dissolve things and leave residue. It just is not advised.

The most common thing to do is fish out junk; some times dirt, or magnetic material from the permanent magnet flakes off. I use tiny strips of tape to fish it out.. Those wayward flakes of magnetic material can cause a needle to stick or have memory and hang at some spots.....

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PostPosted: Apr Tue 27, 2010 3:43 am 
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I was going to try the tape method next on the sprague if the air didn't work. I had the movement removed from the housing, making sure the hair springs weren't being blown around. 1st try blew whatever on the magnet so that the needle would slow down at a different spot! One more try got it right. Someone else on here mentioned that a static electricity charge can mess with a meter too, recommending wiping the meter face with a damp cloth.
These military VTVM movements though, I'm finding have a typical problem ; the needle is able to move fully throughout the scale, not even a hint of catching, by applying a signal/voltage/resistance etc, or by using the adjustment pots. It just seems stubborn, and likes to 'rest' at no particular point (always close to 0) but never quite on 0.


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 Post subject:
PostPosted: Apr Tue 27, 2010 1:54 pm 
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Meter movements are not intended to be lubricated.

Before opening a meter, it is important to check that nothing external is causing the symptoms you are seeing. A static charge on a plastic face cover can make a needle seem like it is sticking. Currents from external circuitry can make it look like the zero isn't set correctly. If a speaker or transformer has been replaced in the unit, its magnetic field may differ from the original part and cause a meter to behave erratically.

If a meter sticks, the first thing to look for is whether or not it sticks at the same place on the scale every time.

If it does, the usual cause is a bit of debris between the coil and the pole faces. This can often be removed without taking the movement completely apart with a piece of tape, or a small feather. Use of compressed air is not recommended due to the chances of damaging the hair springs.

If the meter sticks over an area of the scale, not just one point, there may be pivot damage, pivots that are out of adjustment, or deformed hair springs. If a meter takes a hard fall, it can bend or deform the ends of the pivots. It can also crack the jewels, which are often made of glass on the less expensive movements. These issues are difficult to deal with if you don't have parts and the tools to change them.

However, before condemning a sticky meter, it is wise to check the end shake of the pivots. They do go out of adjustment sometimes due to shock and vibration, tampering, and dimensional changes in the pot metal parts that were sometimes used. On meters with conical pivots, this can be done by gently pushing the pivot sideways. There should be a slight bit of play, perhaps half the thickness of a hair. More than that, and the meter will be sloppy, perhaps hanging in random places on the scale. Less play than that and it will bind.

The way to adjust the pivots is to begin by loosening the lock nut on one of them and turning it out a quarter-turn or so at a time until the end shake becomes really obvious. Then start tightening it an eighth-turn at a time, checking the motion of the needle each time. A point should be reached where the friction is minimized and there is no sticking. You don't want to tighten the pivot any further than that! Just lock it down and test it to make sure nothing changed. Over-tightening meter pivots can ruin them instantly.

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 Post subject:
PostPosted: Apr Tue 27, 2010 5:07 pm 
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quadlibet2001 wrote:
<snip>

Someone else on here mentioned that a static electricity charge can mess with a meter too, recommending wiping the meter face with a damp cloth.


I don't know about damp cloth. An anti-static dryer sheet's worked wonders on every meter face I've had to deal with.

Keep the sheet in the tool box afterwards for that fresh clean scent............. :)

David


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 Post subject:
PostPosted: Apr Tue 27, 2010 5:48 pm 
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Very good info Chris! Thanks.


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 Post subject:
PostPosted: Apr Tue 27, 2010 7:02 pm 
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One thing I used to do, is slightly blow on the meter pointer. This will show if there is any binding. Most meters are a lot more rugged than one would suppose. I have cleaned the bearing surfaces with spray freon (which is no longer produced) while keeping the pointer braced between two fingers to prevent movement. The pointers themselves are quite fragile so don't touch the pointer with your fingers, just use them as a stops.. You might also use alcohol and swish the meter in a container filled with Isopropyl Alcohol.

Use a magnifier to see the pivots and look for debris around or in them.

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 Post subject:
PostPosted: Apr Tue 27, 2010 7:07 pm 
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dholland wrote:
quadlibet2001 wrote:
<snip>

Someone else on here mentioned that a static electricity charge can mess with a meter too, recommending wiping the meter face with a damp cloth.


I don't know about damp cloth. An anti-static dryer sheet's worked wonders on every meter face I've had to deal with.

Keep the sheet in the tool box afterwards for that fresh clean scent............. :)

David
I've found by just exhaling warm breath on a meter glass will usually neutralize any static charge.

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 Post subject:
PostPosted: Apr Tue 27, 2010 9:38 pm 
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CRIPES!!!!! The meters in question are SEALED!!!! :evil: :twisted: :evil: :twisted: :evil: :twisted:


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 Post subject:
PostPosted: Apr Wed 28, 2010 1:42 pm 
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Well,

Time for a hammer to beat the cases off the things.

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 Post subject:
PostPosted: Apr Wed 28, 2010 2:13 pm 
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You need a lesson from the Mark Palmer School of D'Arsonval Meter Movement Repair:

1. Disconnect meter from circuit and remove from equipment.
2. Open meter case. If sealed, pry open with old screwdriver.
3. Stare at meter movement for several minutes and ponder.
4. Flick meter needle a few times with finger.
5. Poke meter spring with small screwdriver.
6. Loosen meter jewel bearings, and re-tighten.
7. Blow in to meter magnets and coils.
8. Hook up meter to test.
9. Discover meter now works 100 times worse than it was before.
10. Toss meter in trash.

-Mark-


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 Post subject:
PostPosted: Apr Wed 28, 2010 2:16 pm 
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Just what are friends for.....?

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 Post subject:
PostPosted: Apr Wed 28, 2010 3:26 pm 
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Location: Westland, MI
There are two things in this hobby that I wished I could learn to do successfully...

1. Rebuild and repair speakers and microphones.

2. Rebuild and repair meter movements.

I didn't realize that I just recently became a graduate of the Mark Palmer school of meter repair while working on one of my VTVMs.

Boy, that *new* VTVM I bought to replace my old one really works nice! :lol:

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 Post subject:
PostPosted: Apr Wed 28, 2010 3:29 pm 
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I hate sealed meters too, but they can be dealt with. Just takes a little more work.

A method of opening them was suggested by Jim Koehler, a "fellow traveler" here on ARF. He clamps them in a lathe and hits the edge with a propane torch until the solder melts. A piece of cloth needs to be handy to catch the glass as it falls.

My method is to use a Dremel with a small burr to dig out as much of the old solder around the glass as possible, then a little light heating with a torch to get the last of the solder to let go. The Dremel should be run at about half speed to make it easier to handle. Takes longer, but less chance of damage to the meter or the glass.

When (if) the meter is ready to be put back together, I simply use non-acid, electronics grade silicone seal to glue the glass back in.

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 Post subject:
PostPosted: Apr Wed 28, 2010 6:33 pm 
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I found liquid fabric softener to be the ultimate in solving my meter face static issue.....

I was selling one of my VTVM's and decided to polish the plastic face. I took it off. It looked great with some fine buffing compound and hand rubbed the heck out of it with a buffing cloth. It was so full of static, steam would not clear it.... nothing would fix it.

The static was so strong I could not get rid of it.... until I got the fabric softener out. I rubbed it on full strength and rinsed with a very small stream of water, while using my fingers to rub it around. After a complete rinse, I used a terrycloth towel to dry, to avoid water spots. I blotted and wiped very carefully (not like I did when I buffed the hell out of it). The end result was static free meter face.

PS I removed the face and treated the inside and front.... I let it finish air dry and re-installed. A bonus.... it smells great.

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 Post subject:
PostPosted: Apr Wed 28, 2010 6:38 pm 
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quadlibet2001 wrote:
CRIPES!!!!! The meters in question are SEALED!!!! :evil: :twisted: :evil: :twisted: :evil: :twisted:


Not if you have time and take them apart. :roll:

I used to do that with even the military sealed meters. And some of those cases were Soldered. :wink:

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 Post subject:
PostPosted: Apr Wed 28, 2010 8:17 pm 
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Mark,

Does your D'Arsonval meter movement repair course offer an official diploma that we can post on our shop wall?


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 Post subject:
PostPosted: Apr Wed 28, 2010 8:33 pm 
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I should print something like that... I have a funny feeling everyone who's taken apart a meter has been through the course at least once :D

-Mark-


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 Post subject:
PostPosted: Apr Wed 28, 2010 10:39 pm 
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OK here ya go

Image

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