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 Post subject: Meter repair - Sprague TO-5
PostPosted: Dec Wed 27, 2006 7:42 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Waterloo, Ontario, Canada
Both the voltage and leakage current read about 25% low on my Sprague TO-5 capacitor checker. I changed the related resistors and still have the problem. I removed the meter and tested it. It requires exactly 6 ma for full deflection. The schematic calls for a 5 ma meter. It seems unlikely that the meter would drift to exactly 6 ma so I am assuming that the wrong meter was installed. Experience tells me that it is more likely that I have made a mistake and Sprague installed the correct meter. I can confirm that it did take 6ma for full deflection.

I have 3 questions:

1/ Is it reasonable to assume that the wrong meter was installed?

2/ I opened it up and there is a shunt resistor. To change this to a 5 ma meter I need to change resistor to a higher value. Can I remove this resistor, measure it, do a bit of math and calculate the new value?

3/ While I can remove the 3 screws on the sides and open the meter and see the shunt resistor, I do not have room for a soldering iron. How do I get the plastic back off. On the back there are 2 recessed smaller nuts but not enough room for a socket. On the face side there are 2 small screws that look like they would only release the face. It looks like I have to disassemble the movement and really really don't want to do that. I need a little advise here.

Thanks
JimT


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 Post subject:
PostPosted: Dec Wed 27, 2006 9:19 pm 
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Location: Maryland, USA
Hi Jim,

Most decent meter movements are held in the case by nuts on the two large studs on the back, where you connect the leads. Remove the front cover and those nuts, and the whole movement including the studs should come forward out of the case.

Cleanliness is very important when working on meter movements (no, not washing your hands :roll: ). Make sure there is nothing magnetic anywhere close. Metal filings will kill the movement. And keep any lint and dust away also.

Better meters are built a bit more sensitive than spec, and a shunt resistor is added to calibrate the final full-scale value. It's possible that your shunt has decreased in value over time, thus increasing the current required for a full-scale reading.

You can change that resistor to bring the sensitivity back up. Put a small variable resistor in place of the shunt and adjust it for the desired 5 mA full scale. You may have to play with the value a bit. Measure the existing shunt and use a variable around twice that value as a starting point.

Start out with the variable adjusted to minimum (zero ohms across the movement) and increase its value to increase the FS reading at a given current. You want to use a fairly high voltage supply and a large value resistor in series with the meter, so this resistor controls the maximum current through the circuit.

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http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


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 Post subject:
PostPosted: Dec Thu 28, 2006 12:45 am 
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Location: Las Vegas, NV, USA 89052
As to your question of it being a correct meter, "Model TO-4" is actually printed on the lower porton of my TO-4's meter face. Check yours; this may be true for the TO-5 model too.


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 Post subject:
PostPosted: Dec Thu 28, 2006 1:14 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 25989
Location: Pocasset, Cape Cod, MA
I have seen "permanent" magnets lose strength. Some meters had magnetic shunts, strips of iron around the outside of the case, which could be moved and locked in place with a screw or nut.

However I agree that the shunt resistor is probably the source of the trouble. I'd expect the basic meter movement to be 1 mA FS.


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 Post subject:
PostPosted: Dec Thu 28, 2006 1:48 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 49
Location: Waterloo, Ontario, Canada
Thanks for all of the answers.

Leigh:
The large nuts do not release the cover. But this no longer matters as I am planning to just snip the shunt out, measure and mount a variable resistor externally. This seems like quite a good idea.

Greg Dan:
The face is labeled correctly. It is for a TO-5. I had guessed that the movement was the wrong one, but I think I must be wrong.

Alan Douglas:
Cool, there IS an iron bar on the top held on by an aluminum "case". I did not even notice it, but its obvious now!! I'm too chicken to play with this so I will just go with the variable shunt, but if I get "spare" meter I will have to play with this.

Thanks for all the help. When I get this finished I will post the values as they may serve as a guide to someone else.

Jim Tremain


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 Post subject:
PostPosted: Dec Thu 28, 2006 2:55 am 
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Posts: 25989
Location: Pocasset, Cape Cod, MA
If you have the magnetic shunt, it's easy. Mark the original location with a pencil, and try moving it. It's not all that critical, and you can't possibly screw anything up.


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 Post subject:
PostPosted: Dec Thu 28, 2006 4:30 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 49
Location: Waterloo, Ontario, Canada
It seems that this meter has the resistor in series, not what I expected. I measured the original resistance of the whole meter at 25.5 Ohms. I removed this resistor, it measures 15.3 Ohms. Across the terminals the meter now measures infinite resistance. When I looked more closely I could see that the resistor was originally in series. I was able to get in with a probe tip and the meter coil measures 10.2 Ohms. I only know a bit about meters with parallel shunts and nothing about this series type. Can someone explain this for me?

Thanks
Jim


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 Post subject:
PostPosted: Dec Thu 28, 2006 6:04 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 49
Location: Waterloo, Ontario, Canada
Alan, I looked more closely at the iron bar magnetic shunt. The iron bar is slightly inset into the magnet on both sides and cannot be moved as it is a good fit. Perhaps this is just structural reinforcement or the magnet shunt was changed by using different thicknesses of iron.


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 Post subject:
PostPosted: Dec Thu 28, 2006 6:31 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Pocasset, Cape Cod, MA
Oh well.

A series resistor (multiplier resistor) affects the meter voltage, but not the current.


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 Post subject:
PostPosted: Dec Thu 28, 2006 8:00 pm 
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Posts: 21033
Location: Maryland, USA
As Alan said, a series resistor changes this into a voltmeter. You can reset the range to 5 mA FS by decreasing the value of this resistor. But be careful.

It's not a good idea to measure the resistance of a meter movement directly. Many ohmmeters can supply enough current to damage the meter movement.

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73 de Leigh W3NLB
http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


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