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 Post subject: Analog Panel Meter Help Needed
PostPosted: Jan Wed 31, 2018 12:40 am 
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Joined: Sep Mon 28, 2009 1:53 am
Posts: 213
Location: Bloomington, IN, 47401
I just got in a 1968 Mercury 990 Tube Tester, and it appears the meter is dead. I have the manual and tube data; when I set the tester for a particular tube, I can see the tube glow, so I know it's getting power. Voltages out of the transformer appear correct.

However, when I move the switch to "Test", I get nothing. A few tests and it's surely dead.

The bezel of the meter is 2-9/16" by 2-3/4"; the body of the meter is a 2.50" (2.43" actual). The parts list in the manual describes it as "1mA, 950 ohm". Can't seem to find a comparable model in the usual spots. I'll need to be able to open the meter so I can swap out the scale/data plate from the old one.

Any links to such a meter?


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 Post subject: Re: WTB - Analog Panel Meter Needed
PostPosted: Jan Wed 31, 2018 7:53 am 
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Joined: Mar Tue 13, 2012 6:56 am
Posts: 1063
Location: San Diego
Did you check it with an ohmmeter?

There are a few at Fair Radio Sales that might work. This one, for instance: https://fairradio.com/product/ma-1-1/

You might have to use a Dremel to swap out the scale.

Matt


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 Post subject: Re: WTB - Analog Panel Meter Needed
PostPosted: Feb Fri 02, 2018 4:54 pm 
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Joined: Sep Mon 28, 2009 1:53 am
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Location: Bloomington, IN, 47401
skylerca wrote:
Did you check it with an ohmmeter?


Yes. Windings are open.

skylerca wrote:
There are a few at Fair Radio Sales that might work. This one, for instance: https://fairradio.com/product/ma-1-1/


I have one on order from China, but I'll keep your link as a back-up. Thanks!


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 Post subject: Re: WTB - Analog Panel Meter Needed
PostPosted: Feb Mon 26, 2018 6:49 am 
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Joined: Sep Mon 28, 2009 1:53 am
Posts: 213
Location: Bloomington, IN, 47401
Got the replacement. Took a bit of work to swap the scale from the old to the new meter. Works, but seems to be reading a bit high (winding resistance may be lower in the new meter than the old).


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 Post subject: Re: Analog Panel Meter Help Needed
PostPosted: Feb Wed 28, 2018 12:21 am 
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I mover here for discussion. Try adding resistance to the new meter to equal the 950 ohms of the original.


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 Post subject: Re: Analog Panel Meter Help Needed
PostPosted: Feb Wed 28, 2018 4:08 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island
Knowing that full scale deflection occurs at 1-mA is only half the information. The other piece is the sensitivity of the meter, which is expressed in its resistance, or sometimes in millivolts. Current, voltage, and resistance are related by Ohm's Law, so if you have two of the three numbers you can calculate the third.

In this case, 950 ohms x 0.001 amp = 0.95 volts, or 950 millivolts across the coil at full scale. Another meter might have more resistance, in which case it would require more voltage to get to 1-mA. That would be a less sensitive meter. A more sensitive meter would have less resistance, i.e. it gets to 1-mA at a lower voltage. The tube tester circuitry was calculated for a meter with a particular sensitivity; if you put a different meter in with more or less sensitivity, the calibration will be off.

You can substitute a more sensitive meter for a less sensitive one by adding a resistor in series. For example, if the new meter is 550 ohms at 1-mA, you could put 400 ohms in series with it, then you'd have a match for the original meter. It is not possible to make a less sensitive meter work in place of a more sensitive one by this method; no resistor you put in series or parallel will make a meter more sensitive than it already is. However, in many instances the meter coil is made for some higher sensitivity and one or more internal resistors are added at the factory to bring it into calibration. You can then remove the factory resistor, and if the basic movement has enough sensitivity, add your own resistor to bring it to the desired place.

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 Post subject: Re: Analog Panel Meter Help Needed
PostPosted: Feb Wed 28, 2018 9:00 pm 
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Joined: Sep Mon 28, 2009 1:53 am
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Location: Bloomington, IN, 47401
Lou deGonzague wrote:
I mover here for discussion. Try adding resistance to the new meter to equal the 950 ohms of the original.


Yeah, I just haven't gotten around to doing the fix yet.

As it turns out, the winding broke at the post. I probably could have taken one turn off the coil and reattached it to the post, but I didn't think to look before I replaced. Only noticed when I was swapping out the faceplates.


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 Post subject: Re: Analog Panel Meter Help Needed
PostPosted: Mar Thu 01, 2018 3:24 am 
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Location: Ashhurst, New Zealand
Have you had the meter apart to verify that it is actually the movement that is open circuit? Often in cheap meters they just make a batch of meters and then calibrate them by adding a series resistor to bring them up to a standard value, often 1kohm or near enough - your 'padder' might be open cct.

DON'T put your ohm-meter directly across the movement - the needle will try to go round three times then give up!

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 Post subject: Re: Analog Panel Meter Help Needed
PostPosted: Mar Sun 04, 2018 4:03 am 
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Joined: Sep Mon 28, 2009 1:53 am
Posts: 213
Location: Bloomington, IN, 47401
When I had the faceplate off, I could see a wire on the coil of the old meter had broken away (24-28 gauge; tiny wire) from a post. Once I was able to get a very close look, it was the beginning of the coil, so it had the rest of the winding on top of it.

I gave a moment's though to unwinding the coil, but figured I'd likely break such a fine wire trying to rewind the coil. So, no hope for that one.

I need to add a resistor to bring it up to the proper ohms, but likely won't get around to it. I've had an offer to sell all the tube testers (except one for my own use) to a guy who has a museum of them, and since no one seems willing to buy the lot, I may have to go that route very soon. Since it will be one of 70+ testers, I doubt the potential buyer is concerned about accuracy.


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